Question

Reaction of HBr with $3$-bromocyclohexene (A) yields only trans-$1,2$-dibromo-cyclohexane. Reaction of $3$-methylcyclohexene (B) with HBr gives a mixture of cis and trans-$1$-bromo-$3$-methylcyclohexene and $1$-bromo-1-methyl cyclohexane. Which statement explains the reactivity differences between A and B?

• A. There is no difference in reactivity. In the case of A the $1,2$ and $1,3$-dibromo product can't be distinguised
• B. The bromonium ion formed in the case of A can be attacked only in the anti manner
• C. A has a chiral center and B has a plane of symmetry
• D. In the case of B, there is a hybride shift following protonation

Answer 1

The correct option is D In the case of B, there is a hybride shift following protonation

In case of A, formation of cyclic bromonium ion takes place which restricts the attack of $B{r}^{-}$ from same side but in the case B, the carbocation fromed gives $1$-bromo-$3$-methyl cyclohexane or hydride shift can occur as shown which converts carbocation into a more stable tertiary carbocation and then, $B{r}^{-}$ addition can take place.

Hence, option D is correct.