---
title: "Free Radical Substitution | Organic Chemistry"
description: "Free radical substitution is a chain reaction where radicals replace a hydrogen in an alkane, usually to make alkyl halides in Organic Chemistry."
canonical: "https://fiveable.me/organic-chem/key-terms/free-radical-substitution"
type: "key-term"
subject: "Organic Chemistry"
unit: "Unit 10"
---

# Free Radical Substitution | Organic Chemistry

## Definition

Free radical substitution is a chain reaction in Organic Chemistry where a radical replaces a hydrogen atom in an alkane, usually forming an alkyl halide. It is the mechanism behind radical halogenation.

## What It Is

Free radical substitution is the Organic Chemistry reaction pattern where a radical removes a hydrogen from an alkane and a new group, usually a halogen, ends up in its place. You will usually see it in radical halogenation, where an alkane is converted into an alkyl halide using chlorine or bromine plus heat or light.

The reaction works as a chain, not a one-step swap. First, the halogen molecule breaks by homolytic fission, so each atom keeps one electron and forms two halogen radicals. That step needs energy from UV light or heat because halogen-halogen bonds do not split on their own very easily.

Once radicals exist, the propagation steps keep the chain going. A halogen radical abstracts a hydrogen atom from the alkane, which makes a hydrogen halide and an alkyl radical. Then the alkyl radical reacts with another halogen molecule to form the alkyl halide product and regenerate a new halogen radical. That regeneration is why one initiation event can lead to many product-forming cycles.

The word substitution matters here because the final product has one C-H bond replaced by a C-X bond. The molecule is not being added to or eliminated from in the usual sense. You are changing an alkane into something more reactive and useful, which is a big reason this reaction shows up when you need a starting material for later synthesis.

A big catch is that alkanes have many similar C-H bonds, so free radical substitution often gives mixtures. The exact product depends on which hydrogen is abstracted and how stable the radical intermediate is. More substituted radicals are usually more stable, so the reaction is often more selective at tertiary positions than at primary ones.

## Why It Matters

Free radical substitution is one of the few ways to turn an unreactive alkane into a useful alkyl halide, so it shows up any time Organic Chemistry moves from simple hydrocarbons to functional group chemistry. Without this step, many alkane-based molecules would be hard to modify because C-H bonds are relatively nonpolar and stubborn.

This term also ties together a lot of mechanism vocabulary you keep seeing. If you can track initiation, propagation, and termination, you can explain why the reaction keeps going, why light or heat is needed, and why the product mix can be messy. That same chain-thinking helps in other radical reactions later in the course.

It also connects to selectivity. When a question asks why one product is major and another is minor, you often have to think about radical stability, halogen reactivity, and which hydrogen atom is being abstracted. That is the difference between naming the reaction and actually predicting what it does.

## Connections

### Radical Halogenation

Free radical substitution is the broader reaction pattern, and radical halogenation is the most common example in Organic Chemistry. In that reaction, a halogen such as chlorine or bromine replaces a hydrogen on an alkane. If a problem mentions UV light, heat, or an alkane turning into an alkyl halide, you are usually looking at this specific type of substitution.

### Homolytic Fission

This is the bond-breaking step that starts the chain. The halogen-halogen bond splits evenly so each atom gets one electron, forming radicals. That is different from ionic bond cleavage, where one atom takes both electrons. If you miss homolytic fission, the rest of the radical mechanism does not make sense.

### [Hydrogen Abstraction](/organic-chem/key-terms/hydrogen-abstraction)

During propagation, a radical pulls a hydrogen atom off the alkane. That step makes a new radical on the carbon skeleton, which is what keeps the chain reaction alive. The stability of the radical formed here strongly affects which product is favored, so this step often controls the mixture you end up with.

### [Halogen Reactivity](/organic-chem/key-terms/halogen-reactivity)

Not all halogens behave the same in radical substitution. Chlorine and bromine differ in how reactive and selective they are, which changes both the speed of the reaction and the product distribution. When you compare halogenations, this term helps explain why one halogen gives a broader mixture while another gives more predictable products.

## On the AP Exam

A quiz or problem set will usually give you an alkane, halogen, and conditions like UV light or heat, then ask for the mechanism or products. Your job is to identify the initiation step, trace the radical chain, and predict where substitution is most likely to happen. If there are multiple possible C-H bonds, you may need to justify the major product using radical stability and the likelihood of hydrogen abstraction. In mechanism questions, label the radical intermediates clearly and remember that the product is an alkyl halide, not an addition product. If the prompt asks why a mixture forms, point to the many different hydrogens on the alkane and the chain nature of the reaction.

## Free Radical Substitution vs Nucleophilic Substitution

Free radical substitution and nucleophilic substitution both replace one part of a molecule with another, but the moving pieces are different. Radical substitution uses radicals and usually starts from alkanes under light or heat, while nucleophilic substitution uses nucleophiles and usually happens at a carbon already attached to a leaving group. If you see a radical chain mechanism, it is not nucleophilic substitution.

## Key Takeaways

- Free radical substitution is the mechanism that replaces a hydrogen in an alkane with another atom or group, most often a halogen.
- It happens as a chain reaction with initiation, propagation, and termination steps, not as a single bond swap.
- Light or heat is needed because the halogen molecule must first undergo homolytic fission to create radicals.
- The reaction often makes a mixture of products because alkanes have several different C-H bonds that can be substituted.
- Radical stability and halogen reactivity help explain which product is formed most often.

## FAQs

### What is free radical substitution in Organic Chemistry?

It is a chain reaction where a radical replaces a hydrogen atom in an alkane, usually producing an alkyl halide. The classic example is halogenation with chlorine or bromine under UV light or heat. The reaction moves through radical intermediates, so the mechanism matters as much as the final product.

### How does free radical substitution start?

It starts with initiation, when the halogen molecule splits by homolytic fission into two radicals. Light or heat provides the energy needed for that split. Once the first radicals form, they trigger the propagation steps that keep the chain going.

### Why does free radical substitution give a mixture of products?

Alkanes usually have several different C-H bonds, so a radical can abstract hydrogen from more than one position. That means more than one alkyl radical can form, leading to more than one alkyl halide. The major product is often the one formed through the more stable radical intermediate.

### Is free radical substitution the same as nucleophilic substitution?

No. Free radical substitution uses radicals and usually happens with alkanes, heat, and halogens. Nucleophilic substitution uses a nucleophile attacking a carbon with a leaving group, which is a different mechanism and usually a different class of starting material.

## Related Study Guides

- [10.2 Preparing Alkyl Halides from Alkanes: Radical Halogenation](/organic-chem/unit-10/preparing-alkyl-halides-from-alkanes-radical-halogenation/study-guide/VxtJ0WhyprjQhQBb)

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