Rearrangement of carbocations

1. hydride shift

The ion that is formed in the first step of an S_N1 reaction is called a carbocation: a group of atoms that contains a carbon atom bearing only six electrons (and therefore, a net positive charge). The carbon bearing the charge can be either primary, secondary or tertiary; hence there are primary, secondary and tertiary carbocations. Of course, these three types of carbocations differ in stability (although they are all very reactive), the primary being the least stable of the three, and the tertiary cation being the most stable.

The difference in stabilities of the various types of carbocations can give rise to rearrangement. In cases where a primary or secondary carbocation is formed, it might undergo a hydride shift or an alkyl shift prior to the attack of the nucleophile. The carbocation resulting from the rearrangement is a more stable one than the one prior to rearrangement: a primary carbocation will rearrange into a secondary or tertiary one; a secondary carbocation will rearrange into a tertiary carbocation.

On the right, the first part of the SN1 reaction of isobutanol is shown. After protonation of the OH-group, H2O leaves the molecule, leaving a primary carbocation. Rearrangement then occurs to result in a tertiary carbocation: a stabler cation. This rearrangement is known as a hydride shift: a migration of hydrogen with a pair of electrons. After the rearrangement, a nucleophile can attack from either side of the carbocation. This is not shown in the animation. Frame 1: protonation of isobutanol