Consider the following statements about optical activity.
(A) A compound that rotates the plane of plane polarised light clockwise is called dextrorotatory, and a positive (+) sign is placed before the degree of rotation.
(B) The angle by which the plane polarised light is rotated is measured by an instrument called a polarimeter.
(C) The two enantiomers of a compound differ in their melting points and boiling points.
(D) Propan-2-ol contains an asymmetric carbon atom and is a chiral molecule.
Which of the statements given above are incorrect? Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (C) and (D) only
An optically active compound rotates the plane of plane polarised light. A molecule with an asymmetric carbon (four different groups on one carbon) is not superimposable on its mirror image, so it is chiral and optically active.
(A) A compound that rotates the plane of plane polarised light clockwise is called dextrorotatory, and a positive (+) sign is placed before the degree of rotation. — Correct statement. Rotation to the right, that is clockwise, is dextrorotatory (the d-form), shown by a (+) sign.
(B) The angle by which the plane polarised light is rotated is measured by an instrument called a polarimeter. — Correct statement. The polarimeter measures the angle of rotation.
(C) The two enantiomers of a compound differ in their melting points and boiling points. — Incorrect statement. Enantiomers have identical physical properties, such as melting point, boiling point and refractive index. They differ only in the direction in which they rotate plane polarised light.
(D) Propan-2-ol contains an asymmetric carbon atom and is a chiral molecule. — Incorrect statement. The central carbon of propan-2-ol, CH3CH(OH)CH3, carries two identical CH3 groups, so it is not asymmetric. Its mirror image can be superimposed on it, so propan-2-ol is achiral.
NCERT (2026-27 edition):
6.7.1 Reactions of Haloalkanes – “…The angle by which the plane polarised light is rotated is measured by an instrument called polarimeter. If the compound rotates the plane of plane polarised light to the right, i.e., clockwise direction, it is called dextrorotatory (Greek for right rotating) or the d-form and is indicated by placing a positive (+) sign before the degree of rotation.”
“As you can see very clearly, propan-2-ol (A) does not contain an asymmetric carbon, as all the four groups attached to the tetrahedral carbon are not different.” … “Thus propan-2-ol is an achiral molecule.”
“Enantiomers possess identical physical properties namely, melting point, boiling point, refractive index, etc. They only differ with respect to the rotation of plane polarised light.”