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

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Answer 1 · 2015-05-25T19:58:22

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.

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

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