Question

Two isomeric alkenes A and B have molecular formula ${C_5}{H_9}Cl$ . On adding ${H_2}$ , A gives an optically inactive compound while B gives a chiral compound. The two isomers are:
A.A is 4-chloro-2-pentene and B is 4-chloro-1-pentene
B.A is 3-chloro-2-pentene and B is 2-chloro-2-pentene
C.A is 2-chloro-3-methyl-2-butene and B is 1-chloro-3-methyl-1-butene
D.A is 3-chloro-1-pentene and B is 1-chloro-2-pentene

Answer 1

We can say an atom that is linked to four non-equivalent atoms or groups is known as the chiral atom. The molecule is optically active because of the asymmetric center. We can say that optical activity is the tendency of chiral molecules to rotate plane polarized light.

Complete answer:
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 ${C_5}{H_9}Cl$ .
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 (B) is correct.

Note:
We have to know that enantiomers contain identical physical properties, but vary in the optical rotation. When plane polarized light is kept they rotate in a contradictory direction, but the magnitude is the same. We have to know that enantiomers contain different configurations at the stereogenic centre as they are mirror images to each other.