Radical chemistry · 7 min read

Radical chain reactions: initiation, propagation and termination

Radical reactions are driven by species with an unpaired electron. They proceed as chains: one radical makes another, which makes another, until the chain is stopped. Every radical mechanism has the same three phases.

The three phases

1. Initiation: a weak bond is broken by heat or light to make the first radicals. Peroxides (O–O) and azo compounds such as AIBN are typical initiators. See radical initiation.
2. Propagation: a radical reacts with a molecule to give a new radical and a product. The new radical continues the chain. Each turn of the chain consumes a reagent and regenerates the carrier. See radical propagation.
3. Termination: two radicals meet and combine, or one abstracts a hydrogen from the other to give an alkane and an alkene (disproportionation). The chain ends. See combination and disproportionation.

Fish-hook arrows

A single-barbed (half-headed) arrow represents the movement of one electron. In homolysis, two fish-hook arrows leave the breaking bond, one to each atom. In a radical attack on a π bond, one arrow shows the radical's electron pairing with one of the π electrons while the other π electron becomes the new radical. For the notation rules, see curved-arrow notation.

Stability and selectivity

• Radical stability: tertiary > secondary > primary > methyl. Allylic and benzylic radicals are especially stable because of resonance.
• Chlorination vs bromination: the hydrogen-abstraction step is fast and exothermic for chlorine, so its transition state comes early and resembles the reactants. It is not very selective. For bromine the step is endothermic, the transition state comes late and resembles the radical, so stability matters a great deal. Bromination of an alkane is highly selective for the most substituted C–H.

Important radical reactions

• Allylic and benzylic bromination: NBS keeps the Br₂ concentration very low so that substitution beats addition. See the Wohl–Ziegler bromination.
• Barton–McCombie deoxygenation: replaces an alcohol C–O by C–H using a tin hydride chain. See Barton-McCombie deoxygenation.
• Radical additions to alkenes: the Giese reaction, thiol–ene, and HBr addition with peroxides (anti-Markovnikov).
• Polymerisation: ATRP and RAFT control chain growth by reversibly capping the radical.
• Biology: vitamin B₁₂ chemistry and cytochrome P450 use radical intermediates.
• Photoredox catalysis: light and a photocatalyst make radicals gently. See Photoredox catalysis.

Why the chain is not endless

Radical concentrations are very low, so two radicals rarely meet. Chain length can reach thousands of cycles. Inhibitors, such as BHT or oxygen, trap radicals and shorten chains. That is why radical reactions are often run under nitrogen and why polymers are stored with a small amount of inhibitor.

Spotting a radical reaction

• Light (hν), heat with a peroxide or AIBN, or NBS in CCl₄.
• Non-polar solvent and no ionic reagents.
• Anti-Markovnikov products.
• Mixtures of products with little regiocontrol (for chlorination).

Practise in the interactive app   Browse all 345 reactions

More study guides

• How to learn reaction mechanisms (without memorising everything)A practical method for learning organic reaction mechanisms: understand electron flow, group reactions into a few patterns, and practise by drawing.
• Curved-arrow notation: rules and the mistakes students makeHow to read and draw curved arrows in organic mechanisms: where they start and end, full-headed versus fish-hook arrows, and the most common errors.
• SN1, SN2, E1 or E2? How to decide which mechanism winsA clear decision guide for substitution versus elimination: substrate, nucleophile or base strength, solvent and temperature, with examples.
• Nucleophiles, electrophiles and leaving groups: what makes a reaction goWhat decides how reactive a nucleophile, an electrophile or a leaving group is, and how solvent, charge, size and resonance change reactivity.