Organic · Reaction Mechanisms
Substitution & Elimination (SN1, SN2, E1, E2)
Exam-focused revision notes on Substitution & Elimination (SN1, SN2, E1, E2) for CSIR-NET Chemical Sciences, GATE Chemistry and IIT-JAM — the core concepts, the formulas worth memorising, the traps that cost marks, and a quick-recall table.
The core idea
A leaving group departs from an sp3 carbon. A nucleophile can take its place (substitution) or a base can pull off a β-hydrogen to form a π bond (elimination). Whether the mechanism is concerted (one step) or stepwise via a carbocation decides everything: rate law, stereochemistry, and which substrates work.
SN2 — bimolecular substitution
- One concerted step: nucleophile attacks from the side opposite the leaving group (backside attack).
- Rate law: rate = k[substrate][Nu] — second order.
- Stereochemistry: inversion of configuration (Walden inversion). A single stereocentre flips.
- Substrate order: methyl > 1° > 2° > 3° (steric hindrance blocks backside attack; 3° essentially does not do SN2).
- Favoured by: strong/small nucleophiles, polar aprotic solvents (DMSO, DMF, acetone), good leaving group.
SN1 — unimolecular substitution
- Two steps: slow ionisation to a carbocation, then fast attack by nucleophile.
- Rate law: rate = k[substrate] — first order (rate depends only on substrate).
- Stereochemistry: planar carbocation is attacked from both faces → racemisation (often with slight excess of inversion).
- Substrate order: 3° > 2° > 1° > methyl (more stable carbocation wins; allylic/benzylic also fast).
- Favoured by: polar protic solvents (water, alcohols) that stabilise the ion, weak nucleophiles, good leaving group.
- Watch for carbocation rearrangements (hydride/alkyl shifts to a more stable cation).
E2 — bimolecular elimination
- One concerted step: strong base removes β-H as the leaving group departs.
- Rate law: rate = k[substrate][base] — second order.
- Geometry: requires anti-periplanar H and leaving group (dihedral ≈ 180°) — key stereoelectronic demand.
- Regiochemistry: normally Zaitsev (more substituted, more stable alkene). A bulky base (t-BuOK, LDA) gives Hofmann (less substituted alkene).
- Favoured by: strong bases, 3° > 2° > 1° substrates, heat.
E1 — unimolecular elimination
- Two steps: ionise to carbocation (slow), then base removes β-H (fast).
- Rate law: rate = k[substrate] — first order; no anti-periplanar requirement.
- Regiochemistry: Zaitsev (most stable alkene predominates).
- Favoured by: 3° substrates, weak base, polar protic solvent, heat — competes with SN1 (same carbocation intermediate).
Choosing the pathway (competition)
Strong bulky base + heat → E2 (Hofmann) · Strong small nucleophile → SN2 · Weak nucleophile/base + protic solvent + 3° → SN1/E1 mixture
- Methyl / 1°: SN2 (or E2 only with a strong bulky base).
- 2°: the battleground — depends heavily on nucleophile/base strength and solvent.
- 3°: no SN2. Weak conditions → SN1/E1; strong base → E2.
- Raising temperature favours elimination over substitution (entropy).
⚠️ Common traps students miss
- SN2 is backside attack → inversion, not racemisation. Racemisation belongs to SN1.
- E2 needs H and leaving group anti-periplanar — if they can't align, the "expected" Zaitsev product may not form (classic in cyclohexanes: both must be axial).
- 3° substrates favour SN1/E1 (or E2 with strong base) — they do not do SN2.
- A strong bulky base (t-BuOK) flips the outcome to Hofmann, not Zaitsev.
- Polar aprotic boosts SN2; polar protic boosts SN1/E1 — don't swap them.
30-second recall table
| Feature | SN1 | SN2 | E1 | E2 |
|---|---|---|---|---|
| Substrate | 3° > 2° | methyl > 1° > 2° | 3° > 2° | 3° > 2° > 1° |
| Rate law | k[sub] | k[sub][Nu] | k[sub] | k[sub][base] |
| Molecularity | 1st order | 2nd order | 1st order | 2nd order |
| Stereochem | racemisation | inversion | Zaitsev alkene | anti-periplanar; Zaitsev* |
| Conditions | weak Nu, protic | strong Nu, aprotic | weak base, protic, heat | strong base, heat |
*E2 gives Hofmann (less substituted alkene) with a bulky base.
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