Question

Which of the following cyclic molecules are meso compounds?

a.

b.

c.

Short answer

1. Not a meso
2. Meso
3. Not a meso

Step-by-step solution

Meso compound 

A compound is said to be meso if it follows two conditions given below:

• Two or more chiral centers.
• Plane of symmetry.

Plane of symmetry

The plane of symmetry is present when a molecule has the same groups at both sides of the plane.

Explanation

a. The molecule has two chiral centers due to the presence of four different groups at each chiral center.

One chiral center has a methyl group above the plane and one methyl group below the plane. Therefore, the molecule does not have any plane of symmetry (neither horizontal nor vertical).

Hence, the molecule is not a meso compound.

Not a meso compound

b. The molecule has two chiral centers, and both the methyl groups are present above the plane. Therefore, a plane passes through the middle of the carbon and divides the molecule into two equal halves.

Hence, the molecule is a meso compound.

Meso compound

c. The molecule has two chiral centers, but the groups at each chiral center are different. Therefore, the molecule does not have any plane. Hence, the molecule is not a meso compound.

Not a meso compound