---
title: "SN² mechanism and difference from SN¹"
subject: "Engineering Chemistry"
chapter: 10
key: "semester_2/engineering_chemistry:sn2-vs-sn1"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/sn2-vs-sn1
site: BCT Engineering
---

# SN² mechanism and difference from SN¹

Engineering Chemistry · Chapter 10: Reaction Mechanism in Organic reactions

- Asked in: 2075 Baisakh, 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-vs-sn1
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-10

## Question

Write the reaction mechanism of SN² reaction with a suitable example. How does it differ from SN¹ reaction?

## Short answer

SN2: one-step concerted backside attack; rate = k[RX][Nu⁻]; inversion (Walden); reactivity order CH₃X > 1° > 2° > 3°. Example: CH₃Br + NaOH → CH₃OH. SN1: two-step with carbocation; rate = k[RX] only; racemization; rearrangement possible; order 3° > 2°; (CH₃)₃CBr + aq. NaOH → (CH₃)₃COH.

## Answer

#### SN2 mechanism
Example: reaction of methyl bromide with aqueous NaOH.

**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).

#### Difference from SN1
| Basis | SN1 | SN2 |
|---|---|---|
| Steps | Two steps; carbocation intermediate | One step; transition state |
| Rate law | Rate = k[RX] (first order) | Rate = k[RX][Nu⁻] (second order) |
| Substrate order | 3° > 2° > 1° | CH₃ > 1° > 2° > 3° |
| Nucleophile | Strength unimportant | Strong nucleophile needed |
| Solvent | Polar protic (water, alcohol) | Polar aprotic (acetone, DMF, DMSO) |
| Stereochemistry | Racemisation (inversion + retention) | Complete inversion (Walden) |
| Rearrangement | Possible | Not possible |
| Example | (CH₃)₃CBr + aq. NaOH | CH₃Br + aq. NaOH |

## 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 side opposite to the leaving group (180 degrees), giving inversion
   B. From the same side as the leaving group, giving retention
   C. From either face of a planar carbocation
   D. Sideways at 90 degrees to the C-X bond
   Answer: A. 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. 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][OH-], second order
   D. Rate = k[CH3Br]^2[OH-]
   Answer: C. Both substrate and nucleophile are in the single rate-determining step, so the reaction is second order overall. (Key point: SN2 rate law)

3. How do the stereochemical outcomes of SN1 and SN2 differ?
   A. SN1 gives complete inversion; SN2 gives racemisation
   B. SN1 gives racemisation; SN2 gives complete inversion
   C. Both give complete retention
   D. Both give complete inversion
   Answer: B. SN1 passes through a planar carbocation (both faces); SN2 is backside attack only. (Key point: Stereochemistry contrast)

4. Which reactivity order holds for SN2?
   A. 3 degree > 2 degree > 1 degree > CH3
   B. All substrates are equally reactive
   C. 2 degree > 1 degree > 3 degree
   D. CH3 > 1 degree > 2 degree > 3 degree (steric hindrance)
   Answer: D. Bulky groups block backside attack, so methyl and primary react fastest in SN2. (Key point: Substrate order)

## Related in Engineering Chemistry

- [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 for hydrolysis; solvent; no carbocation](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Asn2-hydrolysis-solvent-carbocation) — Engineering Chemistry, chapter 10
- [Unimolecular elimination mechanism; difference from E2](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Ae1-mechanism-differ-e2) — Engineering Chemistry, chapter 10, asked 2×
- [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×
- [Bromomethane to methanol; SN² inversion](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Asn2-bromomethane-inversion) — Engineering Chemistry, chapter 10

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