Overview of the Different Types of Isomers

The word 'isomer' originated from the Greek word isomeres (isos=equal and meros=parts). Since isomers have different structural formulae, they have different physical and chemical properties. There are 2 main forms of isomers: structural isomers and stereoisomers. The difference in the main architecture of organic molecules can be seen in the isomers.

Structural isomers differ in the arrangement of atoms, molecules, and functional groups. They may also vary because of the difference in the locations of the double bonds. Structural isomers are different ways of arranging the same atoms.

The more the number of carbon atoms, the more possible ways to arrange the different atoms and molecules. Thus, as the carbon skeleton increases in size, the possible number of isomers also increases. They are also referred to as constitutional isomers, and are of three types, as given here.

An example is butane with its two chain isomers, namely, n-butane (straight chain) and iso-butane (branched chain).

As the name suggests, position isomers are characterized by different positions of the same functional group or substituent, while the structure of the carbon chain remains the same.

For example, propanol (C₃H₇OH) has two position isomers, namely, propanol-1 and propanol-2, which differ in the position of the OH group. While it is attached to the first carbon atom in propanol-1 (n-propyl alcohol), it is attached to the second carbon atom in propanol-2 (iso-propyl alcohol).

Position isomers exist due to different permutations of element positions in a molecule. Position isomers of pentane are possible since there are 3 ways to build a tree of 5 carbon atoms: n-pentane, isopentane, and neopentane.

Functional isomers are two or more compounds that have the same molecular formula, but have different functional groups. A good example is ethanol and methoxymethane. Both have the same molecular formula i.e. C₂H₆O.

However, while one is an alcohol, the other one is an ether. Their structural formula are given below.

Tautomerism is a special case of functional isomerism. Tautomers are two compounds that have the same molecular formula, but have different functional groups. They exist in dynamic equilibrium with each other, and cannot be isolated due to the rapid inter-conversion, which is brought about by the migration of a hydrogen atom. Keto-enol tautomerism in carbonyl compounds such as acetone and acetoacetic ester, is a good example.

Compounds that have the same molecular formula, molecular structure, and bond order, but have different orientation of atoms in space, are called stereoisomers, and they are of two types as given here.

Two compounds are enantiomers if their molecules are "mirror images" of one another that are not superimposable. Not being able to superimpose one molecule on top of the other simply means that the two molecules are not equivalent or identical. For a compound to form an enantiomeric pair, it must have chiral molecules.

Chiral molecules must not have an internal plane of symmetry, and they must have a stereocenter. They look like ball and stick figures, where the middle carbon is called an asymmetric carbon.

This is because it is attached to four different atoms or groups of atoms. Enantiomers have identical physical and chemical properties, but have different chemical reactions with other enantiomers.

The only difference between two such isomers is their ability to rotate plane-polarized light in opposite directions, and this physical property is termed as optical activity. They are also known as optical isomers.

Each isomer of the pair is capable of rotating plane-polarized light, and thus exhibits optical activity. When plane polarized light is passed through solutions of each enantiomer, one rotates its plane in the clockwise direction by "x" number of degrees, while the other rotates it in the counterclockwise direction by the same number of degrees.

A 50:50 mixture of the two enantiomers of a compound is termed as racemic mixture or racemate, and it has neutral effect on plane polarized light.

If the enantiomers are crystalline salts like Pasteur's Tartrate salts, then they will have a different appearance when observed under magnification and one can pick them out to separate them, but most enantiomeric pairs are not salts and therefore look the same.

This makes it extremely difficult to separate the two isomers should they be mixed as often they are. These are extremely important to people in the pharmaceutical industry because two enantiomers of a drug may not be equally effective and may even cause side effects.

Geometric isomers have the same molecular structures but differ in the spatial arrangements of the atoms, and are also known as cis-trans isomers. This type of isomers occur from the inflexibility of the double bonds and ring structures. Cis and trans isomers are called geometric isomers. They differ in their geometrical orientation around a double bond that can't rotate.

The cis isomer has its substituents on the same side of the double bond while the trans isomer has its substituents on opposite sides of the double bond. Geometric isomers are stereoisomers where the two forms are not mirror images of each other.

For example, the compounds cis-2-butene and trans-2-butene both have the same formula, C4H10 and the same atoms are connected to each other in each molecule, but the geometry around the double bond is different. However, the two forms are not mirror images of each other.

This was a brief look at the major types of isomers. In addition to the ones described above, other forms of isomerism may be found in certain compounds. Isomers are used in medicines and beauty products, and have an array of other uses.