Free radical halogenation is a reaction in which an alkane reacts with a halogen, usually chlorine or bromine, to replace a C-H bond with a C-X bond. It matters because alkanes are usually unreactive, yet light or heat can start a chain reaction that converts them into more useful haloalkanes. The reaction is a classic example of how radicals form, react, and disappear through a mechanism with distinct stages.
It also shows why different hydrogen atoms in the same molecule can give different products.
Understanding Chemistry: Free Radical Halogenation
A radical is a particle with one unpaired electron. This makes it highly reactive because the unpaired electron can form a new bond. When chlorine or bromine molecules absorb suitable light, the bond between the two halogen atoms breaks evenly.
Each atom takes one electron from the old bond. This equal splitting is called homolytic bond breaking. It differs from ionic bond breaking, where one atom takes both electrons.
The light source is important because breaking a bond needs energy. Ultraviolet light is often used in laboratory examples, though strong heating can provide the needed energy too.
The reaction continues as a chain because a radical produced near the end of one step starts another cycle. One halogen radical removes a hydrogen atom from an alkane. This creates hydrogen halide and leaves behind a carbon radical.
That carbon radical then reacts with another halogen molecule. A carbon to halogen bond forms, while a fresh halogen radical is released. Many product molecules can form after only a small number of radicals are created at the start.
The chain stops when two radicals collide and form a normal bond. Radical collisions are less common because radicals are present at very low concentrations.
Not every carbon to hydrogen bond is equally easy to break. A carbon radical becomes more stable when nearby carbon groups help spread out the effect of its unpaired electron. This is why hydrogens attached to tertiary carbons are usually removed more readily than hydrogens on secondary, primary, or methyl carbons.
Students should count the different kinds of hydrogen in a structure before predicting products. They should then consider how many of each kind are present.
A molecule may have a more favourable radical site but far more hydrogens at another site. Both radical stability and the number of available hydrogens affect the product mixture.
Chlorination and bromination behave differently in a useful way. Chlorine radicals react quickly, so they often remove hydrogen from several possible positions. Chlorination can give a messy mixture when an alkane has different hydrogen environments.
Bromine radicals are less reactive in the hydrogen removal step. They more strongly favour formation of the more stable carbon radical, so bromination is usually more selective. This difference comes from energy changes during the reaction steps.
In school problems, this helps explain why bromination often gives one major product more clearly than chlorination. Real reactions need controlled light, temperature, and halogen amounts because further substitution can occur.
A product that still contains carbon to hydrogen bonds may react again, producing compounds with two or more halogen atoms. Hydrogen halides formed during the reaction are acidic and harmful, while halogens are toxic, so these reactions require proper ventilation and protective equipment.
Key Facts
- Overall reaction: R-H + X2 -> R-X + H-X, where X is usually Cl or Br.
- Initiation: X2 + hv or heat -> 2 X·.
- Propagation step 1: X· + R-H -> H-X + R·.
- Propagation step 2: R· + X2 -> R-X + X·.
- Termination examples: X· + X· -> X2, R· + X· -> R-X, R· + R· -> R-R.
- Radical stability trend: tertiary radical > secondary radical > primary radical > methyl radical.
Vocabulary
- Free radical
- A free radical is a highly reactive species with an unpaired electron.
- Initiation
- Initiation is the step that creates radicals, usually by breaking a halogen-halogen bond with light or heat.
- Propagation
- Propagation is a repeating set of steps in which one radical reacts to form a product and another radical.
- Termination
- Termination is a step in which two radicals combine, removing radicals from the reaction mixture.
- Selectivity
- Selectivity is the preference for reaction at one type of hydrogen or carbon site over another.
Common Mistakes to Avoid
- Forgetting light or heat in the initiation step is wrong because the halogen-halogen bond usually needs energy to split evenly into radicals.
- Drawing ionic arrows instead of single-headed radical arrows is wrong because radical mechanisms move one electron at a time, not electron pairs.
- Assuming all C-H bonds react equally is wrong because tertiary, secondary, primary, and methyl hydrogens form radicals with different stabilities.
- Treating termination as the main product-forming pathway is wrong because termination stops the chain reaction, while propagation usually produces most of the haloalkane.
Practice Questions
- 1 Write the initiation, two propagation steps, and one termination step for chlorination of methane with Cl2 under light.
- 2 Propane has 6 primary hydrogens and 2 secondary hydrogens. If bromination has relative reactivity 1 for primary H and 82 for secondary H, estimate the percent of 1-bromopropane and 2-bromopropane formed.
- 3 Explain why bromination of an alkane is usually more selective than chlorination, using radical stability and the energy of the hydrogen abstraction step.