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.
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.