To determine the decreasing order of reactivity of the given alkenes towards hydrogen bromide
(HBr) addition, we need to consider the stability of the carbocation intermediate formed during the reaction. The more stable the carbocation, the more reactive the alkene is toward HBr addition.
The stability of carbocations follows the order: tertiary > secondary > primary > methyl. Additionally, resonance stabilization and hyperconjugation can further affect the stability.
Let's analyze each alkene:
Alkene I: This alkene forms a secondary carbocation intermediate after HBr addition.
Alkene II: This alkene forms a primary carbocation intermediate after HBr addition.
Alkene III: This alkene forms a tertiary carbocation intermediate after HBr addition, which is the most stable among the options.
Alkene IV: This alkene can form a secondary carbocation that benefits from resonance stabilization with the adjacent benzene ring, thus enhancing its stability.
Based on the stability of the carbocation intermediates:
Alkene Ill forms the most stable tertiary carbocation.
Alkene IV forms a secondary carbocation stabilized by resonance.
Alkene I forms a secondary carbocation without additional stabilization.
Alkene Il forms a less stable primary carbocation.
Thus, the decreasing order of reactivity towards HBr addition is:
III
> IV
> I
> II
Therefore, the correct option is:
Option D: III > I > II > IV