---
title: "SN1 mechanism with stereochemistry; solvent"
subject: "Engineering Chemistry"
chapter: 10
key: "semester_2/engineering_chemistry:sn1-stereochemistry-solvent"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/sn1-stereochemistry-solvent
site: BCT Engineering
---

# SN1 mechanism with stereochemistry; solvent

Engineering Chemistry · Chapter 10: Reaction Mechanism in Organic reactions

- Asked in: 2068 Bhadra, 4+1 marks
- Video: [SN1 reaction: stereochemistry](https://www.youtube.com/watch?v=jUEzjS0IFxs) — Khan Academy Organic Chemistry
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/sn1-stereochemistry-solvent
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-10

## Question

Discuss the unimolecular nucleophilic substitution reaction mechanism in alkylhalide showing the stereochemistry. What type of solvent favors this type of mechanism?

## Short answer

SN1 mechanism: slow ionization gives planar carbocation, then nucleophile attacks from either face, causing racemization with slight inversion preference. Rate = k[RX]. Polar protic solvents (water, alchols) favor SN1 by solvating and stabilizing the ions.

## Answer

#### Unimolecular nucleophilic substitution (SN1)
Example: hydrolysis of an optically active tertiary halide, R₁R₂R₃C–Br + OH⁻.
```text
Step 1 (slow):  R₃C—Br  →  R₃C⁺ + Br⁻           planar carbocation
Step 2 (fast):  R₃C⁺ + OH⁻  →  R₃C—OH
```
- Rate = k[RX], first order.
- Tertiary halides react because the carbocation is stabilised by hyperconjugation and the +I effect of alkyl groups.

#### Stereochemistry
In SN1 the carbocation formed is **planar** (sp²), so the nucleophile can attack from either face: from the face opposite to the leaving group gives **inversion**, from the same face gives **retention**. This gives a mixture (racemisation). Inversion slightly predominates because the departing leaving group stays near the front face as an ion pair and partly blocks it, so attack from the rear is easier.
```text
              Nu attacks from rear   → inversion  (slightly more)
 planar R₃C⁺ 
              Nu attacks from front  → retention
Overall: partial racemisation of an optically active halide
```

#### Solvent
**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.

## Check yourself

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

1. How does SN1 occur?
   A. Single concerted step
   B. Fast ionisation, slow attack
   C. Slow ionisation to a planar carbocation, then fast nucleophilic attack
   D. Attack first, then ionisation
   Answer: C. Two steps with the carbocation in between. (Key point: SN1 steps)

2. Why does SN1 at a chiral carbon give both inversion and retention?
   A. The nucleophile attacks only from the back in a concerted step
   B. The carbocation is planar so the nucleophile can attack either face
   C. The leaving group never leaves the carbon
   D. The carbocation is tetrahedral and holds its configuration
   Answer: B. The sp2 carbocation has two equivalent faces; rear attack gives inversion and front attack gives retention. (Key point: Planar carbocation in SN1)

3. Why does inversion slightly predominate over retention in SN1?
   A. The departing leaving group stays near the front face as an ion pair and partly blocks it
   B. The nucleophile is attracted to the back by the positive charge on Br
   C. Rear attack is the only pathway allowed for a carbocation
   D. The carbocation is pyramidal with the rear face exposed
   Answer: A. The ion pair partly shields the front face, so attack from the rear is a little easier. (Key point: Why inversion slightly predominates)

4. 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 aprotic, because it hydrogen-bonds to the carbocation
   D. Polar protic, because it stabilises the carbocation and halide ion by solvation
   Answer: D. Polar protic solvents (water, alcohols) stabilise ions through ion-dipole interaction and hydrogen bonding, speeding ionisation. (Key point: Solvent for SN1)

## Related in Engineering Chemistry

- [Solvent favouring SN1; meaning of elimination](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Asn1-solvent-elimination) — 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 reaction; inversion of configuration; SN1](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Asn-inversion) — Engineering Chemistry, chapter 10
- [Mechanism for primary alkyl halide in aqueous medium; solvent](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Asn2-primary-solvent) — Engineering Chemistry, chapter 10
- [3° alkyl halide with alcoholic KOH; solvent](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Atert-alc-koh-solvent) — Engineering Chemistry, chapter 10