Question

The number of structural and configurational isomers of a bromo compound, $C_{5}H_{9}Br$, formed by the addition of $HBr$ to $2-$ pentyne respectively are

• A. $1$ and $2$
• B. $2$ and $4$
• C. $4$ and $2$
• D. $2$ and $1$

Answer 1

Answer: (B)

Addition of $HBr$ to $2$-pentyne gives two structural isomers $(I)$ and $(II)$.
$CH_{3} -C \equiv C-CH_{2}CH_{3} \xrightarrow{ HBr }\underset{\text{(I)}}{{CH_{3}(Br)=CHCH_{2}CH_{3}}} \underset{\text{(II)}}{{+CH_{3}CH=C(Br)CH_{2}CH_{3}}}$
Each one of these will exist as a pair of geometrical isomers as shown below:


Thus, there are two structural and four configurational isomers.