---
title: "SN1 vs SN2 in haloalkane; regulating factors"
subject: "Engineering Chemistry"
chapter: 10
key: "semester_2/engineering_chemistry:sn1-sn2-differ-factors"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/sn1-sn2-differ-factors
site: BCT Engineering
---

# SN1 vs SN2 in haloalkane; regulating factors

Engineering Chemistry · Chapter 10: Reaction Mechanism in Organic reactions

- Asked in: 2069 Bhadra, 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/sn1-sn2-differ-factors
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-10

## Question

How do SN1 and SN2 reactions differ in haloalkane? Mention the factors which regulates the reaction.

## Short answer

SN1 is two-step, carbocation intermediate, first order (k[RX]), favours 3° substrates, polar protic solvents. SN2 is one-step backside attack, second order (k[RX][Nu−]), favours CH3/1° substrates, polar aprotic solvents. Factors: substrate structure, nucleophile strength, leaving group, solent polarity.

## Answer

#### Difference between SN1 and SN2
| 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 |

#### Factors regulating the reaction
1. **Structure of the substrate:** 3° halides favour SN1 (stable carbocation); CH₃ and 1° halides favour SN2 (less steric hindrance).
2. **Nucleophile:** a strong, high-concentration nucleophile favours SN2; a weak one favours SN1.
3. **Leaving group:** a better leaving group (I⁻ > Br⁻ > Cl⁻ > F⁻) speeds up both.
4. **Solvent:** polar protic solvents favour SN1; polar aprotic solvents favour SN2.

## Check yourself

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

1. Which substrate type favours SN1 and which SN2?
   A. CH3 and 1 degree favour SN1; 3 degree favours SN2
   B. 3 degree favours SN1; CH3 and 1 degree favour SN2
   C. Both favour 3 degree
   D. Substrate has no effect
   Answer: B. Stable carbocations drive SN1; low steric hindrance drives SN2. (Key point: Substrate factor)

2. Which nucleophile favours SN2?
   A. A strong, concentrated nucleophile
   B. A weak nucleophile
   C. Only neutral nucleophiles
   D. Nucleophile strength never matters
   Answer: A. SN2 rate depends on the nucleophile; SN1 does not. (Key point: Nucleophile factor)

3. Which is the best leaving group among the halides?
   A. F-
   B. Cl-
   C. I-
   D. Br-
   Answer: C. Order is I- > Br- > Cl- > F-, speeding both SN1 and SN2. (Key point: Leaving group factor)

4. Which solvent pairing is correct?
   A. SN1: polar aprotic; SN2: polar protic
   B. Both: non-polar
   C. Both: polar protic
   D. SN1: polar protic; SN2: polar aprotic
   Answer: D. Protic solvents stabilise ions for SN1; aprotic solvents keep the nucleophile reactive for SN2. (Key point: Solvent factor)

## Related in Engineering Chemistry

- [SN reaction; inversion of configuration; SN1](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Asn-inversion) — Engineering Chemistry, chapter 10
- [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
- [Elimination reactions; E1 vs E2 mechanism](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Ae1-vs-e2) — Engineering Chemistry, chapter 10, asked 2×
- [Solvent favouring SN1; meaning of elimination](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Asn1-solvent-elimination) — Engineering Chemistry, chapter 10
- [SN1 mechanism with stereochemistry; solvent](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Asn1-stereochemistry-solvent) — Engineering Chemistry, chapter 10
- [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×

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