The isomers can be divided into two large groups: structural (or constitutional) isomers and stereoisomers. We will first consider the structural isomers, which can be divided again into three main subgroups: chain isomers, position isomers and isomers of functional groups. Structural isomerism can quickly get out of hand in terms of the number of possible isomers; butane (four carbon atoms) has two possible isomers, decane (ten carbon atoms) has seventy-five, and a simple hydrocarbon containing 40 carbon atoms has an estimate of 62,000,000 structural isomers.
Chain isomers are molecules of the same molecular formula, but different arrangements of carbon skeleton. Organic molecules are based on carbon chains, and for many molecules this chain can be arranged differently either as a continuous chain or as a chain with several branched carbon side groups. The name of the molecule can be changed to reflect this, but we will save naming the molecules for another post. Obviously, there are often many ways to branching the carbon chains in the main chain, which leads to a large number of possible isomers, as the number of carbon atoms in the molecule increases.
Positional isomers are based on the movement of a “functional group” in the molecule. A functional group in organic chemistry is the part of a molecule that gives it reactivity. There are several functional groups, the most common of which have been summarized in a previous post. Nothing else in the molecule changes, simply if the functional group is in it, and the name simply changes slightly to indicate where the molecule is located.
Also called functional group isomers, these are isomers in which the molecular formula remains the same, but the type of functional group in the atom is changed. This is possible by rearranging the atoms in the molecule so that they are linked together in different ways. As an example, a straight-chain alkane (containing only carbon and hydrogen atoms) may have a functional group isomer that is a cycloalkane which is simply the carbon atoms bonded together so as to form a ring. Various functional group isomers are possible for different functional groups.
There are two main types of stereoisomerism: geometric isomerism and optical isomerism. These, as the difference in the name suggests, do not have to do with large-scale rearrangements of the structure of the molecules; instead, they involve different arrangements of parts of the molecule in space. They are a little more complicated to think about than structural isomers, so let’s take a look at each of them in turn.
Geometric isomerism is in fact a “strongly discouraged” term by IUPAC, which prefers “cis-trans” or “E-Z” in the case of specific alkenes. However, “geometric isomerism” is still being used constantly in many level A courses to refer to both, so for that reason I used this name here.
This type of isomerism often involves carbon-carbon double bonds (shown by two lines linking each carbon instead of one). The rotation of these links is restricted, as compared to simple, rotatable bindings. This means that if there are two different atoms or groups of atoms attached to each carbon-carbon double bond carbon, they can be arranged in different ways to give different molecules.
These atoms or groups can receive “priorities” with atoms with higher atomic numbers, with higher priorities. If the groups with the highest priority for each carbon are on the same side of the molecule, the molecule is referred to as the “cis” or “Z” isomer. If placed on opposite sites, it is called the “trans” or “E” isomer.