Carbocation rearrangement in alkene addition reaction

This is a question from my textbook:

Determine a step-wise $S_N1$ mechanism for the overall reaction:

The solution shows the following carbocation rearrangement after Bromine leaves in the first step:

Why does the carbocation rearrange like this, going from a tertiary allylic carbocation to a secondary allylic carbocation? I thought tertiary carbocations are more stable and thus favored to be the major product? I considered Zaitsev's Rule in which the more substituted alkene is the major product, but I thought that only applies in elimination reactions.

Any clarification is appreciated!

1 Answer

The mechanism shown is not a carbocation rearrangement. Those two structures are resonance structures, which means that the chloride anion can attack at either of the carbons that bear a positive charge. Because there are two different alkene products that are possible, the more substituted alkene will be favored. Strictly speaking, you're right, Zaitsev's Rule doesn't apply here, because this isn't an elimination reaction. However, the reasoning is the same: the most substituted, most stable alkene is the favored product.

• A contributing factor might also be the steric effect, the middle carbon is secondary, whereas the 'right'-carbon is tertiary. – Eljee Dec 9 '14 at 14:23