Difference Between Chiral and Achiral

Main Difference – Chiral vs Achiral

Chirality is a term used to describe whether the mirror image of a compound is superimposable with that compound or not. Chiral carbon is the main feature that can be used in order to determine the chirality of a molecule. A chiral carbon is an asymmetric carbon atom present in a compound. The main difference between chiral and achiral is that the mirror image of a chiral is non-superimposable whereas the mirror image of an achiral is superimposable.

Key Areas Covered

1. What is Chiral
      – Definition, Properties
2. What is Achiral
     – Definition, Properties
3. What is the difference between Chiral and Achiral
      – Comparison Of Key Differences

Key Terms: Achiral, Achiral Molecules, Chiral, Chirality, Chiral Carbon, Chiral Center, Enantiomers, Isomerism

What is Chiral

A chiral molecule is asymmetric in such a way that the structure and its mirror image are not superimposable. In order to be chiral, a molecule should have an asymmetric carbon atom. For example, if a carbon atom in a molecule has four different groups attached to it, it is chiral. The presence of different mirror images is known as isomerism. It belongs to the category of stereoisomerism. Chirality may occur in both organic and inorganic compounds.

The mirror images of chiral compounds are known as enantiomers. The enantiomer of a compound has the same structure of the molecule, but the spatial arrangement is different. Therefore, the mirror image of this kind of molecules is not identical and are considered as two different molecules. The compound and the mirror image have the same physical properties except for the direction in which they rotate the polarized light. Other physical properties are the same because the molar mass is the same. (Most of the physical properties are dependent on the molecular mass). But the chemical properties would be sometimes different from each other because the spatial arrangement has a considerable effect on chemical reactions.

A chiral molecule can have more than one chiral center, which is the carbon atom directly bonded to four different groups. This causes the molecule and its mirror image to have distinct properties. Following image gives a good example.

Figure 2: Enantiomers

In the above example, the two enantiomers are named with the prefix “S-“or “R-“. That is according to the direction that molecule rotates the plane polarized light. “S” indicates that the rotation is anticlockwise and “R” indicates that it is a clockwise rotation.

What is Achiral

Achiral is a property of a molecule where its mirror image can be superimposed with the molecule. In other words, if the molecule and its mirror image are identical to each other it is achiral. In achiral molecules, the molecule and its mirror image are not two different molecules, unlike in chiral molecules. They are the same.

There all chemical and physical properties are the same. Unlike chiral molecules, achiral molecules cannot rotate plane-polarized light to any direction. Achiral means that the molecule has no asymmetric carbon atoms present in their structure. If a carbon atom has two different groups attached to it but other two are the same groups, it is considered to be achiral.

Figure 2: CH3OH is an achiral molecule.

Difference Between Chiral and Achiral

Definition

Chiral: Chiral means “asymmetric in such a way that the structure and its mirror image are not superimposable.”

Achiral: Achiral means “symmetric in such a way that it can be superimposed on its mirror image.”

Symmetry

Chiral: Chiral molecules are always asymmetric at one or more centers.

Achiral: Achiral molecules are always symmetric at every center.

Mirror Image

Chiral: In chiral molecules, a particular molecule and its mirror image are two different compounds.

Achiral: In achiral molecules, the molecule and its mirror image are the same.

Superimposition

Chiral: The molecule and its mirror image are non-superimposable in chiral molecules.

Achiral: The molecule and its mirror image are superimposable in achiral molecules.

Rotation of Light

Chiral: Chiral molecules can rotate the plane-polarized light clockwise or anticlockwise.

Achiral: Achiral molecules cannot rotate plane-polarized light in any direction.

Conclusion

Stereoisomerism is a term which is frequently used in organic chemistry. In order to explain this isomerism of compounds, knowing the chirality of compounds is important because some compounds are chiral while some are achiral. The main difference between chiral and achiral is that the mirror image of a chiral is non-superimposable the mirror image of an achiral is superimposable.

References:

1.”Chiral molecules .” Chiral Molecules. N.p., n.d. Web. Available here. 19 June 2017. 
2.“Stereoisomers.” Stereoisomers. N.p., n.d. Web. Available here. 19 June 2017.

Image Courtesy:

1. “Spiroverbindung Chiralität” By NEUROtiker – Own work (Public Domain) via Commons Wikimedia
2. “Caractère achiral du méthanol” By DaraDaraDara – Own work (CC BY-SA 4.0) via Commons Wikimedia