Organic Functional Groups & Key JEE Reactions Guide

Organic Chemistry for JEE Main is fundamentally a language — the language of functional groups. Each group has a characteristic reactivity pattern, and once you internalise those patterns, predicting products and mechanisms becomes systematic rather than rote. JEE Main's Organic section contributes 10–11 marks per session, and most of those marks test your ability to predict what happens when a functional group encounters a specific reagent. This guide covers every major functional group's key reactions in the order the exam actually tests them.

Alcohols, Phenols and Ethers: Reactivity Patterns

Alcohols react with: (1) active metals (Na, K) → alkoxide + H₂; (2) PCC → aldehyde from primary, ketone from secondary, no reaction for tertiary; (3) K₂Cr₂O₇/H₂SO₄ → carboxylic acid from primary, ketone from secondary; (4) SOCl₂ or PCl₅ → alkyl halide (with inversion at the carbon for SN2 conditions); (5) H₂SO₄ (170°C) → alkene (elimination); (6) H₂SO₄ (140°C) → ether (substitution). The oxidation state change (primary → aldehyde → carboxylic acid) is the most tested sequence in JEE.

Phenols are more acidic than alcohols (pKa ≈ 10 vs 16) because of resonance stabilisation of the phenoxide ion. Phenol undergoes electrophilic aromatic substitution ortho/para-directing (−OH is activating). Key reactions: bromination in water → 2,4,6-tribromophenol (complete ring bromination); nitration (dilute HNO₃) → ortho and para nitrophenol; Reimer-Tiemann reaction → salicylaldehyde; Kolbe reaction → sodium salicylate; azo coupling with diazonium salt → azo dye. These are all direct JEE targets. Take a free Organic Chemistry mock to check your functional group speed. For deeper coverage, see our alcohol, phenol and ether guide.

Aldehydes and Ketones: The Rich Reaction Chemistry

Nucleophilic addition is the defining reaction of carbonyl compounds. Key additions: HCN → cyanohydrin; NaHSO₃ → bisulphite adduct (aldehydes and methyl ketones only); RMgX (Grignard) → secondary/tertiary alcohol; H₂NOH → oxime; C₆H₅NHNH₂ → phenylhydrazone. Condensations: Aldol (base-catalysed self-condensation of acetaldehyde or similar), Cannizzaro (no α-H → disproportionation to alcohol + acid), Crossed Aldol, Claisen-Schmidt. The Tollens test (silver mirror) and Fehling's test distinguish aldehyde from ketone — JEE tests this distinction in almost every paper.

Identification reactions: Iodoform test (CHI₃ yellow precipitate) — methyl ketones and acetaldehyde give positive result. 2,4-DNP reagent forms yellow/orange precipitate with any carbonyl. These two colour-change tests appear as predictable one-mark JEE questions. For deeper carbonyl chemistry, see our aldehydes and ketones guide and our carbonyl compounds guide.

Carboxylic Acids and Derivatives

Carboxylic acids: more acidic than phenols (stronger −I effect of C=O). Reacts with: Na₂CO₃/NaHCO₃ → CO₂ + salt (distinguishes from phenol, which only reacts with NaOH). Acid chloride (most reactive) → anhydride → ester → amide → carboxylic acid in order of decreasing reactivity toward nucleophilic substitution. Hell-Volhard-Zelinsky (HVZ) reaction: carboxylic acid + Br₂/P → α-bromo acid. Schmidt rearrangement: acid + HN₃/H₂SO₄ → amine (lose CO₂). These named reactions are tested 1–2 times per session.

Esters undergo saponification (base hydrolysis) → carboxylate + alcohol. Transesterification: ester + alcohol → new ester. Diazomethane converts carboxylic acid to methyl ester under mild conditions — a reagent association question JEE uses. For the full carboxylic acid chemistry, see our carboxylic acids guide.

Nitrogen Compounds: Amines and Diazonium Salts

Amines are basic — primary aliphatic amines are most basic; aromatic amines are less basic because lone pair delocalises into ring. Basicity order: aliphatic 3° > 2° > 1° > NH₃ > aromatic amines. Tests: Hinsberg test (distinguish 1°, 2°, 3°), carbylamine reaction (1° amines → isocyanide, bad odour), mustard oil reaction (2° amines). Diazonium salts (RN₂⁺Cl⁻): formed from 1° aryl amine + NaNO₂ + HCl at 0–5°C; react with CuX (Sandmeyer), Cu₂O, HBF₄, etc. Coupling with active aromatic compounds → azo dyes. The Sandmeyer reaction family and coupling reactions are tested nearly every year. For amine chemistry, see our nitrogen compounds guide.