Arrange the following compounds in the decreasing order of reactivity with HBr:
A
a>b>c
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B
b>a>c
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C
b>c>a
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D
a>c>b
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Solution
The correct option is Bb>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 9α-hydrogens whereas carbocation in (c) is destabilized by −I effect of O.