The word chirality is derived from the Greek word Chair means hands. It is the property of an asymmetrical molecule to rotate the plane of polarized light in a clockwise or anticlockwise direction. These are optically active molecules. Similarly, these are non-superimposable mirror images of each other. They do not have a plane of symmetry.
It is one of the important concepts of stereochemistry. Chirality means handedness. It is exhibited by chiral molecules. They exist in pairs of stereoisomers. These are non-superimposable mirror images of each other. They do not have a plane of symmetry. They can rotate plane-polarized light and are optically active.
Chirality is a property of molecules. It refers to the whole molecule and not an atom. It is related to the symmetry of molecules. It does not depend on the number of substituents attached to the chiral center. The major requirement of chirality is the non-superimposing of mirror images. Chirality depends upon the configurations, not on conformations.
What are chiral molecules?
Chiral molecules are non-superimposable mirror images of each other. These are asymmetrical molecules. They don’t have a plane of symmetry. They can not be superimposed on their mirror images by any means. Similarly, they are optically active. They can rotate the plane polarized light either in a clockwise or anti-clockwise direction.
These molecules are also known as enantiomers. These are the stereoisomers with non-superimposable mirror images. They have the same physical and chemical properties. They differ only in optical activity.
Chiral carbon
When an Sp3 hybridized carbon atom is attached to four different substituents then the carbon atom is called chiral carbon. It cannot be superimposed on its mirror image by any means. This is also known as asymmetrical or dissymmetric carbon. Similarly, it has no plane of symmetry. Chiral carbon is also a stereogenic center because exchanging two of groups can produce new stereoisomers.
What is meant by optical activity?
Optical activity is the property of chiral molecules to rotate plane polarized light either in a clockwise or anti-clockwise direction. The molecule that rotates plane-polarized light in a clockwise direction is dextrorotatory (D) whereas the molecules that rotate the plane polarized light in an anti-clockwise direction are levorotatory (L).
Dextrorotary is also referred to as (+) isomer whereas levorotary is also referred to as (-) isomers. For example D and L glucose. Both of these are mirror images of each other. D enantiomer rotates plane polarized light in a clockwise direction whereas L enantiomer rotates plane-polarized light in the anti-clockwise direction.
Naming of chiral compounds
Initially, the D/L system was used for naming chiral molecules. But nowadays the R/S system is used for naming chiral compounds. The molecule that rotates plane-polarized light in a clockwise direction is R enantiomers whereas S enantiomers rotate plane polarized light in an anticlockwise direction. R-S system is based upon specific rules. These rules are also known as Cahn-Ingold-Prelog (CIP) rules or sequencing rules.
Chirality center
The chirality center is also known as a chiral center. When an atom is bonded to four different groups then the central atom is called a chiral center. It is mostly represented by an asterisk. It is a type of stereogenic center. A molecule having one chirality center must be chiral. Molecules may have more than one chirality center i.e cholesterol has eight chirality centers.
There are molecules that have a chiral center but still, they are achiral. The reason behind this is they have a plane of symmetry. Such molecules are called meso compounds. They have two or more chiral centers but they have planes of symmetry. That’s why they are achiral in nature. They are optically inactive molecules. For example, tartaric acid.
Chirality without chiral center
There are certain molecules that do not possess a chiral center but still, they are showing chirality. As we know that chirality is a property of a molecule. It does not depend upon the number of substituents at the chiral center. These molecules do not have a chiral center but they are optically active. For example,
Substituted allenes
In allenes, two C=C bonds are adjacent to each other. In allenes, central carbon is Sp hybridized whereas terminal carbons are Sp2 hybridized. A central carbon atom can form two Sp-Sp2 𝛔 bonds. Similarly, it also forms two 𝛑 bonds that are mutually perpendicular. So there is no plane of symmetry. Some derivatives of allenes show optical activity. When allenes are differently substituted they behave as chiral molecules. For example,
Substituted biphenyl
biphenyls are compounds in which two aromatic rings are joined together by a single bond. The substituted biphenyls can also show chirality even if they do not have chiral centers. They must be differently ortho disubstituted. The substitution must be necessarily bulky which can restrict rotations. Because of this restriction, these rings become perpendicular to each other. So there is no more plane of symmetry.
This is an exception because the SO3H group is itself too bulky group that can restrict rotation otherwise there must be two different substituents. These rings can not be interconvertible. Very high energy is required for twisting the rings.
Helical molecules
Helical molecules also show chirality. A simple example is a hexahelicene. This is known as screw-shaped chirality. It consists of six fused aromatic rings. This molecule can not be a planer because carbon and hydrogen atoms of the terminal ring occupy the same space. So these rings must bend over or under one another.
Triaryl molecules such as triarylboranes, triarylphosphines, and triarylmethanes are also helical molecules. These are propeller-shaped molecules they also show chirality. In these rotations of aromatic rings is prevented because of steric interaction with other rings.
Spiro compounds
In spiro compounds, two rings share a common atom. These compounds also show chirality. If neither ring contains a plane of symmetry then these are chiral compounds. Their mirror images are non-superimposable.
Prochiral compounds
These are achiral compounds that can be converted into chiral compounds. They have a single-step reaction. They have two identical groups. These groups are heterotopic. They are topologically un equivalent.
Compounds other than carbon showed chirality
Chirality is not only shown by a carbon atom. There are certain other atoms that also show chirality when they are tetrahedrally bonded with four different substituents. For example, nitrogen, phosphorus, silicon, germanium, and sulfur atoms.
Chirality of sulfur
Sulfur atoms can show chirality when tetrahedrally bonded to four groups or trivalently bonded to three substituents In the case of sulphones and sulphoximine sulfur atom is bonded to four different substituents. They show optical activity.
When it is bonded to three groups then lone pair of electrons acts as the fourth group. They have trigonal pyramidal geometry. i.e sulfonium salts, sulfoxides, etc are also chiral compounds.
Chirality of nitrogen
Just like sulfur atom trivalent nitrogen can also shows chirality. It has trigonal pyrimadal geometry. It is bonded to three different groups whereas lone pair of electrons acts as the fourth substituent. Although they exhibit chirality unfortunately their enantiomers are not separate able because of umbrella inversion.
Chirality of phosphorus
Phosphorus atoms can show chirality in tetravalent and trivalent conditions. At trivalent conditions, the lone pair is the fourth substituent. i.e phosphines, phosphonium salts, phosphine oxides, etc.
Examples of chirality/chiral compounds
- First and most commonly used example of chirality is hands. Left and right hands are non-superimposable mirror images. They have no plane of symmetry. The mirror image of the left hand is the right hand whereas, the mirror image of the right hand is the left hand. Both of these are non-superimposable.
- The human body is structurally chiral.
- Except for glycine, all amino acids are chiral in nature. i.e alanine
- DNA shows helical chirality.
- Just like hands feet are also chiral.
- Carbohydrates show chirality.
- Sea shells are chiral in nature
- Gloves, shoes, socks, etc are examples of chirality.
- Many drugs are chiral compounds.
- Natural products like steroids are chiral compounds i.e cholesterol molecules.
Concepts bergs
What is meant by a plane of symmetry?
It is the plane that divides(bisects) objects into symmetrical halves. This is also known as the mirror plane.
What are the basic requirements for chirality?
There are two basic requirements of chirality
- Non-superimposbility of mirror images.
- No plane of symmetry.
Differentiate between the term chirality and chiral.
When an atom is bonded to four different groups then that atom is said to be chiral whereas chirality is a property of an atom. It does not depend on the number of substituents.
Define optical activity.
Optical activity is a rotation of plane-polarized by chiral compounds either in a clockwise or anti-clockwise direction.
How to determine chirality?
We can determine chirality as,
- Atom may have four different substituents.
- There is no plane of symmetry.
- Mirror images are non super imposable.
Define meso compounds.
The achiral compounds having two or more chiral centers and planes of symmetry are meso compounds.
What are the requirements for biphenyl to show chirality?
- It must be ortho disubstituted.
- Substituents must be different.
- They must be the bulky groups to restrict rotation.
What makes a molecule chiral?
Four different substituents that lead to non-superimposable mirror images make a molecule chiral.
References
- 12th edition of Organic Chemistry by T.W Graham Solomons, Craig B. Fryhle, and Scott A. Synder.
- Principle and Application of Stereochemistry by Micheal North.
- 2nd edition of Organic Chemistry by Joseph M. Hornback
