Question

Diastereomers may or may not be chiral – True or False?

• A. True
• B. False

Answer 1

The correct option is A True

What is the meaning of the word “chiral”? It comes from Greek origins and refers to “handedness”. If a compound is non-superimposable on its mirror image, then the compound is chiral. In other words, taking a mirror image gives rise to a different handedness - as in a different “configuration”.
First look at the definition of “stereoisomers”. Stereoisomers are isomers, which have the same chemical formula, the same sequence in which atoms are connected but differ only in the arrangement of atoms in 3d space.
Enantiomers are stereoisomers which are non-superimposable mirror images of each other. Stereoisomers which are not enantiomers are diastereomers. I know all this sounds very confusing. Let us break down the strict definition of diastereomers by means of examples.
1. First, let us take the example of geometrical isomers:
Are these two stereoisomers? Sure. Although they are also cis-trans isomers, these two different compounds have the same formula and the same sequence of connectivity of atoms but differ only in how atoms are arranged or configured in space. These molecules may be planar but still the arrangement in space is different for each compound is it not?
Let us check the definition of diastereoisomers. These two geometrical isomers - are they stereoisomers? Yes.
Are they enantiomers? I want you to take each compound and see if they are mirror images of each other AND non-superimposable also.
Definitely they are not mirror images of each other. They are non-superimposable though. Can we say that these two are not enantiomers? Of course we can!
So stereoisomers which are not enantiomers – they are called diastereomers. And in this case, neither of these compounds are chiral. Why?
Because both these disubstituted alkenes are planar and the plane that contains the molecules is a plane of symmetry. Whenever we have even one single element of symmetry, the compound is achiral.
Can diastereoisomers be chiral? Let us see!
2. Take these two compounds

They are again geometrical isomers or cis-trans isomers. What can you say about the chirality of each of these compounds? Can you find a plane or an element of symmetry?
There are no planes or elements of symmetry. Both these compounds have the same chemical formula. They also share the same order in which atoms are connected but differ in the spatial arrangement of atoms or groups. So they are definitely stereoisomers.

Are they enantiomers? Take the cis isomer:

If we take the mirror image, we get



So we see that the mirror image of the cis isomer is another cis isomer:

But we can clearly see that the above compound is not superimposable on top of

Hence, both these cis isomers are a pair of enantiomers. But what can you say about the original two?

Clearly these two are not enantiomers but eachcompound is optically active. Sure, they are geometrical isomers - as in cis-trans isomers. But these are also stereoisomers which are not enantiomers. Hence these are a pair of diastereomers. So, diastereomers may or may not be chiral - as can be seen from the disubstitutedethenes and the above two compounds!