Two isomeric haloalkenes (A) and (B) have molecular formula C₅H₉Cl (A) gives an optically inactive compound while (B) gives an optically active compound in hydrogenation. The two isomers respectively are

3-chloro-2-pentene and 2-chloro-2-pentene

We know that the compounds which give non-superimposable mirror images of each are known as optical isomers and a chiral molecule shows no plane of symmetry in the molecule. This characteristic feature is called chirality. If four various groups are linked to a tetrahedral atom, the four groups would be arranged in two conceivable ways. The two compounds which lead mirror images of each other are called enantiomers.

We can define an optically inactive compound as a compound that is unable of optical rotation is known to be optically inactive. All pure achiral compounds are optically inactive. An example achiral compound is chloroethane and it does not show rotation of plane of plane-polarized light.

We can say a molecule is achiral if it is superimposable on its mirror image. Most achiral molecules have a symmetrical plane or a symmetry center.

Given data contains the molecular formula of two alkenes A and B as \(C_5H_9Cl\)

 .

By hydrogenation of compound A, an optically inactive compound is formed. So, compound A should contain a plane of symmetry. Compound B on hydrogenation forms a chiral compound and is optically active. So B does not have any plane of symmetry.

So, the two isomeric alkenes are 3-chloro-2-pentene and 2-chloro-2-pentene.

In these two isomeric alkenes, 3-chloro-2-pentene has a plane of symmetry and hence, it is compound A. 2-chloro-2-pentene has no plane of symmetry and so it is compound B.

The hydrogenation of 3-chloro-2-pentene gives 3-chloropentane and the hydrogenation of 2-chloro-2-pentene gives 2-chloropentane.

Therefore, the option (2) is correct.