The Wonderful World Of Isomers

To understand isomers, requires understanding of the two different chemical formulae: molecular and structural. The molecular formula is the number of each element found in a compound, whilst the structural formula illustrates how the atoms of each element are organised through the depiction of bonds. This can either be achieved by a short-hand condensed version where the molecule is broken up into parts and functional groups e.g. -CH₃ for a methyl group, or as a displayed formula where every bond is laid out in a sketch.

There are several types of isomers you encounter at A-Level, these being: structural isomers, geometric isomers, and optical isomers. All are brought under one definition: having the same molecular formula but arrangement of the atoms in space.

Structural Isomers: are the most basic form of isomer as they have the same molecular formula, but different structural formula. Members of many different hydrocarbon groups especially the alkanes and alkenes illustrate this. When the carbon number is 4 or above e.g. butane or butene, structural isomers start appearing. e.g. butane and 2-methylpropane or isobutane.

Geometric Isomers: are about double bonds, especially in organic chemistry. The 4 substituent groups attached, 2 on each end of the double bond in carbon, can be arranged in 2 distinct ways: E/trans where the highest priority groups on each side are opposite each other (seen best in displayed), and Z/cis where the highest priority groups are on the Zame Zide - as corny as that sounds. The Cahn-Ingold-Prelog (CIP) rules are used to assign priority, which basically looks at the atomic number of elements in each group in a systematic way, from nearest to the double bond to atoms furthest away.

Optical Isomers: are stereoisomers, like EZ/geometric isomers, where the structural formula is the same but the positions of atoms in the 3D space are different. Enantiomers, another name for optical isomers, come in pairs, as they are mirror images of each other. In the case of organic chemistry at A-level, these two are non-superimposable molecules due to the presence of 4 distinct groups bonded to a carbon centre. Think of them as a left- and right-hand, therefore not being identical.