Hydration of Alkenes
In general, rearrangements are side reactions in the additions of HX molecules to alkenes. Acid-catalyzed hydrations are no exception. Therefore, regioselectivity (Markovnikov rule) as well as the possibility of rearrangements into geometric isomers have to be considered. The main reason for these rearrangements is the tendency of molecules to occupy the lowest possible energy state. The reaction of 3-methyl-1-butene with HCl is an example of such a rearrangement. The formation of 2-chloro-2-methylbutane as the main product is the result of the migration of a hydride ion together with the bonding electron pair. The rearrangement is called a hydride shift and can be explained by the formation of the energetically-favored tertiary carbenium ion over the secondary carbenium ion. Analogous to the hydride shift, an alkyl group can also migrate the same way. This alkyl shift in the case of cyclic compounds results in a change of ring size.