How can I explain the mechanism for hydrohalogenation with a hydride-shift (carbocation rearrangement)?
A carbocation intermediate that can become more stable by a hydride shift will do so.
We must always consider the possibility of rearrangements when a mechanism involves a carbocation intermediate.
Consider the addition of hydrogen chloride to 3-methylbut-1-ene.
In Step 1, the
This is a 2° carbocation.
Since the new carbocation is more stable, the rearrangement will occur.
Then in Step 3 the