The key difference between chirality and helicity is that chirality refers to the property of asymmetry of molecules having a non-superposable mirror image, whereas helicity refers to the property of asymmetry of molecules having a twisted 3D structure.
Chirality and helicity are two common terms in stereogenic applications. Helicity is also called inherent chirality because the two terms are highly related to each other.
CONTENTS
1. Overview and Key Difference
2. What is Chirality
3. What is Helicity
4. Side by Side Comparison – Chirality vs Helicity in Tabular Form
5. Summary
What is Chirality?
Chirality refers to the property of having a superposable mirror image. This term is mostly used with organic compounds. The point that determines the presence or absence of chirality in a molecule is the chiral centre of that molecule. Chiral centre is a carbon atom of an organic compound that has four different substituents attached to it. Chiral compounds are the compounds containing chiral carbon atoms. Chirality is actually the property of having chiral centres. The chiral centre is essentially sp3 hybridized because it has to bear four different groups of atoms, forming four single covalent bonds.
Figure 01: Two Enantiomers of a Generic Amino Acid that are Chiral
Chiral centres cause the optical isomerism of compounds. In other words, compounds having chiral centres do not superimpose with its mirror image. Therefore, the compound having the chiral centre and the molecule that resembles its mirror image are two different compounds. Together, these two molecules are known as enantiomers.
On the other hand, the term achiral means that there are no chiral centres present. Therefore, a chiral compound has no symmetry. However, it has a non-superimposable mirror image. Since there are no chiral centres in achiral compounds, an achiral compound has superimposable mirror images.
There is also a plane of symmetry in an achiral compound. In other words, an achiral can divide into two identical halves at a certain plane known as the plane of symmetry. However, it is a hypothetical plane. The two symmetrical halves obtained from the plane of symmetry are superimposable mirror images of each other; in other words, one half reflects the other half. Unlike a chiral molecule, an achiral molecule has two or more identical substituents attached to a carbon centre.
What is Helicity?
Helicity is the property of having a twisted, helical structure. This is also called inherent chirality. The molecules showing helicity are asymmetric. But this asymmetry arises not from chiral centres or stereocenters, but from the twisted 3D structure. This concept was first introduced by the scientist Volker Boehmer in 1994.
Figure 02: Helicene Structure
Sometimes, we can observe that some chiral molecules contain a chirality plane or planes across which the molecule is asymmetric. Similarly, some molecules that show helicity contains chirality axes. These axes arise from the axis of the spatial arrangement of the molecule at which the chirality presents.
What is the Difference Between Chirality and Helicity?
Chirality and helicity are two common terms in stereogenic applications. The key difference between chirality and helicity is that chirality refers to the property of asymmetry of molecules having a non-superposable mirror image, whereas helicity refers to the property of asymmetry of molecules having a twisted 3D structure. Moreover, chirality results is a result of the presence of a chiral or stereo center, causing the occurrence of a non-superposable mirror image, while helicity is a result of the presence of a twisted 3D structure, causing the occurrence of a non-superposable mirror image.
Below infographic summarizes the difference between chirality and helicity.
Summary – Chirality vs Helicity
Chirality and helicity are two common terms in stereogenic applications. The key difference between chirality and helicity is that chirality refers to the property of asymmetry of molecules having a non-superposable mirror image, whereas helicity refers to the property of asymmetry of molecules having a twisted 3D structure.
Reference:
1. Helmenstine, Anne Marie. “Chiral Center Definition in Chemistry.” ThoughtCo, Feb. 11, 2020, Available here.
Image Courtesy:
1. “Chirality with hands” By !Original:UnknownVector: — πϵρήλιο – Chirality with hands.jpg (Public Domain) via Commons Wikimedia
2. “10helicene” By Claireotteson – Own work (CC BY-SA 4.0) via Commons Wikimedia