"Markownikow" selectivity was originally observed for the hydrohalogenation of olefins. It was found that for unsymmetrical olefins, when treated with H-X, the favoured product was the alkyl halide that was more substituted. We could represent this for the reaction of propylene with HX:
H_3C-CH=CH_2 + H-Xrarr H_3C-C^(+)H-CH_3 + X^-;
H_3C-C^(+)H-CH_3 + X^(-) rarr H_3C-C(X)H-CH_3 .
Chemists got more of the H_3C-C(X)H-CH_3 product than the alternative H_3C-CH_2-CH_2X 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^+ species, to the secondary carbon, which would give an intermediate of the form:
H_3C-CH_2-C^(+)H_2, which of course would then react with the halide to give a primary alkyl halide.
