Chiral vs Achiral
Both these terms can be discussed under the common term Chirality which was first coined by Lord Kelvin in 1894. The word Chirality has a Greek origin which meant ‘hand.’ The term is commonly used in stereochemistry today and relates to many important fields in Organic, Inorganic, Physical and Computational Chemistry. It is rather a mathematical approach to handedness. When a molecule is said to be chiral, that molecule and its mirror image are non-superimposable which ideally resembles the case with our left and right hands which cannot be superimposable with their respective mirror images.
What is Chiral?
As mentioned above a chiral molecule is a molecule that cannot be superimposed with its mirror image. This phenomenon occurs due to the presence of an asymmetric carbon atom present in the molecule. A carbon atom is said to be asymmetric when there are four different types of groups/atoms joined to that particular carbon atom. Therefore, when considering the mirror image of the molecule it is impossible to make it fit the original molecule. Let’s suppose carbon had two groups similar to each other and the other two being completely different; yet, the mirror image of this molecule can be superimposed with the original molecule after several rounds of rotations. However, in the case of the presence of an asymmetric carbon atom, even after all the possible rotations are performed the mirror image and the molecule cannot be superimposed.
This scenario is best explained through the concept of handedness as mentioned in the introduction. A chiral molecule and its mirror image are called a pair of enantiomers or ‘optical isomers.’ Optical activity relates to the rotation of plane polarized light by the molecular orientation. Therefore, when considering a pair of enantiomers , when one rotates plane polarized light to the left the other does so to the right. Thereby, these molecules can be distinguished by this means. Enantiomers share very similar chemical and physical properties, but in the presence of other chiral molecules they behave very differently. Many of nature’s compounds are chiral, and this has helped a lot in catalysis by enzymes as the enzymes bind only to a particular enantiomer, but not to the other. Therefore, many reactions and pathways in nature are high specific and selective providing platform for variation and uniqueness. Enantiomers are named with different symbols for the convenience of identification. i.e R/S, +/-, d/l etc.
What is Achiral?
An achiral molecule can be superimposed with its mirror image without much effort. When a molecule does not contain an asymmetric carbon or in other words a stereogeniccentre, that molecule can be considered as an achiral molecule. Therefore, these molecules and their mirror images are not two, but the same molecule as they are identical to each other. Achiral molecules do not rotate plane polarized light, hence, are not optically active. However, when two enantiomers are in similar amounts in a mixture, it does not visibly rotate plane polarized light as the light being rotated in similar amounts to the left and the right has the rotation effect cancelled off. Therefore, these mixtures appear to be achiral. Nevertheless, due to this special phenomenon, these mixture are often called racemic mixtures. These molecules also do not have different naming patterns as for the chiral molecules. An atom also can be considered as an achiral object.
What is the difference between Chiral and Achiral?
• A chiral molecule contains an asymmetric carbon atom/stereogeniccentre but an achiral molecule does not.
• A chiral molecule has a non-superimposable mirror image but an achiral molecule does not.
• A chiral molecule and its mirror image are considered as two different molecules called enantiomers, but an achiral molecule and its mirror image is identical.
• A chiral molecule has various prefixes added to the chemical name, but achiral molecules do not contain such prefixes.
• A chiral molecule rotates plane polarized light but an achiral molecule does not.