What Are Carbocation Rearrangements

Rearrangement A reaction in which a carbon or hydrogen atom has shifted its connectivity to another atom within the molecule. 

STEP 1 Add a proton. Proton transfer from the HCl (an electrophile) to the alkene (a nucleophile) gives a 2° carbocation intermediate. 

STEP 3 Reaction of a nucleophile and an electrophile to form a new covalent bond. Reaction of the 3° carbocation intermediate (an electrophile and a Lewis acid) with chloride ion (a nucleophile and a Lewis base) gives the rearranged product. 

The driving force for this rearrangement is the fact that the less stable 2° carbocation is converted to a more stable 3° carbocation. From the study of this and other carbocation rearrangements, we find that 2° carbocations rearrange to 3° carbocations. 1° Carbocations are never observed for reactions taking place in solution and should not be proposed as reaction intermediates. 

Rearrangements also occur in the acid-catalyzed hydration of alkenes, especially when a carbocation formed in the first step can rearrange to a more stable carbocation. For example, the acid-catalyzed hydration of 3-methyl-l-butene gives 2-methyl-2-butanol. In this example, the group that migrates is a hydrogen with its bonding pair of electrons, in effect, a hydride ion H . 

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