Question

Which of the following alkenes will generate the most stable carbocation when it reacts with a proton?

Answer 1

When a proton is abstracted from a strong acid by a nucleophilic `pi` bond, the most reactive `pi` bond will do so.

How do we know which is the most reactive?

Substitution!

Why? Hyperconjugation!

Hence, `("a").` is the answer because it is tetrasubstituted.

But, there is a diene!

Dienes will generally be more reactive than alkenes, barring locked s-trans dienes.

Hence, the general (descending) order of reactivity regarding proton abstraction is,

`"d" > "a" > "b" > "c"`