Organic Reaction Mechanisms (NEET Chemistry)
Why This Chapter Matters
Organic reactions carry 12-18 marks in NEET Chemistry. Named reactions, reaction mechanisms, and identification of products are the most tested topics. This is pure practice-based — the more problems you solve, the better.
Core Concepts
1. Key Named Reactions
Aldol Condensation:
CH₃CHO + CH₃CHO --(NaOH, cold)--> CH₃CH(OH)CH₂CHO (3-hydroxybutanal)
Condition: α-H must be present. Base catalyst.
Product: β-hydroxy carbonyl compound → on heating → α,β-unsaturated carbonyl.
Cannizzaro Reaction:
2HCHO + NaOH --> CH₃OH + HCOONa
Only aldehydes WITHOUT α-H (HCHO, PhCHO, R₃CCHO).
Disproportionation: one molecule oxidised, another reduced.
Clemmensen Reduction: C=O → CH₂ (Zn-Hg/HCl, acidic conditions)
Wolff-Kishner Reduction: C=O → CH₂ (N₂H₄/KOH, basic conditions)
Sandmeyer Reaction:
ArN₂⁺Cl⁻ + CuCl → ArCl + N₂ (diazonium salt + cuprous halide)
ArN₂⁺Cl⁻ + CuCN → ArCN + N₂ (with cuprous cyanide → aryl nitrile)
Balz-Schiemann Reaction:
ArN₂⁺BF₄⁻ --(heat)--> ArF + N₂ + BF₃ (synthesis of aryl fluoride)
Gattermann-Koch Reaction:
ArH + CO + HCl --(AlCl₃, CuCl)--> ArCHO (synthesis of aromatic aldehydes)
Reimer-Tiemann Reaction:
Phenol + CHCl₃ --(NaOH)--> 2-hydroxybenzaldehyde (ortho-substitution, gives o-formyl phenol)
Gabriel Phthalimide Synthesis: Synthesis of primary amines. Phthalimide → N-alkylphthalimide → hydrolysis → primary amine.
Hoffmann Bromamide Reaction:
RCONH₂ + Br₂ + NaOH → RNH₂ + CO₂ (amide → primary amine, carbon chain decreases by 1)
Fehling's Test: Cu²⁺ (blue) → Cu₂O (brick red) with aliphatic aldehydes. Benzaldehyde does NOT give Fehling's test.
Tollen's Test: [Ag(NH₃)₂]⁺ → Ag (silver mirror) with all aldehydes. Useful to distinguish aldehyde from ketone.
Lucas Test: Differentiates 1°, 2°, 3° alcohols using ZnCl₂ + HCl.
3° alcohol: immediately turbid | 2° alcohol: turbid in 5 min | 1° alcohol: no reaction at RT.
2. Nucleophilic Substitution
SN2: Backside attack by nucleophile. Bimolecular. Inversion (Walden inversion). Favoured by 1° substrate and strong Nu.
Rate = k[substrate][nucleophile]
Order: 1° > 2° > 3° (steric hindrance decreases)
SN1: Ionisation to carbocation (rate-limiting). Unimolecular. Racemisation. Favoured by 3° substrate, polar solvent.
Rate = k[substrate]
Order: 3° > 2° > 1°
3. Electrophilic Aromatic Substitution (EAS)
Mechanism: Electrophile attacks ring → Arenium ion intermediate → proton loss → restoration of aromaticity.
Nitration: HNO₃ + H₂SO₄ → NO₂⁺ (nitronium ion, electrophile) → attacks ring
Halogenation: Cl₂ + FeCl₃ → Cl⁺FeCl₄⁻ → Cl⁺ attacks ring
Directing effects:
o/p-directors (activate): -OH, -OR, -NH₂, alkyl groups, halogens (weak director)
m-directors (deactivate): -NO₂, -CN, -CHO, -COOH, -SO₃H
4. Polymers
Classification:
Natural: starch, cellulose, natural rubber, proteins, nucleic acids
Synthetic: nylon, terylene, Bakelite, Buna-S, Buna-N, PTFE (Teflon)
Types by structure:
Linear: nylon, terylene (strong)
Branched: LDPE (soft)
Cross-linked/Network: Bakelite, vulcanised rubber (hard/rigid)
Key polymers:
| Polymer | Monomers | Type | Uses |
|---|
|---|---|---|---|
| Nylon-6,6 | Hexamethylenediamine + Adipic acid | Addition polyamide | Textile fibres |
|---|---|---|---|
| Nylon-6 | Caprolactam | Ring-opening | Fibres |
| Terylene (Dacron) | Ethylene glycol + Terephthalic acid | Condensation polyester | Textile, bottles |
| Bakelite | Phenol + Formaldehyde | Condensation | Electrical insulators |
| Buna-S | Butadiene + Styrene | Copolymer | Tyres |
| LDPE | Ethylene | Free radical addition | Plastic bags |
| HDPE | Ethylene | Ziegler-Natta | Hard plastics, pipes |
| Teflon (PTFE) | Tetrafluoroethylene | — | Non-stick coating |
PYQs (NEET)
NEET 2023: Which reaction converts an amide to a primary amine with one less carbon?
Hoffmann Bromamide Degradation reaction.
RCONH₂ + Br₂/NaOH → RNH₂ + CO₂ + NaBr
NEET 2022: Cannizzaro reaction is given by which aldehyde?
Formaldehyde (HCHO) and Benzaldehyde (PhCHO) — aldehydes WITHOUT alpha hydrogen.
NEET 2021: Lucas test immediately gives turbidity with:
Tertiary (3°) alcohols — instantaneous turbidity because 3° carbocation forms readily.
NEET 2020: Which polymer is used in making non-stick cookware?
Teflon (PTFE — Polytetrafluoroethylene).
MCQ Practice
Q1. Aldol condensation requires:
(A) No α-H (B) α-H present ✓ (C) Only ketones (D) Acid catalyst
Q2. In SN2, the rate depends on:
(A) Only substrate (B) Only nucleophile (C) Both substrate and nucleophile ✓ (D) Solvent only
Q3 (Hard). Product when toluene reacts with Br₂ in presence of FeBr₃ (no light):
(A) Benzyl bromide (B) Ortho and para bromotoluene ✓ (C) Meta bromotoluene (D) Benzaldehyde
[FeBr₃ catalyst = EAS; -CH₃ is o/p director → o-bromotoluene + p-bromotoluene]