In the presence of
peroxides, HBr adds to a given
alkene in an anti-Markovnikov addition fashion. Regiochemistry follows from the reaction mechanism, which exhibits halogen attack on the least-hindered unsaturated carbon. The mechanism for this
chain reaction resembles
free radical halogenation, in which the peroxide promotes formation of the
bromine radical. However, this process is restricted to addition of HBr. Of the other
hydrogen halides (HF, HCl, and HI), only HCl reacts similarly, and the process is too slow for synthetic use. (With HF and HI, the energy released in the halogen-carbon addition does not suffice to cleave another hydrogen-halogen bond. Consequently the chain cannot propagate.) The resulting 1-bromoalkanes are versatile
alkylating agents. By reaction with
dimethyl amine, they are precursors to
fatty tertiary amines. By reaction with tertiary amines, long-chain alkyl bromides such as 1-bromododecane, give
quaternary ammonium salts, which are used as
phase transfer catalysts. With
Michael acceptors the addition is also anti-Markovnikov because now a nucleophilic X− reacts in a
nucleophilic conjugate addition for example in the reaction of HCl with
acrolein. ==Scope==