Question

Which of the double bonds in zingiberene, the compound responsible for the aroma of ginger, is most reactive in an electrophilic addition reaction with HBr?

Short answer

The indicated carbon will form a most stable carbocation.

Step-by-step solution

How reaction proceed

First HBr breaks into H⁺ and Br^-where it will do markoniv addition.

Both will break a double bond and on one carbon H will attach and on second carbon Br will attach.

So the indicated carbon with form most stable carbocation ( a tertiary allylic carbocation) so this bond will break and H and Br will attach on it.

Final answer

The indicated double bond is the most reactive in an electrophilic addition reaction with HBr because addition of an electrophile to this double bond forms are more stable carbo cation (a tertiary allylic cation).