Question

How can I explain the mechanism for hydrohalogenation with a hydride-shift (carbocation rearrangement)?

Answer 1

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.

`"CH"_2"=CH-CH(CH"_3)_2 + "HCl" → "?"`

In Step 1, the `"H"^+` adds to the alkene according to the Markovnikov Rule.

`"H-Cl " + "CH"_2"=CHCH(CH"_3")"_2 → "CH"_3"-"stackrel(+)("C")"H-CH(CH"_3")"_2 + "Cl"^-`

This is a 2° carbocation.

If the `"H"` migrates in Step 2 from C-3 to C-2, we will get a 3° carbocation.

`"CH"_3"-"stackrel(+)("C")"H-CH(CH"_3")"_2 → "CH"_3"-CH"_2"-"stackrel(+)("C")"(CH"_3")"_2`

Since the new carbocation is more stable, the rearrangement will occur.

Then in Step 3 the `"Cl"^-` adds to the carbocation to form the rearranged product.

`"CH"_3"-CH"_2"-"stackrel(+)("C")"(CH"_3")"_2 → "CH"_3"-CH"_2"-C(CH"_3")"_2"Cl"`