"Markownikow"Markownikow selectivity was originally observed for the hydrohalogenation of olefins. It was found that for unsymmetrical olefins, when treated with H-XH−X, the favoured product was the alkyl halide that was more substituted. We could represent this for the reaction of propylene with HXHX:
H_3C-CH=CH_2 + H-Xrarr H_3C-C^(+)H-CH_3 + X^-H3C−CH=CH2+H−X→H3C−C+H−CH3+X−;
H_3C-C^(+)H-CH_3 + X^(-) rarr H_3C-C(X)H-CH_3 H3C−C+H−CH3+X−→H3C−C(X)H−CH3.
Chemists got more of the H_3C-C(X)H-CH_3H3C−C(X)H−CH3 product than the alternative H_3C-CH_2-CH_2XH3C−CH2−CH2X for the reason that the secondary carbocation was stabilized with respect to the alternative primary carbocation, that would result from the addition of the electrophile, the H^+H+ species, to the secondary carbon, which would give an intermediate of the form:
H_3C-CH_2-C^(+)H_2H3C−CH2−C+H2, which of course would then react with the halide to give a primary alkyl halide.
