#"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.
