# SN1 reaction

SN1 is substitution, nucleophilic, unimolecular. The leaving group first departs on its own, forming a carbocation. The nucleophile then attacks. Rate = k[substrate] only. Tertiary substrates react fastest, in polar solvents, and the product is mostly racemic.

Chemistry (university) · Part 10: Organic reactions and mechanisms · Card 3 of 10 · Last updated 2026-10-19

## Analogy

A two-act play: first the actor leaves the stage (slow), then someone new walks on (fast).

## Example

**(CH₃)₃C–Br in water**

Step 1 (slow): loses Br⁻, forms (CH₃)₃C⁺
Step 2 (fast): water attacks cation
Rate depends only on [(CH₃)₃CBr]
Product: (CH₃)₃C–OH

## In practice

Tertiary alcohols turn into tertiary alkyl chlorides in concentrated HCl through an SN1 path.

## Practice

### Flashcards

- **What is the rate law for SN1?** Rate = k[substrate]. The nucleophile does not appear.

### Interview questions

**Compare SN1 and SN2 in terms of rate law, substrate, stereochemistry and solvent.**

- SN2: second order, one step, inversion, primary substrates
- SN1: first order, two steps, racemisation, tertiary substrates
- SN2 favours strong nucleophiles, polar aprotic
- SN1 favours polar protic solvents

## Questions people ask

### Why does SN1 give a racemic mixture?

The carbocation intermediate is flat. The nucleophile can attack from either face with equal chance, giving both mirror-image products in about equal amounts. In practice, a little more inversion product forms, because the leaving group briefly shields one face, so the mixture is not perfectly 50:50.

### How do I choose between SN1 and SN2?

Look at the substrate first. Methyl and primary favour SN2, and tertiary favours SN1. Then look at the nucleophile and solvent: strong nucleophiles with polar aprotic solvents favour SN2, and weak nucleophiles in polar protic solvents favour SN1. Secondary halides can go either way.

## Related concepts

- [SN2 reaction](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-10/chemistry-university-part-10-concept-2)
- [Carbocation stability](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-10/chemistry-university-part-10-concept-4)

Web page: https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-10/chemistry-university-part-10-concept-3
Study this card in the app: https://studyvisually.com/app/learn?category=chemistry-university&card=chemistry-university-part-10-concept-3