Question

The compound which is optically active is
(A) 1-Butanol
(B) 2-Butanol
(C) 1-Propanol
(D) 2-Methyl-1-propanol

Answer 1

In order to find a compound is optically active or not, we have to view the compound completely and check whether it has an element of symmetry, i.e. the plane of symmetry. If it does not have any symmetry element then the compound is optically active.

Complete Solution :
Let us understand what an optically active substance is. When a structure of a molecule cannot be superimposed on its mirror image, then it is called Enantiomers. Enantiomers are said to be optically active. A compound which can optically rotate is said to be optically active. All chiral compounds are said to be optically active. For a molecule to be chiral, it should be attached to four different groups. In other words, we can say chiral molecules are asymmetric. The organic compound should not have any plane of symmetry or centre of symmetry.
- The optically active substance will rotate the plane polarized light in the opposite directions. One that rotates the plane polarized light in the right or in the clockwise direction, it is called dextrorotatory. One which rotates the plane polarized light towards the left or in the anticlockwise direction, it is called laevorotatory. Mixtures of both the enantiomers in equal quantities are called racemic mixtures.
Let us now discuss the given options.
Option (A) 1-Butanol

We can see that this compound does not have a chiral centre. Therefore, it is not optically active.
Option (B) 2-Butanol

We can see that this compound has a chiral center. Therefore, it is optically active.
Option (C) 1-Propanol

We can see that this compound does not have a chiral centre. Therefore, it is not optically active.
Option (D) 2-Methyl-1-propanol

We can see that this compound does not have a chiral centre. Therefore, it is not optically active.
So, the correct answer is “Option B”.

Note: - Optical activity can be measured by polarimetry.
- Polarimeter is a tool which can be used to measure the sugar content. It is mainly used in the sugar industry.