---
title: "SN1 and SN2: Kinetics, Stereochemistry and What Decides the Route"
description: "Two substitution mechanisms differ in whether the leaving group departs before or during the attack, and every observable follows from that. Substrate class, nucleophile, leaving group and solvent eac"
canonical: https://lightmysky.com/learn/science/sn1-and-sn2-kinetics-stereochemistry-and-what-decides-the-route-mt_BiCjJw2eJF
source: https://lightmysky.com/learn/science/sn1-and-sn2-kinetics-stereochemistry-and-what-decides-the-route-mt_BiCjJw2eJF.md
retrieved: 2026-09-12
---

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# SN1 and SN2: Kinetics, Stereochemistry and What Decides the Route

Two substitution mechanisms differ in whether the leaving group departs before or during the attack, and every observable follows from that. Substrate class, nucleophile, leaving group and solvent each push the choice one way.

Subject: Science · Area: Chemistry · Ages 19 to 20
Page: https://lightmysky.com/learn/science/sn1-and-sn2-kinetics-stereochemistry-and-what-decides-the-route-mt_BiCjJw2eJF

## Ready when they can

- States the rate law each mechanism predicts and matches it to measured kinetics
- Predicts the stereochemical outcome of each and explains it from the geometry of the step
- Ranks primary, secondary and tertiary substrates for each route with the reason for the ranking
- Explains how a polar protic solvent favours one route and a polar aprotic solvent the other

## Lesson: Two ways to swap one group

Two substitution routes differ in timing. In the SN1 route the slow step is the substrate ionizing on its own, with the nucleophile arriving only afterwards. Read the rate law from experiment, never from the balanced equation. Doubling the nucleophile leaves an SN1 rate unchanged, while doubling the substrate doubles it. When doubling either partner doubles the rate, both collide in the slow step and the route is SN2.

Stereochemistry tells the routes apart. SN1 passes through a flat carbocation that either face can attack, so ideally a chiral centre gives a racemic mixture. SN2 fires in one concerted motion where the nucleophile must attack from the back side, opposite the leaving group. That gives inversion of configuration.

The substrate class decides which routes are even possible. Tertiary halides ionize readily but block backside attack, so they go SN1 and never SN2. Methyl and primary halides cannot form stable cations but lie open to attack from behind, so they go SN2. Secondary substrates sit on the border and let solvent and nucleophile decide.

**Example.** Take a secondary halide with a strong nucleophile in acetone. Acetone is polar aprotic, so it leaves the nucleophile bare and aggressive. The route is SN2 with inversion. Move the same pair to aqueous ethanol, which is polar protic and cradles ions. Now ionization wins and the route turns SN1 with ideal racemisation.

**Recap.** One slow ionization means SN1 with racemisation in the ideal planar ion model, one concerted collision means SN2 with inversion, and structure plus solvent pick the winner.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Nucleophilic Substitution in Haloalkanes](https://lightmysky.com/learn/science/nucleophilic-substitution-in-haloalkanes-mt_QG-nKlnVvW)
- [Stereochemistry as Mechanistic Evidence](https://lightmysky.com/learn/science/stereochemistry-as-mechanistic-evidence-mt_YtagP8C0Wi)

## Opens up

- [Carbocations: Stability, Rearrangement and Neighbouring Groups](https://lightmysky.com/learn/science/carbocations-stability-rearrangement-and-neighbouring-groups-mt_0r1d2u30PE)