---
title: "Bromomethane to methanol; SN² inversion"
subject: "Engineering Chemistry"
chapter: 10
key: "semester_2/engineering_chemistry:sn2-bromomethane-inversion"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/sn2-bromomethane-inversion
site: BCT Engineering
---

# Bromomethane to methanol; SN² inversion

Engineering Chemistry · Chapter 10: Reaction Mechanism in Organic reactions

- Asked in: 2073 Magh, 3+2 marks
- Video: [Nucleophilic Substitution Reactions | SN1 Reaction and SN2 Reaction](https://www.youtube.com/watch?v=VpFGcpYgwy8) — Najam Academy
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/sn2-bromomethane-inversion
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-10

## Question

Describe the mechanism for the conversion of Bromomethane into methanol in presence of aq. alkali. Why does SN² reaction take place with stereochemical inversion?

## Short answer

Bromomethane converts to methanol by SN2: OH⁻ attacks carbon from the backside, bonds form/break simultaneously, yielding methanol with inversion of configuration (umbrella flip); rate = k[CH3Br][OH⁻].

## Answer

#### Conversion of bromomethane into methanol (SN2)
**Mechanism: SN2 (one step, concerted).**
```text
HO⁻ + H₃C—Br  →  [HO···CH₃···Br]⁻‡  →  HO—CH₃ + Br⁻
                 (transition state)
```
```text
        H                 H                  H
         \                |                 /
 HO⁻ +  H—C—Br  →  [HO····C····Br]⁻  →  HO—C—H  +  Br⁻
         /               / \                \
        H               H   H                H
 backside attack      transition state    inverted product
```
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 like an umbrella in the wind (Walden inversion).
- Reactivity order: CH₃X > 1° > 2° > 3° (steric hindrance).

#### Why SN2 takes place with inversion
In SN2 the nucleophile attacks the carbon from the **backside**, 180° from the leaving group. This is the position of least repulsion between the lone pair of the incoming nucleophile and the electron-rich leaving group, and it gives the best overlap with the C–X σ* orbital. As the new bond forms, the other three groups flatten and then flip to the opposite side, so the configuration is inverted (Walden inversion).

## Check yourself

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

1. How does CH3Br convert to CH3OH with aqueous alkali?
   A. SN1: Br- leaves first to give CH3+
   B. SN2: OH- attacks from the back as Br- leaves in one step
   C. E2: OH- removes H from CH3Br
   D. OH- adds to the carbon and Br- leaves after a long-lived intermediate
   Answer: B. Single concerted step through a transition state [HO...CH3...Br]-. (Key point: Conversion of CH3Br to CH3OH)

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

3. Why does SN2 take place with inversion?
   A. The nucleophile attacks the front face of a planar cation
   B. The leaving group is retained on the carbon
   C. Backside attack makes the other three groups flip like an umbrella
   D. The three groups are replaced by the nucleophile
   Answer: C. As the Nu-C bond forms from the back, the other groups flatten and flip to the opposite side (Walden inversion). (Key point: Why inversion)

4. What favours backside attack?
   A. Greater attraction between OH- and Br
   B. Lower steric hindrance on the Br side
   C. Planarity of the substrate
   D. Least repulsion between nucleophile lone pair and leaving group, with best sigma* overlap
   Answer: D. The rear of C-X has the least electron repulsion and the best orbital overlap. (Key point: Reason for backside attack)

## Related in Engineering Chemistry

- [Mechanism and kinetics of SN2; stereochemical inversion [partly illegible]](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Asn2-kinetics-inversion) — Engineering Chemistry, chapter 10
- [SN¹ retention/inversion vs SN² inversion; mechanism](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Asn1-retention-inversion) — Engineering Chemistry, chapter 10
- [SN reaction; inversion of configuration; SN1](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Asn-inversion) — Engineering Chemistry, chapter 10
- [SN² mechanism and difference from SN¹](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Asn2-vs-sn1) — Engineering Chemistry, chapter 10
- [SN² mechanism for hydrolysis; solvent; no carbocation](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Asn2-hydrolysis-solvent-carbocation) — Engineering Chemistry, chapter 10
- [Primary alkyl halide with alcoholic and aqueous alkali](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Aprimary-halide-alc-aq-alkali) — Engineering Chemistry, chapter 10

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