One of the best pieces of evidence for a carbocation mechanism was discovered during the 1930s by F. C. Whitmore, who found that structural rearrangements often occur during the reaction of HX with an alkene. For example, reaction of HCl with 3-methyl-1-butene yields a substantial amount of 2-chloro-2-methylbutane in addition to the “expected” product, 2-chloro-3-methylbutane: