Stereochemistry is the branch of chemistry that deals with the relative spatial arrangement of atoms forming molecules or chiral molecules. Molecules forming non-superimposable mirror images are known as chiral molecules. In contrast, molecules with superimposable mirror images are known as achiral molecules.
The word chiral is derived from the Greek word cheir, meaning hand. If a molecule is non-superimposable on its mirror image and can be easily distinguished, it is said to be a chiral molecule. The property of non-superimposability of molecules is known as chirality. A chiral molecule must contain an asymmetric carbon atom. Bhutan-2-ol is an example of a chiral molecule.
The two mirror images of a chiral molecule are known as optical isomers or enantiomers.
Enantiomers are the pair of molecules having a non-superimposable mirror image relationship. If both the enantiomers are present in comparable proportions, then the mixture is known as a racemic mixture.
In contrast, if a molecule is superimposable on its mirror image and can not be easily distinguished, it is known as an achiral molecule.
Dutch scientist J. Van’t Hoff and a French scientist, C. Le Bel, were the first to uncover the chirality. They stated that the spatial arrangement around the carbon atom is tetrahedral. Suppose all the substituent attached to the carbon atom is distinct. In that case, the carbon will become asymmetric, which would lack symmetry, and they referred to it as an asymmetric molecule or chiral molecule.
Most of the biological molecules that we encounter are chiral. Chirality plays an essential role in recognising biologically active molecules and its target. Moreover, it plays a vital role in the interactions and binding affinity between the drug and its target, thus shaping the pharmacology of the drug.
Racemisation refers to the conversion of an optically active compound (dextro or laevo) to an optically inactive compound by heating or a chemical reaction. In racemisation, an enantiomer is converted into the comparable proportion of another enantiomer in order to form a racemate. When both dextrorotatory and laevorotatory are present in equal amounts, then the mixture is known as a racemic mixture.
Racemisation is thermodynamically favourable and proceeds spontaneously if an appropriate pathway is available to convert the enantiomers. The racemisation rate relies on the molecule and conditions such as pH and temperature.
Racemisation can be carried about by,
- Heating: Heating can often modify an optically active enantiomer into an optically inactive racemic mixture.
- Chemical reagent: It was observed that the presence of a foreign chemical reagent could transform an optically active enantiomer into an optically inactive racemic mixture.
- Auto racemisation: Sometimes, keeping the substance at room temperature can change an optically active enantiomer into an optically inactive racemic mixture.
Optical Activity of Racemic Mixture
A racemic mixture does not rotate a plane polarised light because the optical activity of one molecule is cancelled out by the other molecule. Based on the direction of rotation, we can classify the optical activity of a molecule into two types.
- Dextrorotatory: A dextrorotatory molecule rotates the plane polarised light in the right, i.e., clockwise direction. It is denoted by writing the (D) or (+) signs before the degree of rotation.
- Laevorotatory: A laevorotatory molecule rotates the plane polarised light on the left, i.e., anticlockwise direction. It is denoted by writing the (L) or (-) signs before the degree of rotation.
A racemic mixture consists of both dextrorotatory and laevorotatory in equivalent proportion. Thus, the mixture will have zero optical activity and will not rotate the plane polarised light in any direction due to racemisation. A polarimeter is an instrument used to measure the degree of rotation.