Free Radical Reactions: Chain Mechanisms and Selectivity

The chain

• Initiation — a bond breaks homolytically, usually under light or heat, generating radicals.
• Propagation — a radical reacts to give a product and a new radical, so the chain continues.
• Termination — two radicals combine, removing them from the chain.

Because propagation regenerates the radical, one initiation event can produce thousands of product molecules. That is why quantum yields for photochemical chain reactions can be enormous, and it is also why a trace of inhibitor stops the reaction entirely by intercepting the chain carriers.

Radical stability

The order matches carbocations, and largely for the same reason — hyperconjugation from adjacent C–H bonds delocalises the unpaired electron. Allylic and benzylic radicals are more stable still, because resonance spreads the unpaired electron over a π system.

Unlike carbocations, however, radicals are neutral, so they are far less sensitive to solvent and do not rearrange readily. That difference is worth stating when comparing radical and ionic mechanisms.

Selectivity: chlorination versus bromination

A question giving a substrate with several types of hydrogen and asking for the major product is answered by this table: with bromine, name the tertiary product; with chlorine, note that a mixture forms and explain why.

Allylic bromination

Reagents that maintain a very low concentration of bromine favour allylic substitution over addition to the double bond. The reason is concentration-dependent: addition requires a reasonable bromine concentration, while radical abstraction does not.

The allylic radical formed is resonance stabilised, and because the unpaired electron is delocalised over two positions, bromine can attach at either — giving two products where the allylic system is unsymmetrical. Predicting both products is the standard question, and giving only one is an incomplete answer.

Radical addition to alkenes

Hydrogen bromide adds anti-Markovnikov in the presence of peroxides because the mechanism becomes radical. A bromine radical adds first, and it adds so as to give the more stable carbon radical — which places bromine on the less substituted carbon, the opposite of the ionic outcome.

The restriction to HBr is important and frequently tested: for HCl and HI, one of the two propagation steps is energetically unfavourable, so the chain does not sustain and the ionic mechanism continues to dominate.

Frequently asked questions

Why do radicals not rearrange like carbocations?

Because the driving force is much smaller. Carbocation rearrangement relieves charge instability, while a radical is neutral and gains far less from rearranging.

Why does a trace of inhibitor stop the whole reaction?

Because it intercepts chain carriers. Since each carrier would have propagated many cycles, removing a small number of them prevents a large amount of product.

What is the Hammond postulate saying here?

That the transition state resembles whichever species it is closer to in energy. For an exothermic step that is the reactants, giving little selectivity; for an endothermic step it is the products, so product stability controls the outcome.

Why do radical reactions need light or heat?

To supply the energy for homolysis in the initiation step. Once started, propagation continues without further input, which is why sustained illumination is often unnecessary.