--- title: "SN² mechanism for hydrolysis; solvent; no carbocation" subject: "Engineering Chemistry" chapter: 10 key: "semester_2/engineering_chemistry:sn2-hydrolysis-solvent-carbocation" source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/sn2-hydrolysis-solvent-carbocation site: BCT Engineering --- # SN² mechanism for hydrolysis; solvent; no carbocation Engineering Chemistry · Chapter 10: Reaction Mechanism in Organic reactions - Asked in: 2067 Mangsir, 3+1+1 marks - Video: [Nucleophilic substitution reaction / SN1 & SN2 (Class 12. Haloalkane and Haloarenes)](https://www.youtube.com/watch?v=JP64GaakOzo) — TOMAR CHEMISTRY CLASSES INDORE - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/sn2-hydrolysis-solvent-carbocation - Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-10 ## Question Explain the SN2 reaction mechanism with reference to hydrolysis of alkylhalide. What type of solvent favors this type of path? How can you say that carbocation is not formed during SN2 reaction? ## Short answer In SN2 hydrolysis, OH- attacks the alkyl halide from the opposite side of the leaving group in one concerted step with Walden inversion; rate = k[RX][OH-]. Polar aprotic solvents are favored. No carbocation forms because the reaction is bimolecular, gives complete inversion, and reactivity is methyl > 1° > 2° > 3°, opposite to carbocation stability. ## Answer #### SN2 hydrolysis of an alkyl halide **Mechanism: SN2 (one step, concerted).** ```text HO⁻ + H₃C—Br → [HO···CH₃···Br]⁻‡ → HO—CH₃ + Br⁻ (transition state) ``` ```text HO⁻ → C ← Br backside attack, 180° from Br /|\ H H H inverted like an umbrella in the wind ``` Overall: CH₃Br + NaOH(aq) → CH₃OH + NaBr - Rate = k[CH₃Br][OH⁻]: second order (bimolecular). - The nucleophile attacks from the side opposite to the leaving group; the three H atoms flip over (Walden inversion). - Reactivity order: CH₃X > 1° > 2° > 3° (steric hindrance). #### Solvent **Solvent:** polar aprotic solvents (acetone, DMF, DMSO, acetonitrile). They dissolve ionic nucleophiles but do not hydrogen-bond to the anion, so the nucleophile remains "naked" and more reactive. Polar protic solvents like water solvate and weaken the nucleophile, slowing SN2. #### No carbocation in SN2 No carbocation is formed in SN2 because: 1. The reaction is a single concerted step (bond formation and bond breaking at the same time), with a transition state, not an intermediate. 2. The rate depends on both the substrate and the nucleophile (second order), whereas carbocation formation would give first-order kinetics. 3. A chiral substrate gives complete inversion; a planar carbocation would give racemisation. 4. Methyl halides react fastest, then primary halides, even though they would form the least stable carbocations; tertiary halides, which form the most stable carbocations, react slowest. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How does the nucleophile attack in an SN2 reaction? A. From the same side as the leaving group, giving retention B. From the side opposite to the leaving group (180 degrees), giving inversion C. From either face of a planar carbocation D. Sideways at 90 degrees to the C-X bond Answer: B. Backside attack is least repulsive and overlaps best with the C-X sigma* orbital, so the configuration is inverted (Walden inversion). (Key point: Backside attack in SN2) 2. Which solvent type favours SN2, and why? A. Polar aprotic, because it leaves the nucleophile 'naked' and more reactive B. Polar protic, because it stabilises the carbocation intermediate C. Polar protic, because it strengthens the nucleophile by hydrogen bonding D. Non-polar, because ionic nucleophiles dissolve best there Answer: A. Aprotic solvents dissolve ionic nucleophiles without hydrogen-bonding to the anion, so it stays reactive. Protic solvents weaken it. (Key point: Solvent for SN2) 3. Which kinetic fact shows SN2 has no carbocation? A. Rate depends on substrate only B. Rate is independent of substrate C. Rate depends only on the solvent D. Rate depends on both substrate and nucleophile (second order) Answer: D. Carbocation formation would give first-order kinetics. (Key point: Kinetic evidence against carbocation) 4. How does stereochemistry show no carbocation in SN2? A. A chiral substrate gives racemisation B. Product retains its configuration C. A chiral substrate gives complete inversion, not racemisation D. Product is always achiral Answer: C. A planar carbocation would give a racemic mixture. (Key point: Stereochemical evidence) ## Related in Engineering Chemistry - [Hydrolysis of 3° alkyl halide by aq. 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