91Ó°ÊÓ

Carbocation rearrangement involves the shift of carbocation from an unstable state to a stable state by various shifts within the molecule. This is by hydride shift or methyl shift.

(a) In the given reaction, the pi bond abstracts a proton from HCl to form a carbocation. The next step is a carbocation rearrangement to form a tertiary carbocation. In the last step, the chloride ion attacks the carbocation forming an addition product. The addition of HX happens in a Markovnikov fashion and the negative part is added to the most substituted carbon.

Mechanism of reaction (a)

(b)In this reaction, the pi bond abstracts a proton from HBr forming a secondary carbocation. The next step involves the attack of bromide ion on the cation forming an addition product. The addition of HBr happens in a Markovnikov fashion and the negative part adds to the most substituted carbon.

The carbocation rearrangement does not happen in this reaction as the adjacent carbon is secondary. The mechanism can be given as:

Mechanism of reaction (b)

(c) In the first step, the pi bond abstracts an acidic proton from the phosphoric acid-generating the most stable tertiary carbocation. In the next step, the iodide ion attack the carbocation forming an addition product. The mechanism can be given as:

Mechanism of reaction (c)