Free-radical addition

These reactions can happen due to the free radicals having an unpaired electron in their valence shell, making them highly reactive.

Under reaction conditions (typically heat or light), some weak bonds homolyse into radicals, which then induce further decomposition in their compatriots before recombination.

[3]: 188, 751  The result is typically anti-Markovnikov addition, a phenomenon Morris Kharasch called the "peroxide effect".

That radical then abstracts a hydrogen atom from another HBr molecule to regenerate the monatomic bromine and continue the reaction.

Thus, for example, sulfonyl, sulfenyl, and other sulfur halides can add radically to give respectively β‑halo sulfones, sulfoxides, or sulfides.

[5]: 211 In the case of silicon, germanium, or phosphorus, the energetics are unfavorable unless the heavy atom bears a pendant hydrogen.

Dinitrogen tetroxide adds to give a mixture: a vicinal dinitro compound, but also a nitro substituent adjacent to a nitrite ester.

[5]: 225 Although aromatic resonance stabilizes aryl radicals, bonds between arenes and their substituents are (in)famously strong.