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

Hint: Isomers are the molecules which have the same molecular formula, but have a different arrangement of the atoms. They have the same empirical formula but they do not necessarily share similar properties.

Complete step by step solution:

Hydrogen bromide and 2-pentyne react together to form 3-bromopent-2-yne and 2-bromopent-2-yne as a final product. The reaction is given as:
$2C{{H}_{3}}-C\equiv C-{{C}_{2}}{{H}_{5}}+2HBr\to C{{H}_{3}}-(Br)C=CH-{{C}_{2}}{{H}_{5}}+C{{H}_{3}}-HC=C(Br)-{{C}_{2}}{{H}_{5}}$

Cis-trans isomerism are shown in a compound when there is a difference in the orientation of the same two groups.
When the two same groups are on the same side of the carbon-carbon bond, the compound exhibits cis isomerism, on the other hand if the same groups lie on the opposite to each other of the C-C bond, they exhibit trans isomerism.

The structure of formed compound is given as:

(E) (Z)

(E) (Z)

Therefore, there are 2 structural isomers. Each of these will lead to Z & E configurational isomers. i.e. total 4.

So, the correct option is (b).

Note: Cis-trans are a type of geometric isomerism which exists when there is a restriction in rotation of a molecule and there are two nonidentical groups on each doubly bonded carbon atom. Cis-trans isomerism are also exhibited by cyclic compounds.