---
title: "E² and SN² mechanisms; factor governing SN²"
subject: "Engineering Chemistry"
chapter: 10
key: "semester_2/engineering_chemistry:e2-sn2-mechanism"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/e2-sn2-mechanism
site: BCT Engineering
---

# E² and SN² mechanisms; factor governing SN²

Engineering Chemistry · Chapter 10: Reaction Mechanism in Organic reactions

- Asked in: 2068 Magh, 4+1 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/e2-sn2-mechanism
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-10

## Question

Explain the reaction mechanism involved in E² and SN² reactions. Mention the one factor governing SN² reaction.

## Short answer

E2: concerted β-H removal and leaving group departure, e.g. CH3CH2Br + KOH(alc) → ethene, rate = k[RX][base]. SN2: backside attack, one-step displacement with inversion, rate = k[RX][Nu−], e.g. CH3Br + OH− → CH3OH. A key factor governing SN2 is substrate steric hinderance: CH3 > 1° > 2° > 3°.

## Answer

#### E2 mechanism
**Mechanism: E2 (one step, concerted).**
```text
 B:⁻  H—CH₂—CH₂—Br   →  [B···H···CH₂···CH₂···Br]⁻‡  →  B–H + CH₂=CH₂ + Br⁻
```
The base removes the β-hydrogen while the C–Br bond breaks and the π bond forms at the same time (the H and Br are anti-periplanar).
Example: CH₃CH₂Br + KOH(alc) → CH₂=CH₂ + KBr + H₂O
- Rate = k[RX][Base]: second order (bimolecular).
- Needs a strong base and a high concentration of it; there is no intermediate.

#### SN2 mechanism
**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).

#### Factor governing SN2
The **structure of the substrate** (steric hindrance) is the main factor: the reactivity falls as CH₃X > 1° > 2° > 3°, because bulky alkyl groups block the backside attack. (Other factors: nucleophile strength, leaving group, solvent.)

## Check yourself

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

1. Which describes E2?
   A. Concerted: base removes beta-H as the halide leaves and the pi bond forms
   B. Two steps via a carbocation
   C. Nucleophile replaces halide with inversion
   D. Slow ionisation followed by fast attack
   Answer: A. E2 is a one-step, anti-periplanar elimination. (Key point: E2 mechanism)

2. How does the nucleophile attack in an SN2 reaction?
   A. From the same side as the leaving group, giving retention
   B. From either face of a planar carbocation
   C. From the side opposite to the leaving group (180 degrees), giving inversion
   D. Sideways at 90 degrees to the C-X bond
   Answer: C. 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)

3. Which main factor governs SN2 reactivity?
   A. Carbocation stability, with 3 degree fastest
   B. Steric hindrance in the substrate: CH3X > 1 degree > 2 degree > 3 degree
   C. Planarity of the nucleophile
   D. Colour of the solvent
   Answer: B. Bulky groups block backside attack. (Key point: Factor governing 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)

## Related in Engineering Chemistry

- [SN reactions; hydrolysis of 3° alkyl halide](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Asn-tertiary-hydrolysis) — Engineering Chemistry, chapter 10, asked 2×
- [SN² mechanism and difference from SN¹](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Asn2-vs-sn1) — 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
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- [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

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