Kharasch addition

The Kharasch addition is an organic reaction and a metal-catalysed free radical addition of CXCl₃ compounds (X = Cl, Br, H) to alkenes.^[1] The reaction is used to append trichloromethyl or dichloromethyl groups to terminal alkenes. The method has attracted considerable interest,^[2] but it is of limited value because of narrow substrate scope and demanding conditions.^[3]

chemical reactions for the Kharasch addition reaction of chloroform to a terminal alkene.

The reaction mechanism involves free radicals of the general formula CXCl₂ (X = Cl, H). For the precursors carbon tetrachloride and chloroform, the requisite radicals can arise by abstraction of a halogen atom by a electropositive metal. The addition proceeds in an anti-Markovnikov fashion. Early work linked the addition to olefin polymerization. ^[4] This addition is a step in a protocol known as atom transfer radical polymerization.^[5]

An example of Kharasch addition is the synthesis of 1,1,3-trichloro-n-nonane from 1-octene and chloroform using an iron-based catalyst:^[6]

CH₃(CH₂)₅CH=CH₂ + HCCl₃ → CH₃(CH₂)₅CH(Cl)−CH₂CHCl₂

History

The reaction was discovered by Morris S. Kharasch in the 1940s.^[7][8][9]