Question

Which of the following compounds is optically active?
A.${\text{C}}{{\text{H}}_{\text{3}}}{\text{C}}{{\text{H}}_{\text{2}}}{\text{COOH}}$
B.${\text{HOOC}} - {\text{C}}{{\text{H}}_{\text{2}}} - {\text{COOH}}$
C.\[{\text{C}}{{\text{H}}_{\text{3}}}{\text{CH(OH)COOH}}\]
D.\[{\text{C}}{{\text{l}}_{\text{2}}}{\text{CHCOOH}}\]

Answer 1

Hint: We should know the condition of optical activity to answer this question. The presence of a chiral centre is necessary for a compound to be optically active. The maximum valency shown by carbon is four. A carbon having four different substituents is known as a chiral centre. We will draw the structure of all given compounds to determine the chiral centre.


Complete answer:
A compound having a chiral centre or we can say a carbon with four different valency will be optically active. A carbon attached with four different type of groups is known as a chiral centre.
We will draw the structure of all the compound to determine the chiral centre as follows:

The propionic acid has two hydrogens, one methyl and one carboxylic group. So, propionic acid has only three types of substituents hence, no chiral centre. So, propionic acid is not optically active.

The malonic acid has two carboxylic, and two hydrogen groups. So, malonic acid has two types of substituents hence, no chiral centre. So, malonic acid is not optically active.

The $2 - $hydroxypropanoic acid has one hydrogen, one methyl, one hydroxy and one carboxylic group. So, $2 - $hydroxypropanoic acid has four different substituents hence, a chiral centre. So, $2 - $hydroxypropanoic acid is optically active.

The $2 - $chloroacetic acid has one hydrogen, two chlorine, and one carboxylic group. So, $2 - $ chloroacetic acid has three types of substituents hence, no chiral centre. So, $2 - $chloroacetic acid is not optically active.So, only $2 - $hydroxypropanoic acid is optically active.
Therefore, option (C) \[{\text{C}}{{\text{H}}_{\text{3}}}{\text{CH(OH)COOH}}\]is correct.

Note: The compound having no chiral centre is known as optical inactive. A chiral molecule can be optical inactive in two conditions: first, if the molecule has symmetry such as tartaric acid having a plane of symmetry. Second, if the molecule exists in two enantiomers and both are present in equal amounts and both rotate the light in opposite directions. This type of mixture when two enantiomers of a chiral molecule present in the same amount and both rotate the light in opposite directions is known as a racemic mixture. The chiral molecule rotates the light in a clockwise direction are known as dextrorotatory and represented by \[\left( + \right)\] sign and the chiral molecule rotate the light in an anticlockwise direction are known as laevorotatory and represented by \[\left( - \right)\] sign.