Free-Radical Halogenation of Alkanes

A representative example — structures drawn live.

Overview: substituting a C–H

Alkanes are unreactive toward most reagents, but under heat or ultraviolet light they react with Cl₂ or Br₂ to swap a hydrogen for a halogen: R–H + X₂ → R–X + HX. The reaction runs by a free-radical chain mechanism, meaning reactive radical intermediates are consumed and regenerated over many cycles. There are three stages: initiation, propagation, and termination.

Initiation

Initiation creates the first radicals. Heat or light (hν) supplies enough energy to break the weak halogen–halogen bond homolytically — each atom leaves with one electron:

• X₂ → 2 X• (two halogen radicals)

This step is where the radical population is born. It happens relatively rarely compared with propagation, but it seeds the chain.

Propagation

Propagation is the heart of the chain — two steps that regenerate a radical so the cycle continues:

• Step 1: X• + R–H → R• + H–X. The halogen radical abstracts a hydrogen atom, creating a carbon radical.
• Step 2: R• + X₂ → R–X + X•. The carbon radical grabs a halogen from X₂, forming the product and releasing a new halogen radical.

Notice that a halogen radical consumed in step 1 is regenerated in step 2, so a single initiation event can drive many product-forming cycles. The hydrogen-abstraction step (step 1) is rate- and selectivity-determining — it decides which C–H reacts.

Termination

The chain ends whenever two radicals combine to form a stable bond, removing radicals from the mixture:

• X• + X• → X₂
• R• + X• → R–X
• R• + R• → R–R (a small amount of coupled by-product)

Because radicals are present in low concentration, termination is comparatively rare, which is exactly why the chain can propagate so many times before it stops.

Selectivity: why bromine beats chlorine

Not all C–H bonds react equally. Tertiary (3°) C–H bonds are weaker and give the most stable radicals, followed by 2°, then 1°. But Cl₂ and Br₂ differ dramatically in how much they care:

• Chlorination is fast but unselective. The Cl• abstraction step is exothermic and has an early, reactant-like transition state (Hammond postulate), so the radical's stability barely matters. Chlorine reacts at all positions roughly in proportion to how many H's are there, giving mixtures.
• Bromination is slow but highly selective. The Br• abstraction step is endothermic with a late, product-like transition state that strongly resembles the carbon radical. The energy difference between forming a 3° versus a 1° radical is therefore fully felt, so Br₂ reacts predominantly at the most substituted (most stable) position — 3° > 2° > 1°.

Practically, if a problem wants a single clean product at the most substituted carbon, choose Br₂. If it shows Cl₂, expect a mixture of constitutional isomers.

Allylic and benzylic positions

C–H bonds next to a double bond (allylic) or an aromatic ring (benzylic) are especially reactive because the radicals they form are stabilized by resonance. These bonds are weaker than ordinary alkane C–H bonds, so halogenation — for example with NBS as a low-concentration bromine source — occurs preferentially at allylic and benzylic sites. This ties directly into radical stability: the more stable the radical, the faster and more selectively that C–H reacts.