Question

Arrange the following compounds in the decreasing order of reactivity with $HBr$:

• A. $a>b>c$
• B. $b>a>c$
• C. $b>c>a$
• D. $a>c>b$

Answer 1

The correct option is B $b>a>c$

During the electrophilic addition of $HBr$ , formation of carbocation is the slowest step.
More the stability of carbocation, more os the reactivity of the alkene.
Carbocations formed are :


In (b) the carbocation is resonance stabilized, whereas there is no conjugation in (a) and (c).
Between (a) and (c),
(a) is more stabilized by 9$\alpha$-hydrogens whereas carbocation in (c) is destabilized by $-I$ effect of $O$.