If we study the structural formula of butane or other higher hydrocarbons of the alkane series, we will observe that it is possible to arrange the atoms in the molecule in more than one way. This means that it is possible to have two or more different structural arrangements for the same molecular formular, a phenomenon known as isomerism. For example, the four carbon atoms and ten hydrogen atoms in the butane molecule can be linked in two different ways which will satisfy the valencies of carbon and hydrogen.
Hexane, C6H14, has five possible isomers, while calculations show that there are 75 isomers for decane, C10H22 and up to 366319 isomers for eicosane, C20H42.
Generally, isomers with the same molecular formula and belonging to the same homologous series tend to have similar chemical properties (as they have the same functional group), but slightly different physical properties as a result of their structural differences. However, it is also possible to have isomers with the same molecular formula but belonging to different homologous series. For example, both ethanol and methoxymethane (dimethylether) have the same molecular formula of C2H6O, but belong to the alkanol and ether series respectively. Such isomers usually have different physical as well as chemical properties because of their different structural formulae and functional groups respectively.
Thus, ethanol is a liquid at room temperature and it reacts readily with phosphorus(V) chloride due to the presence of the hydroxyl group, -OH. Methoxy-methane, which has a lower boiling point, exists as vapor at room temperature and does not react with phosphorus(V) Chloride due to the absence of the hydroxyl group.
We can write the structural formula of butane in two ways, since each carbon atom joined by the single covalent bond has two different atoms/groups attached to it. However, only one form of butane exists because there is free rotation about the C-C single bond.
The situation differs if double bonds are present instead of single bonds. Thus at any given moment, a compound like but-2-ene may exist in two forms as represented by the following structural formulae.
In but-2-ene, the presence of the double bond between the carbon atoms hinders free rotation. So the two forms, cis- and trans- are locked in shape, giving rise to geometric isomerism. Geometric isomerisms are the existence of compounds with the same molecular formula but are not identical because of different spatial arrangement of the compound atoms.
Geometric isomers have similar chemical properties, but their physical properties are different, e.g., the trans- form of 1, 2-dicholoethene boils at 48oC while the cis-form boils at 60oC. Geometric isomers are compounds with the same molecular formula but a different orientation in space.
Geometric isomerism is common among alkenes. An alkene where each carbon atom joined by a double bond is attached to two different atoms or groups can exist in the cis- and trans-forms.
Note: When two heavy or large groups (groups with large molecular masses) are on the same side of the double bond, the molecule is said to have a cis configuration. When two such groups lie on the opposite side of the double bond, the molecule is said to possess a trans configuration.
Optical isomerism is the existence of two or more compounds with the same molecular formula but with different configurations, and because of molecular asymmetry they rotate plane polarized light. Consider white light. This vibrates in many directions (planes), but when passed through a Nicolprism, the light vibrates in one plane only and is said to be plane polarized. A compound that rotates plane polarized light is said to be optically active. For a compound to be optically active, it must have a carbon atom which is substituted by four different groups e.g. Lactic acid.
The compound and its mirror images must not be superimposable on each other. When the compound rotates the plane polarized light to the right, it is said to be dextro-rotatory and is represented by d- or (+). When the light is rotated to the left, the compound is Laevo-rotatory and is designated L- or (-).