- 2070 Magh · 4+1 marks
Mechanism for primary alkyl halide in aqueous medium; solvent
Short answer
Primary alkyl halide reacts by SN2: OH− attacks from the back in one step, displacing Br−; rate = k[RX][OH−]. Polar aprotic solvents (acetone, DMF) favor SN2 because they do not hydrogen-bond the nucleophile, keeping it reactive.
Explain the reaction mechanism of primary alkyl halide in aqueous medium. Which type of solvent favours this reaction, give reason?
A primary alkyl halide reacts with aqueous alkali by the SN2 mechanism to give an alcohol. Example: CH₃CH₂Br + NaOH(aq) → CH₃CH₂OH + NaBr.
HO⁻ + CH₃CH₂—Br → [HO···CH₂(CH₃)···Br]⁻‡ → HO—CH₂CH₃ + Br⁻
- One step: the nucleophile attacks from the back while Br⁻ leaves.
- Rate = k[RX][OH⁻] (second order).
- Primary carbon has little steric hindrance and gives an unstable primary carbocation, so SN1 does not occur.
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.
✓ Answer checked by Claude Opus 5.5 (high effort)
Related questions
- Primary alkyl halide with alcoholic and aqueous alkali · Engineering Chemistry, chapter 10
- Hydrolysis of 3° alkyl halide by aq. NaOH; solvent · Engineering Chemistry, chapter 10
- 3° alkyl halide with alcoholic KOH; solvent · Engineering Chemistry, chapter 10
- 3° alkyl halide with aqueous caustic potash · Engineering Chemistry, chapter 10
- SN reactions; hydrolysis of 3° alkyl halide · Engineering Chemistry, chapter 10 · asked 2×
- Tertiary alkyl halide with aq. NaOH / alc. KOH · Engineering Chemistry, chapter 10