Free-radical addition

In organic chemistry, free-radical addition is an addition reaction which involves free radicals. These reactions can happen due to the free radicals having an unpaired electron in their valence shell, making them highly reactive. Radical additions are known for a variety of unsaturated substrates, both olefinic or aromatic and with or without heteroatoms.

Free-radical reactions depend on one or more relatively weak bonds in a reagent. Under reaction conditions (typically heat or light), some weak bonds homolyse into radicals, which then induce further decomposition in their compatriots before recombination. Different mechanisms typically apply to reagents without such a weak bond.

The basic steps in any free-radical process (the radical chain mechanism) divide into:

In a free-radical addition, there are two chain propagation steps. In one, the adding radical attaches to a multiply-bonded precursor to give a radical with lesser bond order. In the other, the newly-formed radical product abstracts another substituent from the adding reagent to regenerate the adding radical.

In general, the adding radical attacks the alkene at the most sterically accessible (typically, least substituted) carbon; the radical then stabilizes on the more substituted carbon. The result is typically anti-Markovnikov addition, a phenomenon Morris Kharasch called the "peroxide effect". Reaction is slower with alkynes than alkenes.

In the paradigmatic example, hydrogen bromide radicalyzes to monatomic bromine. These bromine atoms add to an alkene at the most accessible site, to give a bromoalkyl radical, with the radical on the more substituted carbon. That radical then abstracts a hydrogen atom from another HBr molecule to regenerate the monatomic bromine and continue the reaction.

Radical addition of hydrogen bromide is a valuable synthetic technique for anti-Markovnikov carbon substitution,^{[citation needed]} but free-radical addition does not occur with the other hydrohalic acids. Radical formation from HF, HCl, or HI is extremely endothermic and chemically disfavored. Hydrogen bromide is incredibly selective as a reagent, and does not produce detectable quantities of polymeric byproducts.

The behavior of hydrogen bromide generalizes in two separate directions. Halogenated compounds with a relatively stable radical can dissociate from the halogen. Thus, for example, sulfonyl, sulfenyl, and other sulfur halides can add radically to give respectively β‑halo sulfones, sulfoxides, or sulfides.