---
title: "Solvent favouring SN1; meaning of elimination"
subject: "Engineering Chemistry"
chapter: 10
key: "semester_2/engineering_chemistry:sn1-solvent-elimination"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/sn1-solvent-elimination
site: BCT Engineering
---

# Solvent favouring SN1; meaning of elimination

Engineering Chemistry · Chapter 10: Reaction Mechanism in Organic reactions

- Asked in: 2072 Magh, 1+1 marks
- Video: [Choosing Between SN2, SN1, E2 and E1 Reactions](https://www.youtube.com/watch?v=zQjenj4B5vM) — Melissa Maribel
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/sn1-solvent-elimination
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-10

## Question

What type of solvent favours the unimolecular nucleophilic substitution reaction mechanism? What is meant by elimination reaction?

## Short answer

Polar protic solvents (e.g., water, ethanol) favor SN1 by stabilizing the carbocation and leaving group through solvation. An elimination reaction removes two atoms or groups from adjacent carbons, forming a double bond; dehydrohalogenation is a typical example.

## Answer

#### Solvent for SN1
**Solvent:** polar protic solvents (water, ethanol, methanol, formic acid). They stabilise the carbocation and the halide ion by solvation (ion–dipole interaction and hydrogen bonding with the leaving group), which lowers the energy of the transition state of the slow step.

#### Elimination reaction
An **elimination reaction** is a reaction in which two atoms or groups are removed from adjacent carbon atoms of a molecule, forming a double (or triple) bond, without being replaced by other groups. **Dehydrohalogenation** of an alkyl halide is the elimination of H and X (as HX) using a strong base such as alcoholic KOH to form an alkene.
Example: CH₃CH₂Br + KOH(alc) → CH₂=CH₂ + KBr + H₂O

## Check yourself

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

1. Which solvent type favours SN1, and why?
   A. Polar aprotic, because it leaves the nucleophile 'naked'
   B. Non-polar, because it avoids solvating the ions
   C. Polar protic, because it stabilises the carbocation and halide ion by solvation
   D. Polar aprotic, because it hydrogen-bonds to the carbocation
   Answer: C. Polar protic solvents (water, alcohols) stabilise ions through ion-dipole interaction and hydrogen bonding, speeding ionisation. (Key point: Solvent for SN1)

2. Which are suitable solvents for SN1?
   A. Water, ethanol, methanol, formic acid
   B. Acetone, DMF, DMSO
   C. Hexane, benzene
   D. Carbon tetrachloride
   Answer: A. These are polar protic solvents. (Key point: Examples of SN1 solvents)

3. What happens in an elimination reaction?
   A. A group is replaced by another group on the same carbon
   B. Two atoms or groups are removed from adjacent carbons, forming a double or triple bond
   C. Two molecules add together to form a larger saturated molecule
   D. A hydrogen is exchanged between two carbons without forming a pi bond
   Answer: B. Elimination removes two groups from neighbouring carbons without replacing them, so a C=C (or triple) bond forms. (Key point: Elimination reaction)

4. What is dehydrohalogenation?
   A. Replacement of X by H
   B. Addition of HX to an alkene
   C. Loss of water from an alcohol
   D. Elimination of H and X (as HX) from an alkyl halide to form an alkene
   Answer: D. A strong base such as alcoholic KOH removes HX. (Key point: Dehydrohalogenation)

## Related in Engineering Chemistry

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