What NEET Asks
- Questions on SN1/SN2 distinction, reactivity order, stereochemistry, and solvent effects are frequent.
- Expect 1-2 questions, carrying 4-8 marks directly or indirectly related to substitution reactions.
- Often integrated with questions on alkyl halides, alcohols, and organic reaction mechanisms.
Key Points
- SN1 Reaction: Unimolecular, 2 steps, carbocation intermediate, racemization (partial). Reactivity: 3° > 2° > 1° alkyl halides. Favored by polar protic solvents.
- SN2 Reaction: Bimolecular, 1 step (concerted), transition state, inversion of configuration (Walden inversion). Reactivity: 1° > 2° > 3° alkyl halides. Favored by polar aprotic solvents.
- Nucleophilicity: Increases down a group in protic solvents (due to better solvation of small ions), but increases with basicity in aprotic solvents.
- Leaving Group Ability: Weaker bases are better leaving groups (e.g., I⁻ > Br⁻ > Cl⁻ > F⁻). Stability of the leaving group is crucial.
- Allylic and benzylic halides show enhanced reactivity in SN1 due to resonance-stabilized carbocations.
- Vinyl and aryl halides are generally unreactive towards nucleophilic substitution under normal conditions.
Must-Know Formula / Reaction
- SN1 Rate Law: Rate = k[Alkyl Halide] (Unimolecular, depends only on substrate concentration in the slow step).
- SN2 Rate Law: Rate = k[Alkyl Halide][Nucleophile] (Bimolecular, depends on both substrate and nucleophile concentration).
Common Mistakes
- Students often confuse the reactivity order for SN1 vs. SN2; remember steric hindrance is key for SN2, while carbocation stability is key for SN1.
- Don't confuse inversion of configuration (characteristic of SN2) with racemization (characteristic of SN1).
- Overlooking the crucial role of solvent (polar protic for SN1 vs. polar aprotic for SN2) in determining the reaction mechanism.
Rapid Revision
- SN1: Carbocation, 3°>2°>1°, racemization, polar protic solvent. Rate = k[R-X].
- SN2: Concerted, 1°>2°>3°, inversion, polar aprotic solvent. Rate = k[R-X][Nu].
- Good leaving groups are weak bases. Allylic/benzylic enhance SN1. Aryl/vinyl are unreactive.