Question

Which of the following compounds is optically active?
A.Butanal
B.Butanol
C.Butan-2 – ol
D.2-methyl propan – 2 – ol

Answer 1

Hint:Only chiral or asymmetric carbon that is the carbon having four different groups on its tetravalencia, containing compounds are referred to as optically active. Drawing the structure of each compound and finding a chiral carbon in it will give the optically active compound.

Complete step-by-step answer:
Chiral molecules can be explained on the basis of plane of symmetry. If all the attached groups to the central carbon atom are different then we can say that there is no plane of symmetry and the molecule is a chiral molecule. The molecule which has a chiral centre can show optical activity.
When a beam of polarised light is passed through an optically active compound, the direction of the plane polarised light changes which shows that the compound is optically active. Let us look at the first given compound, butanal. It has four carbons in it, but none of them has all four different substituents, so it is achiral molecule. Same is the case with butanol.
  

In the third option butan-2-ol, the second carbon of butanol chain has all four different substituents \[ - OH\] , H, \[C{H_3}\] and \[C{H_2}C{H_3}\] . So, it is a chiral molecule and is optically active. The fourth one is 2-methylpropan-2-ol, the central carbon has three methyl groups attached, so it is not chiral.

Hence, the correct option is (C).

Note:This question can also be answered by taking the mirror image of each of the given compounds. If the mirror images are non-superimposable and have chiral centres, we get enantiomers and the compound is optically inactive. But if the mirror image is non-superimposable and non-mirror image with chiral centre, we get diastereomers and the compound is optically active.