Question

Which is the meso isomer of butane-1.2.3.4-tetraol?

(A) 
(B) 
(C) 
(D) None of the above

Answer 1

Hint: You should know that a meso compound or meso isomer is a non-optically active member of a set of stereoisomers, at least two of which are optically active.

Complete step by step answer:
Meso compounds are symmetric compounds that have an internal mirror plane so that the left and right sides of the plane are mirror images of each other.
A meso compound should contain two or more identical substituted stereocenters. Also, it has an internal symmetry plane that divides the compound in half. These two halves reflect each other by the internal mirror.

Here all the structures in the options are of butane-1.2.3.4-tetraol.
In option A, there are two stereocenters. But it is not a meso compound as the rotation of the chiral centres is in the same direction i.e. S.

In option B, there are two stereocenters. It is a meso compound as the rotation of the chiral centres is in the opposite direction i. e. S and R. Hence, these are cancelling out each other.

In option C, there are two stereocenters. But it is not a meso compound as the rotation of the chiral centres is in the same direction i.e. S.


Therefore, we can conclude that the correct answer to this question is option B.

Note: The stereochemistry of stereocenters should "cancel out". What it means here is that when we have an internal plane that splits the compound into two symmetrical sides, the stereochemistry of both left and right sides should be opposite to each other, and therefore, result in optically inactive.
R/S descriptors are assigned by using a system for the ranking priority of the groups attached to each stereocenter. This procedure, often known as the sequence rules, is the heart of the CIP (Cahn–Ingold–Prelog) system.