Organic Chemistry

Learning Objectives

• Understand hybridization (sp³, sp², sp) and tetravalency of carbon in organic compounds
• Apply IUPAC nomenclature rules to name complex organic molecules systematically
• Analyse electron displacement effects — inductive, resonance, hyperconjugation, electromeric
• Distinguish between structural and stereoisomerism with solved NEET examples
• Predict products of hydrocarbon reactions: Markovnikov addition, ozonolysis, electrophilic substitution
• Compare SN1/SN2 and E1/E2 mechanisms with stereochemical outcomes
• Master functional group interconversions: alcohols to aldehydes to acids and beyond
• Identify biomolecules, polymers, and drugs relevant to NEET and everyday chemistry

1. Fundamental Concepts & Nomenclature

Tetravalency & Hybridization

Carbon has electronic configuration 1s² 2s² 2p². It exhibits tetravalency by promoting one 2s electron to the 2p orbital, forming four equivalent sp³ hybrid orbitals in methane. The type of hybridization determines the geometry and bond angles of the molecule.

Carbon can adopt three hybridization states depending on the number of sigma bonds and lone pairs.

Hybridization	Orbitals Used	Geometry	Bond Angle	Examples
sp³	1s + 3p	Tetrahedral	109.5°	CH₄, C₂H₆, C₂H₅OH
sp²	1s + 2p	Trigonal planar	120°	C₂H₄, C₆H₆, HCHO
sp	1s + 1p	Linear	180°	C₂H₂, HCN, CO₂

Sigma (σ) & Pi (π) Bonds

All covalent bonds are classified as σ or π bonds. A σ bond is formed by the head-on (end-to-end) overlap of atomic orbitals along the internuclear axis. It is strong, cylindrically symmetrical, and allows free rotation. A π bond is formed by the lateral (sideways) overlap of unhybridised p-orbitals. It is weaker than a σ bond and restricts rotation (giving rise to geometrical isomerism).

Single bonds (C—C) consist of one σ bond. Double bonds (C=C) consist of one σ + one π bond. Triple bonds (C≡C) consist of one σ + two π bonds (the second π bond is perpendicular to the first). The bond strength order: C≡C > C=C > C—C. The π electrons are more exposed and more polarizable than σ electrons, making alkenes and alkynes more reactive than alkanes towards electrophilic addition.

Bond Type	Orbital Overlap	Strength	Rotation	Electron Location
σ (single)	Head-on (s-s, s-p, p-p)	Strong	Free rotation	Between nuclei (internuclear axis)
π (in double bond)	Sideways (p-p)	Weaker than σ	Restricted	Above and below the internuclear axis
π (in triple bond)	Two perpendicular sideways overlaps	Two π bonds, one σ	Restricted	Around the axis in two planes

IUPAC Nomenclature

The International Union of Pure and Applied Chemistry (IUPAC) system provides a standard method for naming organic compounds. The process involves three steps: 1. Identify the longest continuous carbon chain (parent chain). 2. Number the chain to give the lowest locants to substituents. 3. Name substituents in alphabetical order.

Alkanes, Alkenes, Alkynes

Alkanes end in -ane (methane, ethane, propane). Alkenes end in -ene (ethene, propene) and alkynes end in -yne (ethyne, propyne). For alkenes and alkynes, the position of the multiple bond is indicated by the lowest possible locant.

Functional Groups

When multiple functional groups are present, priority determines the suffix. The principal functional group is used as the suffix; all others are treated as prefixes.

Priority	Functional Group	Formula	Suffix	Prefix
1	Carboxylic acid	—COOH	-oic acid	carboxy
2	Sulfonic acid	—SO₃H	-sulfonic acid	sulfo
3	Ester	—COOR	-oate	alkoxycarbonyl
4	Acyl halide	—COX	-oyl halide	halocarbonyl
5	Amide	—CONH₂	-amide	carbamoyl
6	Nitrile	—C≡N	-nitrile	cyano
7	Aldehyde	—CHO	-al	formyl / oxo
8	Ketone	—CO—	-one	oxo
9	Alcohol	—OH	-ol	hydroxy
10	Amine	—NH₂	-amine	amino

Priority Rules

When numbering, the principal functional group gets the lowest number. If there is a tie, the first point of difference rule is applied. Substituents are listed alphabetically (ignoring prefixes di-, tri-, tetra-).

Common Names vs IUPAC Names

Many organic compounds are still widely known by their common (trivial) names. NEET often tests both. Important examples: HCHO — Formaldehyde (methanal); CH₃CHO — Acetaldehyde (ethanal); C₆H₅CHO — Benzaldehyde; HCOOH — Formic acid (methanoic acid); CH₃COOH — Acetic acid (ethanoic acid); C₆H₅OH — Phenol (benzenol); CHCl₃ — Chloroform (trichloromethane); CCl₄ — Carbon tetrachloride (tetrachloromethane); CH₂=CH₂ — Ethylene (ethene); CH≡CH — Acetylene (ethyne); C₆H₆ — Benzene; CH₃COCH₃ — Acetone (propanone); CH₃CH₂OH — Ethyl alcohol (ethanol).

Common Name	IUPAC Name	Formula
Formaldehyde	Methanal	HCHO
Acetaldehyde	Ethanal	CH₃CHO
Acetone	Propanone	CH₃COCH₃
Formic acid	Methanoic acid	HCOOH
Acetic acid	Ethanoic acid	CH₃COOH
Chloroform	Trichloromethane	CHCl₃
Ethylene	Ethene	CH₂=CH₂
Acetylene	Ethyne	HC≡CH
Glycerol	Propane-1,2,3-triol	HOCH₂CH(OH)CH₂OH
Aniline	Benzenamine	C₆H₅NH₂

Electron Displacement Effects

These effects govern the electron density distribution in a molecule, influencing reactivity, acidity, basicity, dipole moment, and orientation in substitution reactions. Understanding these effects is essential for predicting reaction outcomes in NEET organic chemistry. There are four main types: inductive, resonance (mesomeric), hyperconjugation, and electromeric.

Inductive Effect (I-effect)

Permanent polarization along a sigma bond due to electronegativity difference. It is a through-bond effect that diminishes with distance (practically negligible after 3-4 bonds). Groups that withdraw electrons via sigma bonds are -I groups: —NO₂, —CN, —COOH, —F, —Cl, —Br, —I, —OH, —OR, —NH₃⁺, —CHO, —COR. Groups that donate electrons via sigma bonds are +I groups: —CH₃, —C₂H₅, —CH(CH₃)₂, —C(CH₃)₃ (alkyl groups), —O⁻, —COO⁻.

Consequences of Inductive Effect: (1) Acidity of carboxylic acids: Electron-withdrawing groups increase acidity by stabilizing the conjugate base. Cl₃CCOOH > Cl₂CHCOOH > ClCH₂COOH > CH₃COOH. (2) Basicity of amines: Electron-donating alkyl groups increase electron density on nitrogen, enhancing basicity. (3) Dipole moment: —I groups create a dipole with the positive end on the group.

Resonance / Mesomeric Effect (M-effect)

Delocalization of pi electrons or lone pairs through conjugated systems. It is represented by drawing multiple contributing structures (resonance structures) connected by double-headed arrows (↔). The actual molecule is a hybrid of all contributing structures and is more stable than any individual contributing structure. Rules for resonance: (1) Only π electrons and lone pairs move; σ bonds are never broken. (2) All contributing structures must have the same atomic framework. (3) The most stable structure has the maximum number of covalent bonds and minimal charge separation. +M groups donate electrons via resonance: —OH, —NH₂, —OR, —NHR, —NR₂, —X (F, Cl, Br, I). -M groups withdraw electrons via resonance: —NO₂, —CN, —CHO, —COR, —COOH, —COOR, —CONH₂.

Note: The mesomeric effect is stronger than the inductive effect. For example, —OH is strongly electron-donating by resonance (+M) but weakly electron-withdrawing by induction (-I). The net effect determines the overall electron density distribution.

Hyperconjugation (Baker-Nathan Effect)

The interaction between sigma (σ) electrons of a C—H (or C—C) bond with an adjacent empty or partially filled p-orbital or pi (π*) system. Also called no-bond resonance because it can be represented by resonance structures where the C—H bond is broken. The number of α-hydrogen atoms directly correlates with the extent of hyperconjugation: more α-H means greater delocalization and more stabilization. Applications: (1) Carbocation stability: (CH₃)₃C⁺ (9 α-H) > (CH₃)₂CH⁺ (6 α-H) > CH₃CH₂⁺ (3 α-H) > CH₃⁺ (0 α-H). (2) Alkene stability: more highly substituted alkenes are more stable (R₂C=CR₂ > R₂C=CHR > R₂C=CH₂ > RHC=CH₂). (3) Free radical stability: 3° > 2° > 1° > CH₃·.

Electromeric Effect (E-effect)

The complete transfer of a pi-electron pair to one of the atoms at the demand of an attacking reagent. It is a temporary, polarizable effect — it occurs only at the moment of attack by a reagent and ceases once the reagent is removed. +E effect: The π-electron pair moves towards the attacking reagent (observed in alkenes during electrophilic addition). -E effect: The π-electron pair moves away from the attacking reagent (observed in carbonyl compounds during nucleophilic addition). The electromeric effect is represented by a curved arrow showing the direction of electron movement.

Effect	Type	Permanent / Temporary	Transmission
Inductive	σ-electron displacement	Permanent	Through sigma bonds
Resonance	π-electron delocalization	Permanent	Through conjugated pi system
Hyperconjugation	σ → π interaction	Permanent	Through overlapping orbitals
Electromeric	π-electron transfer	Temporary	At the point of attack

Reactive Intermediates

Carbocations, Carbanions, Free Radicals

Carbocations (R₃C⁺) are electron-deficient, planar (sp²), and stabilized by resonance and hyperconjugation. Stability order: 3° > 2° > 1° > methyl. Carbanions (R₃C⁻) are electron-rich, pyramidal (sp³), and stability is reverse: 1° > 2° > 3°. Free radicals (R₃C·) are neutral species with an unpaired electron; stability: 3° > 2° > 1° > methyl.

Intermediate	Hybridization	Geometry	Stability Trend
Carbocation	sp²	Trigonal planar	3° > 2° > 1° > CH₃⁺
Carbanion	sp³	Pyramidal	CH₃⁻ > 1° > 2° > 3°
Free radical	sp² (approx.)	Trigonal planar	3° > 2° > 1° > CH₃·

2. Isomerism

Isomers are different compounds that have the same molecular formula but differ in the arrangement of atoms. Isomerism is broadly classified into structural isomerism (different connectivity) and stereoisomerism (same connectivity, different spatial arrangement). This topic is highly scoring in NEET, with frequent questions on chiral centres, geometrical isomerism, and conformational analysis.

Structural Isomerism

Chain, Position, Functional, Metamerism

Chain isomerism: Different carbon skeleton (n-butane vs isobutane). Branched isomers have lower boiling points due to reduced surface area. Position isomerism: Same skeleton, different position of substituent or functional group (1-propanol vs 2-propanol). Position isomers often have different chemical reactivity — 1° alcohols oxidize to aldehydes/acids while 2° alcohols oxidize to ketones. Functional group isomerism: Same molecular formula but different functional group (C₂H₅OH alcohol vs CH₃OCH₃ ether; C₂H₄O₂ can be acetic acid, methyl formate, or glycolaldehyde). Metamerism: Different alkyl groups on either side of a polyvalent functional group such as —O—, —CO—, —NH— (C₂H₅—O—C₂H₅ vs CH₃—O—C₃H₇). Tautomerism: A special type of functional group isomerism where two isomers (tautomers) exist in dynamic equilibrium, differing in the position of a proton and a double bond. Keto-enol tautomerism is the most common type. The keto form is generally more stable except when the enol form is stabilized by resonance (phenol) or intramolecular hydrogen bonding (β-diketones).

Type	Example Pair	Formula	Key Feature
Chain	n-butane / isobutane	C₄H₁₀	Different carbon skeleton
Position	1-chloropropane / 2-chloropropane	C₃H₇Cl	Same skeleton, different substituent position
Functional group	Ethanol / Dimethyl ether	C₂H₆O	Alcohol vs ether functional groups
Metamerism	Diethyl ether / Methyl propyl ether	C₄H₁₀O	Different alkyl groups around O
Tautomerism	Keto / Enol (acetoacetic ester)	C₄H₆O₂	Proton and double bond shift
Ring-chain	Cyclopropane / Propene	C₃H₆	Cyclic vs open-chain structure

Stereoisomerism

Stereoisomers have the same structural formula (same connectivity) but differ in the three-dimensional arrangement of atoms. They are classified into geometrical (cis-trans/E-Z), optical (enantiomers/diastereomers), and conformational isomerism.

Geometrical Isomerism (Cis-Trans / E-Z Isomerism)

Arises due to restricted rotation about a double bond (C=C) or in cyclic compounds. For a compound to exhibit geometrical isomerism, each of the two doubly bonded carbons must have two different substituents. In the cis-trans system, identical groups on the same side = cis; opposite sides = trans. The E-Z system (Cahn-Ingold-Prelog priority rules) assigns priority based on atomic number: if higher-priority groups are on the same side → Z (zusammen, German for \"together\"); opposite sides → E (entgegen, \"opposite\"). The E-Z system is preferred when the cis-trans system is ambiguous (e.g., 1-bromo-1-chloropropene has Br and Cl on one carbon and CH₃ and H on the other). Geometrical isomers differ in physical properties (melting point, boiling point, dipole moment) and sometimes in chemical reactivity.

Optical Isomerism

Compounds that rotate plane-polarized light are optically active. A carbon with four different substituents is a chiral centre (stereocenter, asymmetric carbon). Enantiomers are non-superimposable mirror-image molecules. They have identical physical properties (melting/boiling point, density, solubility in achiral solvents) but differ in the direction of rotation of plane-polarized light — one is dextrorotatory (+), the other levorotatory (−). A 1:1 mixture of enantiomers is a racemic mixture (racemate), which is optically inactive. The R/S system (Cahn-Ingold-Prelog) assigns absolute configuration: assign priority (1→4) based on atomic number, orient the lowest priority group (4) away from view, then check the direction of 1→2→3; clockwise = R (rectus), anticlockwise = S (sinister). Diastereomers are stereoisomers that are not mirror images; they differ in physical properties (melting point, boiling point, solubility). Meso compounds: Achiral compounds with multiple chiral centres but an internal plane of symmetry (e.g., meso-tartaric acid). They are optically inactive. Number of optical isomers: For a compound with n chiral centres, the maximum number of optical isomers = 2ⁿ (if no meso form exists). If meso forms are possible, the number is less than 2ⁿ.

Conformational Isomerism

Different spatial arrangements achieved by rotation about single (σ) bonds. Conformers interconvert rapidly at room temperature. Newman projections visualize conformations by looking along a C—C bond axis. Ethane: Staggered conformation (more stable, torsional strain minimized, 0° dihedral angle between bonds) and eclipsed conformation (less stable, torsional strain ~12 kJ/mol due to bond-bond repulsion). Butane: Anti (most stable, dihedral angle 180°), gauche (less stable, 60°, steric strain from gauche butane interaction), and fully eclipsed (least stable). Cyclohexane: Chair conformation (most stable, all C—C bond angles ~109.5°, no angle strain), boat (less stable due to flagpole interactions), and twist-boat. Axial bonds (↑↓, parallel to the axis of the ring) and equatorial bonds (↗↘, pointing outward) interchange via ring flip. For substituted cyclohexanes, the bulkier substituent prefers the equatorial position to minimize 1,3-diaxial interactions. The preference is quantified by the A-value (energy difference between axial and equatorial). For methyl, A ≈ 1.74 kcal/mol; for tert-butyl, A ≈ 5.0 kcal/mol (essentially locks the ring).

3. Hydrocarbons

Hydrocarbons are compounds composed exclusively of carbon and hydrogen. They are classified as aliphatic (alkanes, alkenes, alkynes) and aromatic (benzene and its derivatives). Hydrocarbons form the fundamental building blocks of organic chemistry and are a core topic in the NEET syllabus, with questions on preparation, reactions, and mechanisms.

Alkanes (CₙH₂ₙ₊₂)

Preparation of Alkanes

1. Catalytic hydrogenation (Sabatier-Senderens): Alkene/alkyne + H₂ (Ni/Pt/Pd, 200-300°C) → alkane. This is a syn-addition of H₂. 2. Wurtz reaction: 2R—X + 2Na (dry ether) → R—R + 2NaX. Suitable only for symmetrical alkanes. Cross-coupling gives a mixture of three products. 3. Reduction of alkyl halides: R—X + Zn/HCl → R—H + ZnX₂ (or LiAlH₄/NaBH₄). 4. Decarboxylation (soda lime): R—COONa + NaOH/CaO (heat) → R—H + Na₂CO₃. The product alkane has one carbon less than the parent carboxylic acid. 5. Kolbe's electrolysis: Electrolysis of concentrated aqueous solution of sodium/potassium salt of carboxylic acid yields alkane at the anode: 2RCOO⁻ → R—R + 2CO₂ + 2e⁻. The alkane formed has double the number of carbon atoms. 6. Frankland reaction: 2R—X + Zn (in presence of water) → 2R—H + ZnX₂. 7. Grignard reagent with water: RMgX + H₂O → R—H + MgX(OH).

Reactions of Alkanes

1. Free radical halogenation: Alkanes + Cl₂/Br₂ (UV light or heat, 300-400°C) → haloalkane + HX. The reactivity of halogens: F₂ > Cl₂ > Br₂ > I₂ (I₂ does not react). The ease of H-abstraction: 3° > 2° > 1°. Selectivity increases from Cl₂ (low selectivity, almost statistical ratio) to Br₂ (high selectivity, predominantly 3° substitution). The reaction proceeds via free radical chain mechanism (initiation, propagation, termination). 2. Combustion: Alkane + O₂ → CO₂ + H₂O + heat. Incomplete combustion gives CO and soot. 3. Pyrolysis (cracking): High-temperature (500-700°C) decomposition in the absence of air yields smaller alkanes, alkenes, and H₂. 4. Isomerization: n-Alkanes → branched alkanes using AlCl₃/HCl catalyst. 5. Aromatization: Alkanes with 6+ carbons (Pt, 500°C) → aromatic compounds. Hexane → benzene + 4H₂.

Reaction	Reagent / Condition	Product	Key Feature
Wurtz reaction	2RX + 2Na (dry ether)	R—R + 2NaX	Symmetrical alkanes; cross-coupling gives mixture
Sabatier-Senderens	Alkene + H₂ (Ni, 200-300°C)	Alkane	Catalytic hydrogenation, syn addition
Decarboxylation	RCOONa + NaOH/CaO, heat	R—H + Na₂CO₃	One carbon less than parent acid
Kolbe's electrolysis	2RCOOK + H₂O (electrolysis)	R—R + 2CO₂ + H₂ + 2KOH	Doubles the number of carbon atoms
Frankland reaction	RX + Zn + H₂O (trace)	R—H + ZnX(OH)	Reduction of alkyl halide
Halogenation	Cl₂/Br₂ + hv or heat	R—X + HX	Free radical chain mechanism

Alkenes (CₙH₂ₙ)

Preparation of Alkenes

1. Dehydration of alcohols: R—CH₂—CH₂OH + conc. H₂SO₄ (170°C) or Al₂O₃ (350°C) → R—CH=CH₂ + H₂O. Follows E1 (for 2°/3° alcohols) or E2 (for 1° alcohols) mechanism. Zaitsev product (more substituted alkene) is favoured. 2. Dehydrohalogenation of alkyl halides: R—CH₂—CHX—R' + alc. KOH (heat) → R—CH=CH—R' + KX + H₂O. E2 elimination with strong base. Favours more substituted alkene (Zaitsev) except with bulky bases (K-tBuO) that favour Hoffmann product. 3. Dehalogenation of vicinal dihalides: R—CHX—CHX—R' + Zn (alcohol) → R—CH=CH—R' + ZnX₂. 4. Partial reduction of alkynes: Lindlar's catalyst (Pd/CaCO₃ + quinoline, H₂) → cis-alkene. Na/NH₃ (Birch reduction) → trans-alkene.

Addition Reactions — Markovnikov & Peroxide Effect

Markovnikov's rule: In the addition of HX (HCl, HBr, HI) to an unsymmetrical alkene, the hydrogen (the electropositive part) adds to the carbon with more hydrogen atoms (the less substituted carbon), and the halogen adds to the carbon with fewer hydrogen atoms (the more substituted carbon). This is because the reaction proceeds through the more stable carbocation intermediate. Peroxide effect (Kharasch effect): In the presence of organic peroxides (ROOR), HBr adds against Markovnikov's rule via a free radical mechanism. The Br· radical (formed by homolytic cleavage of HBr initiated by the peroxide) adds to the terminal (less substituted) carbon to form a more stable carbon radical. HCl and HI do not show the peroxide effect (HCl has a bond too strong to cleave homolytically; HI is easily oxidized by peroxides).

Other addition reactions: Hydration (H₂O/dil. H₂SO₄ → Markovnikov alcohol via carbocation, may involve rearrangement). Oxymercuration-demercuration [Hg(OAc)₂/H₂O then NaBH₄] → Markovnikov alcohol without rearrangement. Hydroboration-oxidation [BH₃/THF then H₂O₂/OH⁻] → anti-Markovnikov alcohol (syn addition of H and OH). Halogenation (Br₂/CCl₄ → vicinal dibromide; brown colour of Br₂ is discharged — test for unsaturation). Ozonolysis (O₃ then Zn/H₂O → aldehydes/ketones; structure of alkene deduced from ozonolysis products). Hydrogenation (H₂/Ni → alkane, syn addition). Anti-dihydroxylation (cold KMnO₄ → vicinal diol, Baeyer's test; or OsO₄ → syn dihydroxylation). Hot KMnO₄ cleaves the double bond.

Reaction	Reagent / Conditions	Product	Stereochemistry / Orientation
Markovnikov addition	HX (HCl, HBr, HI), no peroxide	Alkyl halide	H⁺ to C with more H (via carbocation)
Anti-Markovnikov	HBr + peroxide (ROOR)	Alkyl bromide	Br· to C with more H (free radical mechanism)
Hydration	H₂O + dil. H₂SO₄	Alcohol	Markovnikov; may involve rearrangement
Oxymercuration-demercuration	Hg(OAc)₂/H₂O then NaBH₄	Alcohol	Markovnikov, no rearrangement; syn addition
Hydroboration-oxidation	BH₃/THF then H₂O₂/OH⁻	Alcohol	Anti-Markovnikov; syn addition
Halogenation	Br₂ in CCl₄ (room temp)	vic-Dibromide	Anti addition (trans); brown colour discharged
Ozonolysis (reductive)	O₃ then Zn/H₂O	Aldehydes / Ketones	Cleaves C=C; useful for structural elucidation
Syn-dihydroxylation	Cold, dilute KMnO₄ (OH⁻) or OsO₄	vic-Diol (glycol)	Syn addition of two —OH groups (Baeyer test)
Catalytic hydrogenation	H₂ + Ni/Pt/Pd	Alkane	Syn addition of H₂

Alkynes (CₙH₂ₙ₋₂)

Acidic Nature & Metal Acetylides

Terminal alkynes (R—C≡C—H) are weakly acidic due to the high s-character (50%) of the sp-hybridized carbon. The pKa of acetylene is ~25 (compared to ~44 for ethene, ~50 for ethane). Terminal alkynes react with strong bases (NaNH₂, NaH, RMgX) to form metal acetylides: HC≡CH + NaNH₂ → HC≡C⁻ Na⁺ + NH₃. Silver acetylide (white precipitate with ammoniacal AgNO₃) and cuprous acetylide (red precipitate with ammoniacal Cu₂Cl₂) are used to distinguish terminal alkynes from internal alkynes (distinctive test). Internal alkynes (R—C≡C—R') lack the acidic H and do not form these precipitates.

Addition Reactions of Alkynes

Alkynes undergo two-step addition (adding one mole of reagent, then a second). 1. Hydrogenation: Lindlar's catalyst (Pd/CaCO₃ + quinoline, H₂) → cis-alkene. Na/NH₃ (Birch) → trans-alkene. Excess H₂/Ni → alkane. 2. Halogenation: Br₂ (1 eq, CCl₄) → trans-dibromoalkene; Br₂ (2 eq) → tetrahaloalkane. 3. Addition of HX: HX (1 eq) → vinyl halide (Markovnikov); HX (2 eq) → geminal dihalide (two halogens on the same carbon). 4. Hydration (Kucherov reaction): Alkyne + H₂O + HgSO₄/H₂SO₄ → enol (unstable) → tautomerizes to ketone. For HC≡CH, the product is CH₃CHO (acetaldehyde). For R—C≡CH, the product is R—CO—CH₃ (methyl ketone). For internal alkynes, the product is a mixture of ketones. 5. Ozonolysis: Alkynes + O₃ then H₂O → carboxylic acids + CO₂. 6. Polymerization: Passing acetylene through a red-hot tube gives benzene (cyclic trimerization). Linear polymerization gives polyacetylene (conducting polymer).

Aromatic Hydrocarbons

Benzene, Huckel's Rule & Aromaticity

Benzene (C₆H₆) is the parent aromatic compound — a planar, hexagonal molecule with 6 sp²-hybridized carbons and a delocalized π-electron system (6 π electrons, 3 double bonds). Each carbon contributes one p-orbital perpendicular to the ring; these p-orbitals overlap to form a continuous π-cloud above and below the plane. Hückel's rule: A planar, monocyclic, fully conjugated molecule with (4n+2) π electrons is aromatic (n = 0, 1, 2, 3...). n=1 → 6π (benzene); n=2 → 10π (naphthalene). Non-aromatic: Does not satisfy Hückel's rule (e.g., cyclooctatetraene, C₈H₈, 8π — tub-shaped, non-planar, non-aromatic). Antiaromatic: Planar, cyclic, fully conjugated with 4n π electrons (e.g., cyclobutadiene, 4π — extremely unstable). Aromatic ions: Cyclopentadienyl anion (C₅H₅⁻, 6π) is aromatic. Cyclopropenyl cation (C₃H₃⁺, 2π) is aromatic. Tropylium cation (C₇H₇⁺, 6π) is aromatic. Heterocyclic aromatics: Pyridine (C₅H₅N, 6π), Pyrrole (C₄H₅N, 6π with lone pair on N), Furan (C₄H₄O, 6π), Thiophene (C₄H₄S, 6π).

Electrophilic Aromatic Substitution (EAS)

Benzene's characteristic reaction is electrophilic substitution, not addition (addition would destroy the stable aromatic ring). The mechanism involves two steps: Step 1 (Slow, RDS): The electrophile (E⁺) attacks the π-electron cloud, forming a resonance-stabilized arenium ion (σ-complex, Wheland intermediate). The positive charge is delocalized over three carbon atoms of the ring. Step 2 (Fast): Loss of H⁺ from the arenium ion restores aromaticity. Base (e.g., HSO₄⁻) abstracts the proton.

EAS Reaction	Reagents	Electrophile (E⁺)	Product
Nitration	Conc. HNO₃ + Conc. H₂SO₄	NO₂⁺ (nitronium ion)	Nitrobenzene
Halogenation (Cl₂)	Cl₂ + FeCl₃/AlCl₃ (anhydrous)	Cl⁺ (or polarized Cl—FeCl₃)	Chlorobenzene
Halogenation (Br₂)	Br₂ + FeBr₃/AlBr₃	Br⁺ (or polarized Br—FeBr₃)	Bromobenzene
Sulphonation	Fuming H₂SO₄ (SO₃ + H₂SO₄)	SO₃ (or SO₃H⁺)	Benzene sulphonic acid
Friedel-Crafts alkylation	R—Cl + AlCl₃ (anhydrous)	R⁺ (carbocation, may rearrange)	Alkylbenzene
Friedel-Crafts acylation	R—CO—Cl + AlCl₃	R—C≡O⁺ (acylium ion, no rearrangement)	Phenyl alkyl ketone

Directing Effects of Substituents

Activating groups (ortho-para directing): —OH, —NH₂, —NHR, —NR₂, —OCH₃, —CH₃, —C₂H₅, —C₆H₅. These donate electrons (+M and/or +I), increasing electron density at ortho and para positions. Deactivating groups (meta directing): —NO₂, —CN, —CHO, —COR, —COOH, —COOR, —SO₃H, —NH₃⁺. These withdraw electrons (-M and/or -I), decreasing overall ring electron density, with the meta positions being least deactivated. Halogens (—F, —Cl, —Br, —I): Ortho-para directing but deactivating. The +M effect (lone pair donation) directs ortho-para, but the stronger -I effect reduces overall ring electron density, making them deactivating. This is a unique case that NEET frequently tests.

4. Haloalkanes & Haloarenes

Haloalkanes (alkyl halides) and haloarenes (aryl halides) are compounds containing halogen atoms (F, Cl, Br, I) bonded to sp³ and sp² hybridized carbons respectively. The polarity of the C—X bond (δ⁺ on C, δ⁻ on X) makes the carbon electrophilic, enabling nucleophilic substitution. The reactivity difference between alkyl and aryl halides is a key NEET concept.

SN1 — Unimolecular Nucleophilic Substitution

Mechanism: Two-step process. Step 1 (slow, RDS): Heterolytic cleavage of C—X bond forms a planar carbocation intermediate (R⁺) and X⁻. Step 2 (fast): The nucleophile attacks the carbocation from either face. Kinetics: Rate = k[RX] (first order). The rate depends only on the concentration of the alkyl halide, not on the nucleophile. Stereochemistry: Racemization — the planar carbocation is attacked from the top or bottom with equal probability, giving a 50:50 mixture of R and S. However, some net inversion is often observed due to ion-pairing (the leaving group partially blocks one face before complete dissociation). Factors favouring SN1: (1) Tertiary alkyl halides (3° > 2° > 1°; methyl does not undergo SN1). (2) Weak nucleophiles (H₂O, ROH, CH₃COOH). (3) Polar protic solvents (H₂O, ROH) stabilize the carbocation intermediate by solvation. (4) Good leaving groups (I⁻, Br⁻, OTs⁻). Rearrangement: Carbocation intermediates frequently undergo 1,2-hydride shifts or 1,2-alkyl shifts to form more stable carbocations, leading to rearranged products (a common NEET exam trick).

SN2 — Bimolecular Nucleophilic Substitution

Mechanism: One-step, concerted process. The nucleophile attacks the electrophilic carbon from the back side (opposite to the leaving group) while the C—X bond breaks simultaneously. The transition state is pentacoordinated (sp²-like) with the nucleophile and leaving group partially bonded. Kinetics: Rate = k[RX][Nu⁻] (second order). The rate depends on both the alkyl halide and the nucleophile. Stereochemistry: Complete inversion of configuration (Walden inversion). (R)-configuration reacts to give (S)-product and vice versa. This is a defining characteristic of SN2. Factors favouring SN2: (1) Primary alkyl halides (CH₃X > 1° > 2° > 3°; 3° is extremely slow due to steric hindrance). (2) Strong nucleophiles (OH⁻, CN⁻, OR⁻, I⁻, HS⁻). (3) Polar aprotic solvents (DMSO, DMF, acetone, CH₃CN) — the cation is solvated but the nucleophile anion is not strongly solvated, making it more reactive. (4) Good leaving groups. Steric hindrance: The rate decreases sharply with increasing substitution on the α-carbon because bulky groups hinder the backside attack.

Feature	SN1	SN2
Number of steps	Two (with carbocation intermediate)	One (concerted, transition state only)
Kinetics	First order: Rate ∝ [RX]	Second order: Rate ∝ [RX][Nu⁻]
Intermediate	Carbocation (planar, sp²)	None (pentacoordinated transition state)
Stereochemistry	Racemization (mostly)	Complete inversion (Walden inversion)
Nucleophile	Weak (neutral: H₂O, ROH, CH₃COOH)	Strong (anionic: OH⁻, CN⁻, OR⁻, I⁻)
Alkyl halide preference	3° > 2° > 1° (stable carbocation needed)	CH₃ > 1° > 2° > 3° (steric hindrance prevents)
Solvent	Polar protic (H₂O, ROH)	Polar aprotic (DMSO, DMF, acetone, CH₃CN)
Rearrangement	Possible (hydride/alkyl shift to more stable carbocation)	Not possible (no intermediate to rearrange)
Leaving group requirement	Good LG (I⁻ > Br⁻ > Cl⁻ > F⁻; OTs⁻, OMs⁻)	Same requirement

Elimination Reactions (E1 & E2)

Elimination reactions compete with substitution reactions. A strong base and high temperature favour elimination over substitution. E1 mechanism: Two-step process via carbocation intermediate (similar to SN1). The leaving group departs first to form a carbocation, followed by loss of a β-proton to a weak base. Favours 3° RX. Gives the more substituted alkene (Zaitsev product). E2 mechanism: One-step, concerted process. A strong base (OH⁻, RO⁻) abstracts a β-proton while the leaving group departs simultaneously. Requires anti-periplanar geometry of the H and the leaving group (the H and X must be on opposite sides of the C—C bond, 180° apart). Favours 1° and 2° RX. Zaitsev vs Hoffmann: With a normal base (e.g., alc. KOH), the more substituted alkene (Zaitsev) predominates. With a bulky base (e.g., K-tert-butoxide), the less substituted alkene (Hoffmann) predominates because the bulky base cannot access the sterically hindered β-proton. E2 is stereospecific: The anti-periplanar requirement means that the stereochemistry of the reactant determines the stereochemistry of the alkene product.

Feature	E1	E2
Mechanism	Two-step (carbocation intermediate)	One-step (concerted)
Base requirement	Weak base (solvent acts as base)	Strong base (OH⁻, RO⁻, NH₂⁻)
Kinetics	First order: Rate ∝ [RX]	Second order: Rate ∝ [RX][Base]
Alkyl halide	3° > 2° > 1°	1° > 2° > 3°
Rearrangement	Possible (carbocation rearrangement)	Not possible (concerted)
Stereochemistry	Not stereospecific	Stereospecific (anti-periplanar required)
Product selectivity	Zaitsev (more substituted alkene)	Zaitsev (normal base) or Hoffmann (bulky base)

Haloarenes (Aryl Halides)

Aryl halides (Ar—X) are much less reactive towards nucleophilic substitution than alkyl halides. The reasons are: (1) Resonance stabilization: The lone pair of the halogen is delocalized into the aromatic ring, giving the C—X bond partial double-bond character (making it stronger and more difficult to break). (2) sp² hybridization: The C—X bond is shorter and stronger (more s-character) than in alkyl halides. (3) No backside attack possible: The planar aromatic ring prevents the backside approach required for SN2. Haloarenes undergo nucleophilic substitution only under harsh conditions via addition-elimination (SNAr) or benzyne mechanisms. Electron-withdrawing groups (—NO₂, —CN) at ortho and para positions activate the ring towards nucleophilic substitution by stabilizing the Meisenheimer complex intermediate.

5. Alcohols, Phenols & Ethers

Alcohols (R—OH) are classified as 1° (e.g., ethanol), 2° (e.g., isopropyl alcohol), or 3° (e.g., tert-butyl alcohol) based on the number of alkyl groups attached to the carbon bearing the —OH group. Phenols have —OH directly attached to an aromatic ring. Ethers (R—O—R') have an oxygen bridging two alkyl/aryl groups. Alcohols have significantly higher boiling points than corresponding alkanes due to intermolecular hydrogen bonding. Methanol, ethanol, and propanol are miscible with water; solubility decreases as the alkyl chain length increases.

Preparation of Alcohols

1. Hydration of alkenes: Alkene + H₂O/dil. H₂SO₄ → alcohol (Markovnikov addition, via carbocation). May involve rearrangement. 2. Hydroboration-oxidation: Alkene + BH₃/THF then H₂O₂/OH⁻ → alcohol (anti-Markovnikov, syn addition, no rearrangement). 3. Reduction of carbonyl compounds: Aldehydes → 1° alcohols; Ketones → 2° alcohols (NaBH₄ or LiAlH₄). Carboxylic acids and esters → 1° alcohols (requires LiAlH₄; NaBH₄ is not strong enough). 4. Grignard synthesis: RMgX + HCHO → 1° alcohol (after hydrolysis). RMgX + R'CHO → 2° alcohol. RMgX + R'COR" → 3° alcohol. This is a versatile method that forms new C—C bonds. 5. Fermentation: C₆H₁₂O₆ (glucose) → 2C₂H₅OH + 2CO₂ (yeast, anaerobic conditions).

Reactions of Alcohols

1. Dehydration: Conc. H₂SO₄ (170°C) or Al₂O₃ (350°C) → alkene + H₂O. Reactivity: 3° > 2° > 1°. Follows E1 for 2°/3°, E2 for 1°. 2. Oxidation: 1° alcohols (PCC/CH₂Cl₂) → aldehyde; (K₂Cr₂O₇/H⁺, KMnO₄) → carboxylic acid. 2° alcohols → ketone. 3° alcohols are resistant to mild oxidation (no α-H at the —OH carbon). 3. Lucas test: R—OH + ZnCl₂ + HCl (conc.) → R—Cl (insoluble, appears as turbidity/cloudiness). 3°: immediate turbidity. 2°: turbidity in 5-10 min. 1°: no turbidity at RT (requires heating). 4. Esterification (Fischer): R—OH + R'COOH (H₂SO₄, heat) → R'COOR + H₂O. Reversible reaction. 5. Reaction with metals: 2R—OH + 2Na → 2R—ONa + H₂↑ (sodium alkoxide). 6. Conversion to alkyl halides: R—OH + HX (ZnCl₂ catalyst for 1°) → R—X + H₂O. Also using PX₃, PX₅, or SOCl₂ (SOCl₂ + pyridine is the best method for 1° alcohols — high yield, no rearrangement).

Phenols

Acidity: Phenols (pKa ≈ 10) are more acidic than alcohols (pKa ≈ 16) but less acidic than carboxylic acids (pKa ≈ 4-5). The phenoxide ion (C₆H₅O⁻) is stabilized by resonance delocalization of the negative charge into the aromatic ring. Effects on acidity: Electron-withdrawing groups (—NO₂) at ortho and para positions significantly enhance acidity (picric acid, 2,4,6-trinitrophenol, is a strong acid with pKa ≈ 0.3). Electron-donating groups (—CH₃) decrease acidity. Kolbe-Schmitt reaction: Phenol + CO₂ (NaOH, 125°C, 4-7 atm) → sodium salicylate → salicylic acid (aspirin precursor). Reimer-Tiemann reaction: Phenol + CHCl₃ + NaOH (50-70°C) → o-hydroxybenzaldehyde (salicylaldehyde) as the major product. Electrophilic substitution: —OH is strongly activating (+M), so phenol undergoes EAS readily even under mild conditions. Bromination with Br₂/H₂O gives 2,4,6-tribromophenol (white precipitate) — a characteristic test for phenol. Coupling with diazonium salts: Phenol couples with benzene diazonium chloride in alkaline medium to give a brightly coloured azo dye (p-hydroxyazobenzene).

Ethers

Williamson's synthesis: R—O⁻Na⁺ + R'—X → R—O—R' + NaX. This is the best method for preparing both symmetrical and unsymmetrical ethers. The alkoxide should be used as the nucleophile, and the alkyl halide should be primary (secondary/tertiary alkyl halides undergo elimination). For preparing aryl alkyl ethers (e.g., anisole, C₆H₅OCH₃): sodium phenoxide + CH₃I → anisole + NaI. Cleavage of ethers: R—O—R' + HI (hot, conc.) → R—I + R'—OH. If excess HI is used, both products are converted to alkyl iodides. The cleavage mechanism is SN2 (for 1° alkyl groups) or SN1 (for 3° alkyl groups). The order of cleavage efficacy: HI > HBr > HCl. Anisole: Methyl phenyl ether undergoes EAS at ortho and para positions (the —OCH₃ group is strongly activating, strong +M effect). This is used in the preparation of p-methoxyacetophenone via Friedel-Crafts acylation.

6. Aldehydes, Ketones & Carboxylic Acids

The carbonyl group (C=O) is one of the most versatile functional groups in organic chemistry. The carbon is sp² hybridized, and the strong π-bond is polarized (δ⁺ on C, δ⁻ on O), making the carbonyl carbon highly electrophilic. Aldehydes are more reactive than ketones towards nucleophilic addition due to less steric hindrance and greater electrophilicity of the carbonyl carbon (ketones have two +I alkyl groups that stabilize the carbonyl carbon). Formaldehyde (HCHO) is the most reactive aldehyde.

Aldehydes & Ketones — Nucleophilic Addition

1. Addition of HCN: Aldehyde/ketone + HCN (catalytic amount of base) → cyanohydrin (α-hydroxy nitrile). The CN⁻ nucleophile attacks the carbonyl carbon. Cyanohydrins are useful synthetic intermediates — the —CN group can be hydrolysed to —COOH or reduced to —CH₂NH₂. 2. Addition of NaHSO₃: Aldehyde/ketone + saturated NaHSO₃ solution → white crystalline bisulphite addition product. This is a reversible reaction used for the purification of carbonyl compounds. Methyl ketones and cyclic ketones react readily; the product can be regenerated with dil. Na₂CO₃ or dil. HCl. 3. Addition of RMgX (Grignard reagents): HCHO → 1° alcohol (after hydrolysis); RCHO → 2° alcohol; RCOR' → 3° alcohol. This is a powerful C—C bond-forming reaction. 4. Addition of alcohols: Aldehyde + ROH (dry HCl gas) → hemiacetal (unstable) → acetal (stable). Ketones form ketals. Acetals/ketals are used as protecting groups for the carbonyl function. 5. Addition of ammonia derivatives (condensation): NH₂OH → oxime; NH₂NH₂ → hydrazone; 2,4-DNPH (2,4-dinitrophenylhydrazine) → orange/red precipitate of 2,4-dinitrophenylhydrazone (used for identification/characterization of carbonyl compounds); NH₂—NH—CS—NH₂ (thiosemicarbazide) → thiosemicarbazone.

Reduction of Aldehydes & Ketones

1. Metal hydrides: NaBH₄ (mild, selective for C=O, works in aqueous/methanolic solution) or LiAlH₄ (strong, reduces C=O, C≡N, COOH, esters). Aldehydes → 1° alcohols; Ketones → 2° alcohols. 2. Catalytic hydrogenation: H₂/Ni (Pt, Pd, Ru) → alcohol. 3. Wolff-Kishner reduction: NH₂NH₂ (hydrazine) + KOH (heat, ethylene glycol) → R—CH₃ (alkane from aldehyde) or R₂CH₂ (alkane from ketone). The C=O is reduced to CH₂. 4. Clemmensen reduction: Zn(Hg) + HCl (conc., heat) → alkane. Used for acid-sensitive carbonyl compounds (the Wolff-Kishner uses strong base, so Clemmensen is preferred for base-sensitive compounds).

Oxidation of Aldehydes & Ketones

Aldehydes are easily oxidized to carboxylic acids by mild oxidizing agents. Tollens' test: Aldehyde + ammoniacal AgNO₃ → Ag mirror (Ag⁺ reduced to Ag⁰). Fehling's test: Aldehyde + Cu²⁺/tartrate (in NaOH) → red precipitate of Cu₂O. Aromatic aldehydes do not react with Fehling's solution. Benedict's test: Similar to Fehling's, used clinically to detect glucose in urine. Schiff's test: Aldehyde + Schiff's reagent (rosaniline decolourized by SO₂) → pink/magenta colour. Ketones do not give any of these tests. Ketones require strong oxidizing conditions (hot KMnO₄, hot HNO₃) and undergo C—C bond cleavage, giving a mixture of carboxylic acids.

Aldol & Cannizzaro Reactions

Aldol condensation: Carbonyl compounds with at least one α-hydrogen undergo condensation in the presence of dilute base (NaOH) to form β-hydroxy carbonyl compounds (aldols). On heating, the aldol dehydrates to form an α,β-unsaturated carbonyl compound. Crossed aldol (Claisen-Schmidt reaction) between two different carbonyl compounds gives a mixture of four products unless one of them has no α-H (e.g., benzaldehyde + acetaldehyde gives a single product — cinnamaldehyde). Cannizzaro reaction: Aldehydes without α-H (HCHO, C₆H₅CHO, (CH₃)₃CCHO) undergo disproportionation in concentrated base: 2 molecules → 1 alcohol (reduced) + 1 carboxylic acid (oxidized). In a crossed Cannizzaro reaction, HCHO (which is more electrophilic) is always oxidized to formic acid (or formate), while the other aldehyde is reduced to the corresponding alcohol.

Reagent	Detects	Observation
Tollens' reagent [Ag(NH₃)₂]⁺OH⁻	Aldehyde	Silver mirror
Fehling's solution (Cu²⁺ + tartrate)	Aldehyde	Red precipitate of Cu₂O
Benedict's solution	Aldehyde	Red precipitate
Iodoform test (I₂ + NaOH)	CH₃CO— or CH₃CH(OH)—	Yellow precipitate of CHI₃
2,4-DNP	Aldehyde / Ketone	Orange precipitate
Schiff's reagent	Aldehyde	Pink / magenta colour

Carboxylic Acids

Acidity: Carboxylic acids (pKa ~4-5) are stronger than phenols due to resonance stabilization of the carboxylate anion. Derivatives: Acid chlorides (SOCl₂), esters (alcohol + H⁺), amides (NH₃), anhydrides (P₂O₅). Hell-Volhard-Zelinsky (HVZ) reaction: α-halogenation using X₂/P. Decarboxylation: Heat with soda lime → alkane.

Carboxylic Acids (R—COOH)

Carboxylic acids are characterized by the carboxyl group (—COOH). They are the most acidic class of neutral organic compounds (pKa ~4-5). The acidity is due to resonance stabilization of the conjugate base (carboxylate anion, RCOO⁻), where the negative charge is delocalized over two equivalent oxygen atoms. Preparation: (1) Oxidation of 1° alcohols/aldehydes (KMnO₄, K₂Cr₂O₇/H⁺). (2) Hydrolysis of nitriles (RCN + H₂O/H⁺ → RCOOH + NH₄⁺). (3) Carbonation of Grignard reagents (RMgX + CO₂ → RCOOMgX → RCOOH). (4) Hydrolysis of esters and amides. Reactions: (1) Esterification (Fischer, ROH/H⁺, reversible). (2) Formation of acid chlorides (SOCl₂, PCl₅, PCl₃). (3) Formation of amides (NH₃ → ammonium salt → heat → amide). (4) Reduction (LiAlH₄ → 1° alcohol; BH₃/THF selectively reduces COOH without affecting NO₂ or ester groups). (5) HVZ reaction (Hell-Volhard-Zelinsky): Br₂ + catalytic red P → α-bromo acid (then with NH₃ → α-amino acid). (6) Decarboxylation: RCOONa + NaOH/CaO (soda lime, heat) → R—H + Na₂CO₃. (7) Arndt-Eistert synthesis: RCOOH → RCH₂COOH (increases chain by one carbon).

Distinction Tests — Aldehydes vs Ketones

Test / Reagent	Target	Positive Result	Notes
Tollens' [Ag(NH₃)₂]⁺OH⁻	Aldehydes	Silver mirror on test tube	Ag⁺ → Ag⁰; ketones do not react
Fehling's (Cu²⁺ + tartrate in NaOH)	Aldehydes (aliphatic)	Red Cu₂O precipitate	Aromatic aldehydes do not react
Benedict's (Cu²⁺ + citrate)	Aldehydes / reducing sugars	Red Cu₂O precipitate	Clinical test for glucose in urine
Iodoform (I₂ + NaOH)	CH₃CO— or CH₃CH(OH)—	Yellow CHI₃ precipitate	Acetaldehyde: +; other aldehydes: —; Methyl ketones: +
2,4-DNPH	Aldehydes & Ketones	Orange/red precipitate	General test for carbonyl compounds
Schiff's reagent	Aldehydes	Pink/magenta colour	Ketones do not restore colour

7. Amines

Amines are derivatives of NH₃ where one or more hydrogen atoms are replaced by alkyl (aliphatic amines) or aryl (aromatic amines) groups. Classified as 1° (primary, RNH₂), 2° (secondary, R₂NH), 3° (tertiary, R₃N), and quaternary ammonium salts (R₄N⁺X⁻). Amines are basic due to the lone pair of electrons on nitrogen. This is a high-yield NEET topic with questions on basicity order, Hinsberg test, and diazonium chemistry.

Basicity of Amines

Aliphatic amines: Alkyl groups are electron-donating (+I), increasing electron density on nitrogen, making aliphatic amines stronger bases than NH₃. In aqueous solution, the basicity order is: 2° > 1° > 3° > NH₃. Secondary amines are the strongest base due to a balance of inductive effect (two alkyl groups donate more electron density) and solvation of the conjugate acid (R₂NH₂⁺ is well solvated by water). Tertiary amines are weaker than secondary despite having three alkyl groups because the bulky R₃NH⁺ ion is poorly solvated, destabilizing it. In the gas phase (no solvation effects), the order is: 3° > 2° > 1° > NH₃. Aromatic amines: Aniline (C₆H₅NH₂) is much weaker (pKb ≈ 9.4) than aliphatic amines (pKb ≈ 3-4) because the lone pair on nitrogen is delocalized into the aromatic ring via resonance (the +M effect of —NH₂). Substituent effects on aniline basicity: EWG (—NO₂) decrease basicity; EDG (—CH₃, —OCH₃) increase basicity. ortho-Substituted anilines are always weaker than aniline regardless of the nature of the substituent (ortho effect — steric hindrance to solvation).

Important Reactions of Amines

1. Carbylamine reaction (isocyanide test): 1° amine + CHCl₃ + alc. KOH (heat) → R—NC (isocyanide) + 3KCl + 3H₂O. The isocyanide has a characteristic foul smell. Only primary amines (aliphatic and aromatic) give this test; 2° and 3° amines do not react. 2. Hinsberg test: 1° amine + C₆H₅SO₂Cl (benzenesulphonyl chloride) + NaOH → N-substituted sulphonamide (soluble in alkali because the N—H is acidic, forming a salt) → clear solution. 2° amine → N,N-disubstituted sulphonamide (no N—H, insoluble in alkali) → precipitate. 3° amine → no reaction (no N—H to react) → immiscible oily layer. 3. Acylation: 1° and 2° amines + RCOCl or (RCO)₂O → amide (R—CO—NR'R"). 3° amines do not have an N—H bond and do not undergo acylation. 4. Reaction with HNO₂ (nitrous acid): 1° aliphatic amine + HNO₂ → N₂ gas (bubbles, test for 1° aliphatic amine). 1° aromatic amine + HNO₂ (0-5°C) → diazonium salt (stable at low temperature). 2° amine + HNO₂ → N-nitrosamine (yellow oil). 3° amine + HNO₂ → trialkylammonium nitrite salt (may give C-nitroso compound in aromatic cases). 5. Hoffmann elimination (exhaustive methylation): 1° amine + excess CH₃I (methylation) → quaternary ammonium iodide → AgOH (wet Ag₂O) → quaternary ammonium hydroxide → heat → alkene + H₂O + trimethylamine. The least substituted alkene (Hoffmann product) is formed.

Diazonium Salts

Primary aromatic amines react with NaNO₂ + HCl at 0-5°C to form benzene diazonium chloride (Ar—N₂⁺Cl⁻). These are versatile synthetic intermediates. Substitution reactions (replacement of —N₂⁺): Sandmeyer (CuCl/HCl → ArCl; CuBr/HBr → ArBr; CuCN/KCN → ArCN). Gattermann (Cu + HX → ArX). With KI → ArI. With H₂O (heat) → ArOH. With H₃PO₂ (hypophosphorous acid) → ArH. With HBF₄ (Balz-Schiemann reaction) → ArF (fluorobenzene). Coupling reactions: Diazonium salts couple with phenols (in alkaline medium) and aromatic amines (in acidic medium) to form brightly coloured azo compounds (Ar—N=N—Ar'). This is the basis of azo dye chemistry. The coupling occurs at the para position (if occupied, then ortho).

Important Named Reactions of Amines

Carbylamine reaction: 1° amine + CHCl₃ + alc. KOH → foul-smelling isocyanide (RNC). Hinsberg test: Benzenesulphonyl chloride distinguishes 1°, 2°, 3° amines. Hoffmann elimination: 4° ammonium hydroxide on heating yields alkene + H₂O + amine.

8. Biomolecules

Biomolecules are organic compounds essential for life: carbohydrates, proteins, nucleic acids, lipids, and vitamins. This is a high-yield NEET topic with 2-3 questions per exam, covering structure, properties, and biological significance.

Carbohydrates

Carbohydrates are polyhydroxy aldehydes or ketones with the general formula Cₓ(H₂O)ᵧ. Classified as monosaccharides (simplest sugars, cannot be hydrolysed — glucose, fructose, galactose, ribose), disaccharides (two monosaccharides joined by a glycosidic bond — sucrose, maltose, lactose), and polysaccharides (many monosaccharides — starch, cellulose, glycogen). Glucose (C₆H₁₂O₆): An aldohexose, reducing sugar, forms a pentaacetate (5 —OH groups). The open-chain form has an aldehyde group; the cyclic forms (α and β anomers) exist in equilibrium via mutarotation. The pyranose (6-membered) and furanose (5-membered) ring forms arise from internal hemiacetal formation. Glucose reduces Tollens' reagent (Ag mirror) and Fehling's solution (red Cu₂O). With phenylhydrazine, glucose forms a characteristic osazone (needle-shaped crystals, melting point 205°C). Fructose: A ketohexose (sweetest sugar), reducing sugar, forms the same osazone as glucose (because the carbonyl group is at C-2 and the stereochemistry of C-3, C-4, C-5 is the same as glucose). Sucrose (cane sugar): α-D-glucose + β-D-fructose linked by α(1→2) glycosidic bond. Non-reducing sugar (both anomeric carbons are involved in the glycosidic bond). Hydrolysis (invertase or dil. HCl) gives invert sugar (glucose + fructose, reducing). Maltose: α-D-glucose + α-D-glucose linked by α(1→4) bond. Reducing sugar. Lactose: β-D-galactose + β-D-glucose linked by β(1→4) bond. Reducing sugar (milk sugar). Starch: Polymer of α-D-glucose — amylose (linear, α(1→4) links) + amylopectin (branched, α(1→6) links). Gives blue colour with iodine. Cellulose: Polymer of β-D-glucose (linear, β(1→4) links). Structural component of plant cell walls. Not digestible by humans (lack β-glucosidase).

Proteins & Amino Acids

Amino acids have the general formula H₂N—CH(R)—COOH. The 20 standard α-amino acids are the building blocks of proteins. At the isoelectric point (pI), amino acids exist as zwitterions (⁺H₃N—CH(R)—COO⁻) with no net charge. Peptide bond: Formed between the —COOH of one amino acid and the —NH₂ of another (—CO—NH—), with elimination of H₂O. The peptide bond is planar with partial double-bond character. Protein structure: Primary (amino acid sequence), Secondary (α-helix and β-pleated sheet stabilized by H-bonds), Tertiary (3D folding stabilized by H-bonds, disulphide bridges —S—S—, hydrophobic interactions, ionic bonds), Quaternary (multiple polypeptide chains — e.g., haemoglobin has 4 subunits). Denaturation: Loss of secondary, tertiary, and quaternary structure (by heat, pH change, heavy metal salts) without breaking the primary structure. Tests for proteins: Biuret test (violet colour with CuSO₄/NaOH — detects peptide bonds), Ninhydrin test (blue-purple colour with α-amino acids, proline gives yellow), Xanthoproteic test (yellow colour with HNO₃, turning orange with alkali — detects aromatic amino acids like tyrosine, tryptophan), Millon's test (red colour with Hg(NO₃)₂/HNO₃ — detects phenolic —OH group in tyrosine).

Nucleic Acids

DNA (deoxyribonucleic acid): Double-stranded helix (Watson-Crick model). Each strand has a sugar-phosphate backbone (deoxyribose + phosphate) and four nitrogenous bases: Adenine (A), Guanine (G), Cytosine (C), Thymine (T). Complementary base pairing: A≡T (2 H-bonds), G≡C (3 H-bonds). The two strands are antiparallel. RNA (ribonucleic acid): Single-stranded, has ribose sugar and Uracil (U) instead of Thymine. Types: mRNA (carries genetic code from DNA to ribosomes), tRNA (carries amino acids to ribosomes for protein synthesis, has anticodon loop), rRNA (structural component of ribosomes). Nucleosides vs Nucleotides: Nucleoside = base + sugar (e.g., adenosine, guanosine). Nucleotide = base + sugar + phosphate (e.g., AMP, ADP, ATP). ATP (adenosine triphosphate) is the energy currency of the cell. Chargaff's rule: In DNA, A = T and G = C (the number of purines equals the number of pyrimidines).

Lipids

Lipids are naturally occurring organic compounds insoluble in water but soluble in organic solvents. Fats and oils: Triesters of glycerol with fatty acids (triglycerides). Oils are unsaturated (liquid at RT); fats are saturated (solid at RT). Saponification: Fat/oil + NaOH → glycerol + sodium salts of fatty acids (soap). Iodine number: Measures the degree of unsaturation — higher iodine number = more unsaturation. Phospholipids: Major components of cell membranes (e.g., lecithin has glycerol + 2 fatty acids + phosphate + choline). Steroids: Lipids with a characteristic four-ring structure (e.g., cholesterol, testosterone, oestrogen, cortisol).

Vitamins

Vitamins are essential organic compounds required in small amounts for normal physiological function. They are classified as fat-soluble (Vitamins A, D, E, K — stored in liver and fatty tissues, can accumulate to toxic levels) and water-soluble (Vitamin B-complex and Vitamin C — not stored, excreted in urine, need regular intake). Vitamin A (Retinol): Night blindness, xerophthalmia. Source: carrots, green leafy vegetables. Vitamin B₁ (Thiamine): Beriberi (affects nervous system and heart). Present in whole grains, meat. Vitamin B₂ (Riboflavin): Cheilosis, angular stomatitis. Vitamin B₃ (Niacin): Pellagra (dermatitis, diarrhoea, dementia). Vitamin B₆ (Pyridoxine): Anaemia, convulsions. Vitamin B₁₂ (Cobalamin): Pernicious anaemia. Only vitamin containing cobalt. Vitamin C (Ascorbic acid): Scurvy (bleeding gums, poor wound healing). Powerful antioxidant. Vitamin D (Cholecalciferol): Rickets in children, osteomalacia in adults. Synthesized in skin on exposure to sunlight. Vitamin E (Tocopherol): Antioxidant, fertility. Vitamin K (Phylloquinone): Required for blood clotting (prothrombin synthesis).

Biomolecule	Monomer (Building Block)	Type of Bond	Functions / Examples
Carbohydrates	Monosaccharides (e.g., glucose, fructose)	Glycosidic bond (—O—)	Energy source: starch, cellulose, glycogen; structural: cellulose
Proteins	α-Amino acids (20 types)	Peptide bond (—CO—NH—)	Enzymes, structural (keratin, collagen), transport (haemoglobin), antibodies
Nucleic acids	Nucleotides (base + sugar + phosphate)	Phosphodiester bond	Genetic information storage and transfer: DNA, RNA
Lipids	Fatty acids + Glycerol (triglycerides)	Ester bond	Energy storage, cell membrane components: fats, oils, phospholipids, steroids

9. Polymers

Polymers are high molecular mass substances (macromolecules) composed of repeating structural units called monomers, linked by covalent bonds. The process of forming a polymer is called polymerization. NEET typically asks 1-2 questions on polymer classification, monomers, and uses.

Classification of Polymers

Based on source: Natural (cotton, silk, wool, natural rubber, starch), Semi-synthetic (rayon, celluloid — chemically modified natural polymers), and Synthetic (nylon, polythene, PVC, terylene — man-made). Based on structure: Linear (thermoplastics, melt on heating — polythene, PVC), Branched (LDPE), and Cross-linked (thermosetting, do not melt on heating — bakelite, melamine-formaldehyde). Based on mode of polymerization: Addition (chain growth) and Condensation (step growth).

Addition (Chain Growth) Polymerization

Monomers containing C=C double bonds (alkenes and derivatives) undergo chain growth polymerization via free radical, cationic, anionic, or coordination (Ziegler-Natta) mechanisms. The three steps are: initiation (generation of reactive radical/ion), propagation (successive addition of monomers, each addition regenerating the reactive centre), and termination (combination of two growing chains, disproportionation, or chain transfer). The polymer has the same elemental composition as the monomer (no by-product). Examples: polyethene (LDPE — free radical, high pressure; HDPE — Ziegler-Natta, low pressure), polypropylene, polystyrene, PVC, Teflon, polyacrylonitrile (PAN/Orlon), polybutadiene, neoprene.

Condensation (Step Growth) Polymerization

Bifunctional or polyfunctional monomers react with the elimination of small molecules (H₂O, HCl, NH₃, CH₃OH). The polymer does NOT have the same composition as the monomer. Requires functional groups on both ends of the monomers. Examples: polyesters (terylene/Dacron — ethylene glycol + terephthalic acid), polyamides (nylon-66 — hexamethylenediamine + adipic acid; nylon-6 — caprolactam), phenolic resins (bakelite — phenol + formaldehyde), epoxy resins, polyurethanes. Copolymers vs Homopolymers: Homopolymers have only one type of monomer (e.g., polythene). Copolymers have two or more different monomers (e.g., Buna-S, nylon-66, terylene).

Important Polymers for NEET

Polymer	Monomer(s)	Polymerization Type	Applications
Low Density Polyethene (LDPE)	CH₂=CH₂ (ethene)	Addition (free radical, high pressure)	Plastic bags, squeeze bottles, films
High Density Polyethene (HDPE)	CH₂=CH₂	Addition (Ziegler-Natta, low pressure)	Bottles, pipes, containers, buckets
Polyvinyl chloride (PVC)	CH₂=CHCl (vinyl chloride)	Addition	Pipes, flooring, cable insulation, synthetic leather
Polytetrafluoroethene (PTFE / Teflon)	CF₂=CF₂ (tetrafluoroethene)	Addition	Non-stick cookware, gaskets, bearings
Polystyrene	C₆H₅CH=CH₂ (styrene)	Addition	Packaging, disposable cups, thermal insulation
Polyacrylonitrile (PAN / Orlon / Acrilan)	CH₂=CH—CN (acrylonitrile)	Addition	Synthetic fibres (acrylic wool, sweaters, blankets)
Polymethyl methacrylate (PMMA / Plexiglass)	CH₂=C(CH₃)COOCH₃ (methyl methacrylate)	Addition	Shatterproof windows, lenses, acrylic sheets
Neoprene	CH₂=CCl—CH=CH₂ (chloroprene)	Addition	Synthetic rubber, wetsuits, gaskets, conveyor belts
Buna-S (SBR — Styrene Butadiene Rubber)	Butadiene + Styrene	Addition (copolymer)	Synthetic rubber (automobile tyres, footwear)
Buna-N (Nitrile rubber)	Butadiene + Acrylonitrile	Addition (copolymer)	Oil-resistant seals, O-rings, hoses
Polybutadiene	Butadiene	Addition	Synthetic rubber
Polyisoprene (Natural rubber)	Isoprene (2-methylbuta-1,3-diene)	Addition (natural, cis-1,4 polymer)	Natural rubber, tyres, gloves
Vulcanized rubber	Natural rubber + Sulphur (heat)	Cross-linking (by S bridges)	More durable rubber (tyres) — Charles Goodyear
Nylon-66	Hexamethylenediamine + Adipic acid	Condensation	Fabrics, ropes, parachutes, gears, brushes
Nylon-6	Caprolactam (ring-opening polymerization)	Condensation	Fabrics, carpets, fishing nets
Nylon-6,10	Hexamethylenediamine + Sebacic acid	Condensation	Bristles, brushes
Terylene (Dacron / PET)	Ethylene glycol + Terephthalic acid	Condensation	Polyester fibres, PET bottles, film
Bakelite (Phenol-formaldehyde resin)	Phenol + Formaldehyde	Condensation	Electrical switches, handles, telephones, adhesives
Melamine-formaldehyde resin	Melamine + Formaldehyde	Condensation	Unbreakable crockery, decorative laminates
Glyptal (Alkyd resin)	Ethylene glycol + Phthalic acid	Condensation	Paints, coatings, varnishes
Polyurethane	Polyol + Diisocyanate	Condensation	Foam (mattresses, cushions), elastomers, coatings
Kevlar	p-Phenylenediamine + Terephthaloyl chloride	Condensation	Bulletproof vests, high-strength fibres

10. Chemistry in Everyday Life

This chapter applies organic chemistry concepts to real-world substances: drugs and pharmaceuticals, chemicals in food, and cleansing agents. NEET typically asks 1-2 questions from this topic — often on drug classification, antiseptics vs disinfectants, or artificial sweeteners.

Drugs & Pharmaceuticals

Analgesics (painkillers): Non-narcotic (Aspirin — acetylsalicylic acid, analgesic, antipyretic, anti-inflammatory; Paracetamol — acetaminophen, analgesic, antipyretic; Ibuprofen). Narcotic (Morphine — from opium poppy, acts on CNS, habit-forming). Antipyretics: Reduce fever (Paracetamol, Aspirin, Ibuprofen). Antibiotics: Bactericidal (kill bacteria — Penicillin, Aminoglycosides) vs Bacteriostatic (inhibit growth — Tetracycline, Chloramphenicol, Erythromycin). Broad-spectrum (effective against Gram-positive and Gram-negative — Chloramphenicol, Tetracycline, Ampicillin). Narrow-spectrum (Penicillin G — mainly Gram-positive). Penicillin was discovered by Alexander Fleming. Penicillin contains a β-lactam ring; penicillianase (β-lactamase) is an enzyme produced by resistant bacteria that cleaves this ring. Antiseptics: Applied to living tissues (Dettol — chloroxylenol + terpineol; Boric acid; Iodine tincture 2%; Hydrogen peroxide; Iodoform). Disinfectants: Applied to inanimate objects (Phenol — 1% solution is antiseptic, 5% is disinfectant; Chlorine — water disinfection; SO₂ — fumigation). Tranquilizers (anxiolytics/sedatives): Reduce anxiety and induce calm (Barbiturates — phenobarbital, Veronal; Benzodiazepines — Valium/diazepam, Librium; Meprobamate — Equanil). Antacids: Neutralize stomach acid (Mg(OH)₂ milk of magnesia, Al(OH)₃ gel, NaHCO₃). H₂-receptor antagonists (Ranitidine, Cimetidine, Famotidine) reduce acid secretion by blocking histamine receptors in the stomach. Antihistamines: Block H₁ histamine receptors, used for allergies (Loratadine, Cetirizine, Diphenhydramine — causes drowsiness; the newer drugs are non-sedating).

Chemicals in Food

Preservatives: Prevent microbial spoilage. Common examples: Sodium benzoate (C₆H₅COONa, effective in acidic foods like pickles, jams, soft drinks), Sodium metabisulphite (Na₂S₂O₅, used in fruit juices, wine), Sorbic acid and potassium sorbate (used in cheese, baked goods), Citric acid, Vinegar (acetic acid). Artificial sweeteners: Provide sweetness without calories. Saccharin (oldest, 550× sweeter than sucrose, has bitter aftertaste), Aspartame (180× sweeter, not heat-stable, used in cold drinks and foods; contraindicated in phenylketonuria — PKU), Sucralose (600× sweeter, heat-stable, used in cooking), Alitame (2000× sweeter), Neotame. Food colours (synthetic): Sunset Yellow (FD&C Yellow 6), Tartrazine (FD&C Yellow 5, an azo dye), Indigo carmine, Fast Green FCF, Allura Red (Red 40). Natural colours: Carotene/Carotenoids (orange/yellow), Chlorophyll (green), Anthocyanins (red-purple, found in berries).

Cleansing Agents

Soaps: Sodium or potassium salts of long-chain fatty acids (RCOO⁻Na⁺, where R = C₁₁ to C₁₇). Soaps work by forming micelles — the hydrophobic (tail) dissolves grease/oil while the hydrophilic (head) faces water, emulsifying the dirt. Soaps are ineffective in hard water because Ca²⁺/Mg²⁺ ions form insoluble scum (calcium/magnesium stearate). Synthetic detergents: Anionic (SDS — sodium dodecylbenzenesulphonate; sodium lauryl sulphate), Cationic (Cetyltrimethylammonium bromide, CTAB — also has germicidal properties), Non-ionic (Polyethylene glycol alkyl ethers). Detergents work in both hard and soft water because the sulphonate/sulphate anions do not form insoluble precipitates with Ca²⁺/Mg²⁺. Biodegradability: Branched-chain alkylbenzenesulphonates (hard detergents, non-biodegradable, cause water pollution — foaming in rivers). Linear alkylbenzenesulphonates (soft detergents, biodegradable, environmentally friendly).

Category	Compound / Example	Mechanism / Use
Antacid	Mg(OH)₂, Al(OH)₃, NaHCO₃, Ranitidine (H₂ blocker)	Neutralizes acid or blocks histamine H₂-receptors
Antihistamine	Loratadine, Cetirizine, Diphenhydramine	Blocks H₁ receptors, treats allergies
Analgesic (non-narcotic)	Paracetamol, Aspirin, Ibuprofen	Pain relief, anti-inflammatory, antipyretic
Analgesic (narcotic)	Morphine, Codeine	CNS acting, may cause addiction
Antibiotic (broad-spectrum)	Chloramphenicol, Tetracycline, Ampicillin	Kills/inhibits both Gram+ and Gram− bacteria
Antibiotic (narrow-spectrum)	Penicillin G	Mainly Gram+ bacteria
Antiseptic	Dettol, Boric acid, Iodine, H₂O₂	Applied to living tissue (wounds, cuts)
Disinfectant	Phenol (5%), Chlorine (water), SO₂	Inanimate objects, surfaces, water
Tranquilizer	Valium (diazepam), Barbiturates, Equanil	Anxiety, stress, sleep disorders
Antibacterial	Prontosil (sulfonamide prodrug)	First synthetic antibacterial
Artificial sweetener	Saccharin, Aspartame, Sucralose, Alitame	Low-calorie sweetening of foods/drinks
Food preservative	Sodium benzoate, Sorbic acid, Na₂S₂O₅	Prevents microbial growth

Quick Reference: Functional Group Interconversions

Starting Material	Reagent(s)	Product
Alkene	H₂ / Pd	Alkane
Alkene	H₂O / H⁺	Alcohol
Alkene	O₃ then Zn/H₂O	Aldehyde or Ketone
1° Alcohol	PCC / K₂Cr₂O₇ + H⁺	Aldehyde
1° Alcohol	K₂Cr₂O₇ / H⁺ (excess)	Carboxylic acid
2° Alcohol	K₂Cr₂O₇ / H⁺	Ketone
Aldehyde	Tollens / Fehling	Carboxylic acid
Aldehyde	NaBH₄ / LiAlH₄	1° Alcohol
Ketone	NaBH₄ / LiAlH₄	2° Alcohol
Carboxylic acid	LiAlH₄	1° Alcohol
Carboxylic acid	SOCl₂	Acid chloride
Acid chloride	NH₃	Amide
Amide	Br₂ + NaOH (Hoffmann)	1° Amine (−1C)
Nitrile (RCN)	H₂ / Ni or LiAlH₄	1° Amine
Ester	H₂O / H⁺ or OH⁻	Carboxylic acid + Alcohol

Master List: Named Reactions for NEET

Reaction Name	Starting Material	Reagent / Conditions	Product	Key Feature
Wurtz	R—X (alkyl halide)	2Na, dry ether	R—R (alkane)	Doubles C atoms; symmetrical alkane
Wurtz-Fittig	R—X + Ar—X	2Na, dry ether	R—Ar (alkylbenzene)	Coupling of alkyl and aryl halides
Fittig	2Ar—X	2Na, dry ether	Ar—Ar (diaryl)	Coupling of two aryl halides
Sandmeyer	Ar—N₂⁺Cl⁻	CuCl/HCl or CuBr/HBr or CuCN/KCN	Ar—Cl, Ar—Br, Ar—CN	Replacement of —N₂⁺ by —Cl, —Br, —CN
Gattermann	Ar—N₂⁺Cl⁻	Cu + HX (X = Cl, Br)	Ar—X (haloarene)	Similar to Sandmeyer using Cu powder
Balz-Schiemann	Ar—N₂⁺Cl⁻	HBF₄, then heat	Ar—F (fluorobenzene)	Introduction of fluorine into aromatic ring
Kucherov	Alkyne (terminal)	HgSO₄ + H₂SO₄, H₂O	Enol → Ketone	Markovnikov hydration of alkynes
Lindlar reduction	Alkyne	H₂, Pd/CaCO₃ + quinoline	cis-Alkene	Partial reduction to cis-alkene
Birch reduction	Alkyne	Na, NH₃ (liq.)	trans-Alkene	Partial reduction to trans-alkene
Ozonolysis	Alkene	O₃ then Zn/H₂O	Aldehydes / Ketones	Cleaves C=C; useful for structural determination
Kolbe's electrolysis	RCOOK (conc. aq.)	Electrolysis, C/Pt electrodes	R—R (alkane) + CO₂	Anode: decarboxylation + coupling
Frankland	R—X	Zn + H₂O (trace)	R—H (alkane)	Reduction to alkane
Rosenmund	RCOCl (acyl chloride)	H₂, Pd/BaSO₄ (poisoned catalyst)	RCHO (aldehyde)	Selective reduction of acid chloride to aldehyde
Stephen	RCN (nitrile)	SnCl₂ + HCl, then H₂O	RCHO (aldehyde)	Reduction of nitrile to imine, then aldehyde
Cannizzaro	HCHO / ArCHO (no α-H)	Conc. NaOH/KOH (heat)	Alcohol + Carboxylic acid	Disproportionation of aldehydes
Aldol condensation	RCHO / RCOR' (with α-H)	Dil. NaOH, then heat	β-Hydroxy carbonyl / α,β-unsaturated	C—C bond formation via enolate
Perkin	ArCHO + (CH₃CO)₂O	CH₃COONa (heat)	ArCH=CHCOOH (cinnamic acid)	Aldehyde + anhydride → α,β-unsaturated acid
Claisen condensation	2RCOOR' (esters)	Na/NaOEt (base), then H⁺	β-Keto ester	Ester + ester → ketone
Hoffmann bromamide	RCONH₂ (amide)	Br₂ + NaOH	RNH₂ (1° amine)	Rearrangement; product has one less C
Curtius	RCOCl (acyl chloride)	NaN₃, then heat, then H₂O	RNH₂ (1° amine)	Acyl azide intermediate; rearrangement
Schmidt	RCOOH + HN₃	H₂SO₄ (conc.)	RNH₂ (1° amine)	Carboxylic acid → amine with loss of CO₂
Koble-Schmitt	Phenol (C₆H₅OH)	CO₂ + NaOH (125°C, 4-7 atm)	Salicylic acid	CO₂ introduced ortho to OH
Reimer-Tiemann	Phenol	CHCl₃ + NaOH (50-70°C)	Salicylaldehyde	Formyl group introduced ortho to OH
Williamson's synthesis	R—ONa + R'—X	Heat	R—O—R' (ether)	Best method for ether preparation
Fischer esterification	RCOOH + R'OH	H₂SO₄ (conc.), heat	RCOOR' (ester) + H₂O	Reversible, needs excess of one reagent
Hell-Volhard-Zelinsky (HVZ)	RCH₂COOH	Br₂ + P (red), then H₂O	RCHBrCOOH (α-bromo acid)	α-Halogenation of carboxylic acids
Gabriel phthalimide	R—X (primary alkyl halide)	Potassium phthalimide + H⁺ (hydrolysis)	RNH₂ (1° amine, pure)	Primary amine without 2°/3° contamination
Clemmensen reduction	C=O (carbonyl)	Zn(Hg) + HCl (conc.)	—CH₂— (alkane)	Reduction of C=O to CH₂ in acid
Wolff-Kishner reduction	C=O (carbonyl)	NH₂NH₂ + KOH (heat, ethylene glycol)	—CH₂— (alkane)	Reduction of C=O to CH₂ in base
Hinsberg test	1°/2°/3° amine	C₆H₅SO₂Cl + NaOH (aq.)	Different products each	Distinguishes 1°, 2°, 3° amines
Carbylamine (isocyanide)	1° amine (RNH₂)	CHCl₃ + alc. KOH (heat)	R—NC (isocyanide)	Foul smell; only 1° amines react
Mendius	RCN (nitrile)	H₂/Ni or Na/EtOH or LiAlH₄	RCH₂NH₂ (1° amine)	Reduction of nitrile to primary amine
Hoffmann elimination	R₄N⁺OH⁻ (quat. ammonium hydroxide)	Heat (strong base)	Alkene (least substituted) + R₃N + H₂O	Hoffmann product (less substituted alkene)

Important Reagents — Quick Reference

Reagent	Used For	Key Observation / Product
Tollens' reagent [Ag(NH₃)₂]⁺OH⁻	Aldehyde detection	Silver mirror
Fehling's solution Cu²⁺/tartrate	Aliphatic aldehyde detection	Red Cu₂O precipitate
Benedict's solution	Reducing sugars / aldehydes	Red Cu₂O precipitate
Schiff's reagent	Aldehyde detection	Pink/magenta colour
I₂ / NaOH (iodoform)	CH₃CO— / CH₃CH(OH)— groups	Yellow CHI₃ precipitate
2,4-DNPH	Aldehydes / Ketones	Orange/red precipitate
NaHSO₃ (saturated)	Purification of carbonyl compounds	White crystalline adduct
Br₂ / CCl₄	Test for unsaturation (C=C, C≡C)	Brown colour discharged
Baeyer's reagent (cold KMnO₄)	Test for unsaturation	Purple colour discharged; diol formed
Lucas reagent (ZnCl₂ + HCl)	Distinguish 1°/2°/3° alcohols	Cloudiness: 3° immediate, 2° 5-10 min, 1° no reaction
Hinsberg reagent (C₆H₅SO₂Cl)	Distinguish 1°/2°/3° amines	1°: clear soln; 2°: ppt; 3°: oily layer
Carbylamine (CHCl₃ + alc. KOH)	Test for primary amines	Foul-smelling isocyanide (RNC)
Biuret reagent (CuSO₄/NaOH)	Detection of peptide bonds (proteins)	Violet colour
Ninhydrin	Detection of α-amino acids	Blue-purple colour (proline gives yellow)
Soda lime (NaOH + CaO)	Decarboxylation of carboxylic acids	Alkane (R—H) + Na₂CO₃
Grignard reagent (RMgX)	C—C bond formation; alcohol synthesis	1°/2°/3° alcohol depending on carbonyl
LiAlH₄ / NaBH₄	Reduction of carbonyls and other groups	LiAlH₄: strong; NaBH₄: mild
PCC (pyridinium chlorochromate)	Mild oxidation of 1° alcohols	Stops at aldehyde (does not overoxidize)
SOCl₂ (thionyl chloride)	Conversion of RCOOH to RCOCl	Acid chloride + SO₂ + HCl (gaseous by-products)
O₃ (ozone) then Zn/H₂O	Ozonolysis of alkenes	Aldehydes / Ketones
Lindlar catalyst (Pd/CaCO₃ + quinoline)	Partial reduction of alkynes	cis-Alkene
Na/NH₃ (Birch reduction)	Partial reduction of alkynes	trans-Alkene

Quick Mnemonics for Common NEET Traps

Mnemonic	Meaning
"Rich get richer"	Markovnikov: the carbon with more H gets the H⁺
"SN1 — Tertiary, Two-step, Through carbocation"	SN1 favours 3° alkyl halides
"SN2 — Primary, Preferred with Polar aprotic"	SN2 favours 1° alkyl halides
"E2 — anti-periplanar"	H and LG must be 180° apart in E2
"3-2-1 Lucas test"	3° immediate cloudiness; 2° 5-10 min; 1° no reaction
"A-T: 2 bonds; G-C: 3 bonds"	DNA base pairing: A=T (2), G≡C (3)
"Activating = Ortho-Para; Deactivating = Meta"	EAS directing effects; except halogens (ortho-para but deactivating)
"Aromatics: 4n+2 = Happy"	Hückel's rule for aromaticity
"EWG increases acidity; EDG decreases it"	Substituent effect on carboxylic acid/phenol acidity
"Tollens, Fehling, Benedict — All for Aldehydes"	Aldehyde distinction tests; ketones do not react
"Primary gives Carbylamine; Secondary/tertiary do not"	Carbylamine test is only for 1° amines
"Nylon-66: 6 + 6 = two monomers each with 6C"	Nylon-66 is from hexamethylenediamine (6C) + adipic acid (6C)

Product Prediction — Practice Examples for NEET

Organic Chemistry Revision Roadmap for NEET

Week 1 — Fundamentals & Nomenclature: Master IUPAC naming, functional group priority, hybridization, electron displacement effects (I, R, H, E), and reactive intermediates. Week 2 — Isomerism & Hydrocarbons: Structural isomerism, geometrical/optical isomerism, conformations. Alkanes (preparation, free radical halogenation), Alkenes (addition, Markovnikov, anti-Markovnikov, ozonolysis), Alkynes (acidity, Kucherov, partial reduction), Aromatic HCs (Hückel, EAS, directing effects). Week 3 — Haloalkanes, Alcohols, Ethers: SN1/SN2/E1/E2 mechanisms, Lucas test, Williamson's synthesis, phenol chemistry (Kolbe, Reimer-Tiemann), distinction tests. Week 4 — Carbonyl Compounds: Nucleophilic addition, aldol, Cannizzaro, reduction (Wolff-Kishner, Clemmensen), oxidation (Tollens, Fehling, iodoform), carboxylic acid derivatives. Week 5 — Amines, Biomolecules, Polymers, Everyday Chemistry: Basicity of amines, Hinsberg, diazonium salts, carbohydrates (reducing/non-reducing), proteins (peptide bond, denaturation), DNA base pairing, polymers (addition vs condensation), drugs and classification, soaps vs detergents. Week 6 — Practice: Solve 20-25 NEET-level MCQs daily, review named reactions, practice product prediction, and focus on distinction tests and common traps.

Key Numeric Constants for NEET Organic Chemistry

Concept	Value	Notes
Hückel's rule π electrons	4n + 2	n = 0, 1, 2... (6, 10, 14...)
Maximum optical isomers	2ⁿ (n = chiral centres)	Reduced if meso forms exist
Peptide bonds in polypeptide	(α-amino acids) − 1	50 amino acids → 49 peptide bonds
sp³ bond angle	109.5°	Tetrahedral geometry
sp² bond angle	120°	Trigonal planar geometry
sp bond angle	180°	Linear geometry
DNA H-bonds: A—T, G—C	2, 3	A=T (2), G≡C (3)
pKa of carboxylic acids	~4-5	Stronger than phenols (pKa ~10)
pKa of terminal alkynes	~25	Acidic due to 50% s-character
Diazotization temperature	0-5°C	Above 5°C, diazonium salt decomposes
Iodine value measure	Unsaturation of fats/oils	Higher = more unsaturation
Saponification number	mg KOH/g fat	Higher = shorter fatty acid chains

Mastering NEET Organic Chemistry requires understanding reaction mechanisms (not just memorizing products), practicing regularly with timed MCQs, and reviewing common exam traps. Focus on high-weightage topics: named reactions, stereoisomerism, EAS, SN1/SN2, biomolecules, and polymers. Good luck!