---
title: "Free-Radical Substitution in Alkanes"
description: "Ultraviolet light splits a halogen molecule into radicals, and those radicals attack an alkane in a chain that keeps itself going. The mechanism is why the product is always a mixture."
canonical: https://lightmysky.com/learn/science/free-radical-substitution-in-alkanes-mt_QkreYC9e2d
source: https://lightmysky.com/learn/science/free-radical-substitution-in-alkanes-mt_QkreYC9e2d.md
retrieved: 2026-09-12
---

> **Agent view.** This is the Markdown twin of the page, for tools and assistants.
> When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md
> API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md
> Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt
> Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json
> Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`.

# Free-Radical Substitution in Alkanes

Ultraviolet light splits a halogen molecule into radicals, and those radicals attack an alkane in a chain that keeps itself going. The mechanism is why the product is always a mixture.

Subject: Science · Area: Chemistry · Ages 16 to 17
Page: https://lightmysky.com/learn/science/free-radical-substitution-in-alkanes-mt_QkreYC9e2d

## Ready when they can

- Writes initiation, propagation and termination steps with the radical dots in the right places
- Explains how one photon can lead to many product molecules
- Explains why further substitution, and so a mixture, cannot be avoided
- Connects the same radical chemistry to chlorine radicals breaking down ozone

## Lesson: How a flash of light starts a chain

Alkanes are usually unreactive, but a burst of ultraviolet light changes that. The light splits a halogen molecule into two radicals, and each radical carries an unpaired electron looking for a partner. That split is called initiation, and it takes only one flash to begin.

**Example.** For methane with chlorine, the two propagation steps run as follows. A chlorine radical grabs a hydrogen from methane, giving hydrogen chloride plus a methyl radical. The methyl radical then attacks a chlorine molecule, giving chloromethane plus a fresh chlorine radical. Write every radical dot in the right place, because the dots show where the unpaired electron sits.

Each propagation turn regenerates a chlorine radical, so the chain repeats itself many times from a single photon. One flash can therefore lead to hundreds of product molecules before anything stops. The chain stops in termination, when two radicals meet and pair up: two methyl radicals join to ethane, or two chlorine radicals reform chlorine.

**Tip.** You never get one clean product from this reaction. Chloromethane can meet another radical and substitute again, so dichloromethane, trichloromethane and tetrachloromethane build up alongside unreacted methane. The same radical chain chemistry matters in the sky: chlorine radicals from older refrigerants attack ozone the same way, with each radical destroying many ozone molecules in a chain.

**Recap.** Light splits the halogen, the chain feeds itself, and the mixture is unavoidable.

## Practice

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

## Needs first

- [Isomerism: Same Formula, Different Molecule](https://lightmysky.com/learn/science/isomerism-same-formula-different-molecule-mt_DvD0coeKZe)
- [The Electromagnetic Spectrum: Uses and Dangers](https://lightmysky.com/learn/science/the-electromagnetic-spectrum-uses-and-dangers-mt_P_NXNSo1zQ)

## Opens up

- [Electrophilic Addition to Alkenes](https://lightmysky.com/learn/science/electrophilic-addition-to-alkenes-mt_1RB5nj8zR8)
- [Chain Reactions, Branching and Explosion Limits](https://lightmysky.com/learn/science/chain-reactions-branching-and-explosion-limits-mt_8SDv_5iRMu)
- [Radical Selectivity, Stability and Allylic Positions](https://lightmysky.com/learn/science/radical-selectivity-stability-and-allylic-positions-mt_tgIlRLvc9x)