---
title: "Mechanism for primary alkyl halide in aqueous medium; solvent"
subject: "Engineering Chemistry"
chapter: 10
key: "semester_2/engineering_chemistry:sn2-primary-solvent"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/sn2-primary-solvent
site: BCT Engineering
---

# Mechanism for primary alkyl halide in aqueous medium; solvent

Engineering Chemistry · Chapter 10: Reaction Mechanism in Organic reactions

- Asked in: 2070 Magh, 4+1 marks
- Video: [Nucleophilic Substitution of Halogenoalkanes with Hydroxide Ions, OH- (A-level Chemistry)](https://www.youtube.com/watch?v=hri0ig1nW3M) — Chemistry Student
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/sn2-primary-solvent
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-10

## Question

Explain the reaction mechanism of primary alkyl halide in aqueous medium. Which type of solvent favours this reaction, give reason?

## 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.

## Answer

A primary alkyl halide reacts with aqueous alkali by the **SN2 mechanism** to give an alcohol. Example: CH₃CH₂Br + NaOH(aq) → CH₃CH₂OH + NaBr.

```text
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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Which mechanism does a primary halide follow with aqueous alkali?
   A. SN1 via a primary carbocation
   B. E1 elimination
   C. SN2, giving an alcohol
   D. E2 elimination only
   Answer: C. CH3CH2Br + NaOH(aq) gives CH3CH2OH + NaBr by SN2. (Key point: Mechanism for primary halide in aqueous alkali)

2. Why does SN1 not occur for primary halides?
   A. A primary carbocation is unstable
   B. Primary carbons are too hindered
   C. Primary halides cannot ionise in any solvent
   D. SN1 needs a strong base
   Answer: A. Instability of the carbocation rules SN1 out, and the carbon is unhindered, favouring SN2. (Key point: Why not SN1)

3. Which solvent type favours SN2, and why?
   A. Polar protic, because it stabilises the carbocation intermediate
   B. Polar aprotic, because it leaves the nucleophile 'naked' and more reactive
   C. Polar protic, because it strengthens the nucleophile by hydrogen bonding
   D. Non-polar, because ionic nucleophiles dissolve best there
   Answer: B. Aprotic solvents dissolve ionic nucleophiles without hydrogen-bonding to the anion, so it stays reactive. Protic solvents weaken it. (Key point: Solvent for SN2)

4. What is the rate law for SN2 hydrolysis of CH3Br by OH-?
   A. Rate = k[CH3Br], first order
   B. Rate = k[OH-], first order
   C. Rate = k[CH3Br]^2[OH-]
   D. Rate = k[CH3Br][OH-], second order
   Answer: D. Both substrate and nucleophile are in the single rate-determining step, so the reaction is second order overall. (Key point: SN2 rate law)

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