What are Stereochemistry and Isomerism?  

Stereochemistry deals with the three-dimensional structure and properties of molecules. It explains the arrangement of atoms and their manipulation within a molecule in a 3D structure. Stereochemistry can be dealt with all types of molecules. Based on the stereochemistry of a molecule, the physical and chemical properties of the molecules vary.  

There are few compounds that have the same molecular formula, i.e. the same number of atoms, but differ in their chemical and physical properties due to the difference in the arrangements of the atoms in the 3D structure of the molecule. These compounds are called isomers and the study dealing with these compounds is termed isomerism. 

Isomerism 

Isomerism is classified into two types: Structural or constitutional isomerism and stereoisomerism or spatial isomerism. 

Structural isomers 

Pair of compounds that show structural isomerism, i.e., contain the same molecular formula but differ in their connectivity are termed as structural isomers.  

Structural isomers are further classified as chain isomerism, position isomerism, functional group isomerism, metamerism, tautomerism, and ring chain isomerism.  

Chain isomersism 

This type of isomerism is generally seen in the arrangement of carbon to the carbon chain of the molecule. These isomers contain the same number of carbon atoms but the arrangement of carbon atoms in the chain differs i.e. the main carbon skeleton varies such as straight-chain and branching chains. It is also called skeletal isomers.   

Example: Hexane shows five structural chain isomers or skeletal isomers.  

Position isomerism 

This type of isomerism is generally noticed when the molecules have the same molecular formula with the same functional group but differ in the position of the functional groups in the main skeletal of the molecule.  

Example: Pentene shows two positional isomers.  

Functional group isomerism 

This type of structural isomers are noticed with the compounds whose molecular formula is the same but differ completely in the function group present i.e. the two molecules completely differ in their basic family.  

Metamerism 

This type of structural isomerism is noticed in molecules containing few specific functional groups such as ethers, thioethers, amide, esters, etc. This is very rarely noticed isomerism and is found in the molecules with divalent atoms such as oxygen, sulfur, etc., bonded to alkyl groups. The molecules which show metamerism are termed metamers.  

Example: Pentanone shows two metamers.  

Tautomerism 

Tautomerism is a type of structural isomerism in which the molecular formula remains the same as well as the arrangement and the number of carbon atom chains, but differs in the position of one hydrogen atom or a proton or at times the position of an electron. They are readily interconvertible to each other. This concept of interconverting between two isomers is named tautomerization. Among the pair of tautomers, one form is very stable and the other form is least stable.  

Ring-chain isomerism 

Ring chain isomerism is a type of structural isomerism in which compounds have the same molecular form but the structure might be an open chain and a cyclic ring chain.  

Stereoisomers 

Pair of compounds which show stereo isomerism i.e. the molecular formula and the bonding are same but differ in the position of the atoms are termed as stereo isomers. Stereoisomers are further classified into geometrical isomers and optical isomers.  

Geometrical isomers  

The compounds which have same number of atoms and bonding i.e. same molecular formula but differ in the arrangement of the atoms in the molecule are geometrical isomers. The arrangements differ due to the rotation of the bonds in the molecule. This type of isomerism is found between the atoms bonded with unrestricted rotation. Further this type of isomers can be noticed only with the molecules which have chiral carbon atoms. Geometrical isomers are of two types: cis and trans isomers. The word “cis” and “trans” are coined from Latin which means “this side of” and “the other side of”, respectively. If in a molecule, one of the functional group is oriented on the opposite side of the other, it is called a trans isomer and if they are oriented on the same side, then it is a cis isomer. Among the two isomers, the trans isomer is said to more stable than the cis isomer due to less steric hindrance in trans isomers.   

Optical isomers 

The word chirality comes from Greek word “Kheir” which means “hand”. The human hand is said to be the best example for a chiral object, i.e. the right hand acts as a non-superimposable mirror image of left hand. An object, a substance, a molecule or an atom is said to be chiral if it is different when it is superimposed on its mirror image. Thus, a chiral object or a system cannot be superimposed on its mirror image. The pair of compounds i.e. a chiral object and its mirror image, are called enantiomers and they are said to be optically active. Diastereomers are the compounds which are chiral and not mirror images and they are not optically active. In a molecule, chirality plays a role in the presence of a chiral carbon. A chiral carbon is said to show chirality if it is bonded to different groups i.e. it should not be bonded to same group or substituents.  

Based on the rotation of the plane-polarized light by the chiral molecule, the optical isomers or the enantiomers can be classified as dextrorotatory (d) and levorotatory (l) molecules. If a chiral molecule rotates the plane-polarized light clockwise or towards the right, then it is dextrorotatory and if it rotates it anticlockwise or towards the left, then it is levorotatory. The (+) and (-) symbols indicate that the specific rotation values for dextrorotatory compounds are positive and that levorotatory compounds are negative. Further, they can also be mentioned in R and S notation where R is “Rectus” in Latin which means right and S is “Sinister” which means left.

Context and Applications    

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for Bachelors and Masters in Chemistry.