Free-radical addition

Free-radical addition is an addition reaction in organic chemistry that involves free radicals, species with an unpaired electron in their valence shell, making them highly reactive. These reactions occur with a variety of unsaturated substrates, including olefinic or aromatic compounds, with or without heteroatoms. The process depends on one or more relatively weak bonds in a reagent; under conditions such as heat or light, these bonds homolyse into radicals, which then induce further decomposition before recombination.

Mechanism and regiochemistry

The mechanism follows a radical chain process divided into initiation, where a radical is created from a non-radical precursor; propagation, where a radical reacts with a non-radical to produce a new radical; and termination, where two radicals combine to form a non-radical species. In free-radical addition specifically, two propagation steps occur: the adding radical attaches to a multiply-bonded precursor, yielding a radical with reduced bond order, and then this new radical abstracts a substituent from the adding reagent to regenerate the original radical.

The adding radical typically attacks the most sterically accessible, or least substituted, carbon of an alkene, with the radical stabilizing on the more substituted carbon, leading to anti-Markovnikov addition—a phenomenon termed the "peroxide effect" by Morris Kharasch. Reaction with alkynes is slower than with alkenes. In a paradigmatic example, hydrogen bromide radicalizes to monatomic bromine, which adds to an alkene at the most accessible site to form a bromoalkyl radical; this radical then abstracts a hydrogen from another HBr molecule, regenerating the bromine atom.

Compounds that add radically

This anti-Markovnikov addition is synthetically valuable for hydrogen bromide, but the other hydrohalic acids do not undergo radical addition due to highly endothermic radical formation. The behavior generalizes to halogenated compounds with stable radicals, such as sulfur halides, which add to yield β-halo sulfones, sulfoxides, or sulfides, and to unsubstituted compounds like thiols, which can add across double bonds but risk catalyzing polymerization.

For silicon, germanium, or phosphorus, energetics are unfavorable unless a pendant hydrogen is present. Nitrogen oxides add to give mixtures, such as vicinal dinitro compounds and nitro-nitrite esters.