Organic Functional Groups: JEE Main Master Guide

Organic chemistry in JEE Main is organised by functional group — every reaction, every test, every physical property follows directly from the functional group present in the molecule. This master guide maps all the major functional groups to their structural features, nomenclature rules, physical properties, and the key reactions JEE Main tests. Use this as a consolidated reference for your final organic chemistry revision.

Alkanes, Alkenes, and Alkynes

Alkanes (C-C single bond): sp³ hybridised, tetrahedral geometry, free rotation about C-C bond. Reactions: combustion (oxidation), halogenation by free radical mechanism (most reactive at 3° carbon), thermal cracking. Alkenes (C=C double bond): sp² hybridised, planar, restricted rotation — source of geometric (cis-trans) isomerism. Reactions: electrophilic addition (Markovnikov's rule for unsymmetric alkenes), anti addition (bromine test for unsaturation), Baeyer's test (KMnO₄ decolourises in cold — positive for alkene). JEE Main tests Markovnikov addition most frequently: HBr + propene → 2-bromopropane (H adds to less substituted carbon, Br to more substituted).

Alkynes (C≡C triple bond): sp hybridised, linear at the triple bond. Terminal alkynes are acidic (pKa ~25) — react with NaNH₂ to form alkynide ion. Reactions: electrophilic addition (two steps with HX — first Markovnikov), hydration with HgSO₄/H₂SO₄ gives ketone (or aldehyde from terminal alkyne). For reaction mechanisms underlying these reactions, see our Reaction Mechanisms Guide.

Halogen Compounds

Haloalkanes (alkyl halides): C-X bond polar (X is more electronegative). Reactions: SN1 (3° substrates, carbocation mechanism), SN2 (1° substrates, backside attack, inversion of configuration). Competition between substitution and elimination: SN2 vs. E2 — bulky base favours E2. Grignard reagent (RMgX): formed from alkyl halide + Mg in dry ether — reacts with H₂O, CO₂, C=O, C≡N. Haloarenes: C-X bond stronger than in haloalkanes, less reactive in substitution. Reactions: nucleophilic aromatic substitution (requires electron-withdrawing groups activating the ring). Take a free mock test on organic chemistry to practise functional group identification and reaction prediction.

Alcohols, Phenols, Carbonyl Groups

Alcohols (−OH on sp³ carbon): hydrogen bonding, high boiling point. Key reactions: esterification, Lucas test (3°>2°>1° reactivity), dehydration to alkene. Phenols (−OH on benzene): aromatic, highly activated for EAS, more acidic than alcohols. Key reactions: bromination with Br₂/H₂O (no catalyst), Reimer-Tiemann, Kolbe's reaction. For detailed coverage, see our Alcohols, Phenols & Ethers Guide.

Aldehydes (−CHO) and Ketones (C=O): electrophilic carbonyl carbon undergoes nucleophilic addition. Aldehydes are more reactive than ketones (less steric hindrance, more electrophilic). Tests: Tollens' reagent (Ag mirror — only aldehydes), Fehling's solution (red ppt — only aliphatic aldehydes), 2,4-DNP (orange ppt — both). Aldol condensation: base-catalysed self-condensation of aldehyde or methyl ketone. Cannizzaro: non-α-H aldehydes undergo disproportionation with strong base.

Nitrogen-Containing Groups

Amines (−NH₂, −NHR, −NR₂): basic, react with acids to form salts. Primary amines react with HNO₂ to form diazonium salts (important — basis for Sandmeyer and related reactions). Nitro group (−NO₂): strongly electron-withdrawing, deactivates benzene ring, directs meta. Reduction of nitro gives amine: ArNO₂ → ArNH₂ (Sn/HCl or Fe/HCl). Amides (−CONH₂): weakly basic, Hofmann degradation gives primary amine one carbon shorter. Nitriles (−C≡N): hydrolysis gives amide → carboxylic acid. JEE Main frequently asks for product when a nitrile is hydrolysed in acidic or basic medium.

Functional Group Tests: Quick Reference

Tollens' test: silver mirror → aldehyde. Fehling's test: red precipitate → aliphatic aldehyde. Baeyer's test: KMnO₄ decolourised → C=C or C≡C. Lucas test: turbidity with ZnCl₂/HCl → 3° immediate, 2° slow, 1° no reaction. Bromine water test: decolourisation → unsaturation or phenol. FeCl₃ test: violet colour → phenol. Carbylamine test: isocyanide smell with CHCl₃/KOH → primary amine. Lassaigne's test: sodium fusion → identifies N, S, halogens. JEE Main tests these identification reactions — know each test, what gives a positive result, and what it identifies.

Exam Strategy

Functional group questions in JEE Main are almost always identification (which reagent gives this product?) or test-based (which reagent differentiates A from B?). Build a table: functional group → test → positive result. This table covers 30-40% of all JEE Main organic questions. For the named reactions that use these functional groups, see our Organic Name Reactions Guide. Upgrade for ₹149/month for comprehensive organic chemistry question banks organised by functional group.