Take a pair of shoes and examine them. The two shoes might look the same, except if you try to wear right leg shoes on your left leg, it just isn’t going to work. It won’t feel comfortable, so though they appear identical, they aren’t the same. They are the opposite. They are the perfect mirror image of each other. They are also non-superimposable, i.e. although they are mirror images of each other, they can’t be rotated to the same shape.
Similarly, the molecule which can not be superimposed is called chiral molecules, while a molecule that can be superimposed are called Achiral molecules.
In this section, we will discuss chiral and achiral molecules in detail.
Table of Content
Chiral Molecules
The word chiral emanates from the Greek word cheir, which means hand. A molecule or ion is chiral if it is non-superimposable, i.e. it can not be superimposed on its mirror image. This geometric property of a molecule is called chirality.
- Chiral molecules don’t have a plane of symmetry or a centre of symmetry.
Chiral molecules exist in two stereoisomers, non-superimposable mirror images of each other, commonly known as enantiomers. Enantiomers are classified as “right-handed” or “left-handed” based on the absolute configuration.
Enantiomers were found to have identical chemical properties unless reacting with another chiral species. They were also found to have comparable physical properties. A homogenous mixture of two enantiomers in a relative amount is called a racemic mixture. A racemic mixture shows different properties from enantiomer.
Achiral Molecules
Achiral is the reverse of chiral. A molecule or ion is achiral if it is superimposable, i.e. it can be superimposed on its mirror image. They have a plane of symmetry or a centre of symmetry.
- Achiral molecules having a stereocenter are known as Meso molecules.
(meso)-2,3-dibromobutane and trans-1,2-dichloro-1,2-ethanediol are examples of achiral molecules.
Chiral vs Achiral
|
S No. |
Chiral |
Achiral |
|---|---|---|
|
1 |
Chiral molecules are asymmetric. |
Achiral molecules are symmetric. |
|
2 |
Chiral molecules are non-superimposable mirror images of each other. |
Achiral molecules are superimposable mirror images of each other. |
|
3 |
Chiral molecules are asymmetric at one or more centres. |
Achiral molecules are symmetric at all the centres. |
|
4 |
Chiral molecules rotate plane polarised light in a clockwise or anticlockwise direction. |
Achiral molecules don’t rotate plane polarised light in any direction. |
|
5 |
The mirror image of a chiral molecule is a different molecule. |
The mirror image of an achiral molecule is an identical molecule. |
What is an Enantiomer?
Enantiomers are pairs of molecules having non-superimposable mirror images. Enantiomers are identified by passing plane polarised light on them. They have similar melting and boiling points.
Enantiomers are classified as “right-handed” or “left-handed” based on the absolute configuration. Enantiomers were found to have identical chemical properties unless reacting with another chiral species. A homogenous mixture of two enantiomers in a relative amount is called a racemic mixture. A racemic mixture shows different properties from enantiomer.
Frequently Asked Questions on Chiral vs Achiral
What are chiral molecules?
A molecule or ion is chiral if it is non-superimposable, i.e. it can not be superimposed on its mirror image. This geometric property of a molecule is called chirality. Chiral molecules don’t have a plane of symmetry or a centre of symmetry.
What are achiral compounds?
A molecule or ion is achiral if it is superimposable, i.e. it can be superimposed on its mirror image. Achiral molecules have a plane of symmetry or a centre of symmetry. Achiral molecules having a stereocenter are known as Meso molecules.
(meso)-2,3-dibromobutane and trans-1,2-dichloro-1,2-ethanediol are examples of achiral molecules.
What is a racemic mixture?
A homogenous mixture of two enantiomers in a relative amount is called a racemic mixture. A racemic mixture shows different properties from enantiomers.
What is an enantiomer?
Enantiomers are pairs of molecules having non-superimposable mirror images. Enantiomers are identified by passing plane polarised light on them. They have similar melting and boiling points.
Enantiomers are classified as “right-handed” or “left-handed” based on the absolute configuration. Enantiomers were found to have identical chemical properties unless reacting with another chiral species.
Are chiral and achiral molecules identical?
No, chiral and achiral are different molecules. Achiral are superimposable mirror images, while chiral are non-superimposable mirror images.
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