Families of compounds which have similar structures can form homologous series - each member of the series shares similar chemical properties and related formulae.

There are 7 ways of representing the structure of an organic molecule


Isomerism

Isomerism occurs because atoms in a molecule have choices of where to bond. Isomers are molecules with the same molecular formulae but different arrangements in space. There are two main types of isomerism - structural isomerism (2 or more different structures) and stereoisomerism (only two different structures).

Chain isomerism occurs when there are two or more ways of arranging the carbon skeleton of a molecule

With position isomerism, the isomers have the same carbon skeleton but the functional group is attached at different places on the chain.

Functional group isomerism occurs when different functional groups are present in compounds which have the same molecular formula.

Geometrical isomerism occurs because of restricted rotation at the C=C bond, cis and trans forms (geometrical isomers) occur when there is suitable substitution of one or more of the hydrogen atoms. It is not possible to have geometrical isomerism when there are two identical groups joined to the same carbon atom in a double bond. Restricted rotation about a C=C bond arises due to the interaction between the two adjacent p-orbitals of the carbon atoms, forming a zt-bond. Disruption of the zt-bond requires significantly more energy than is available at room temperature.



Optical isomerism occurs when four different atoms or groups are attached to a carbon atom, the molecule has no centre of symmetry, plane of symmetry or ands of symmetry. The molecule is said to be chiral and to possess an asymmetric carbon atom. Two tetrahedral arrangements in space are possible so that one is the mirror image of the other.

Stereoisomers of this kind are known as enantiomers. It is not possible to superimpose one enantiomer on the other. Enantiomers have the same physical properties except for their effect on the plane of plane-polarised light and, because of this difference, are said to be optically active. When plane-polarised light, which is made up of waves vibrating in one plane only, passes through a solution of a chiral compound, the light emerges with its direction of polarisation changed. One enantiomer will rotate the plane of plane-polarised light in a clockwise direction; it is termed (+) or dextrorotatory. Its mirror-image form will rotate the plane of plane-polarised light by the same amount in a counterclockwise direction; it is termed (-) or laevorotatory. A mixture of equal amounts of both enantiomers is optically inactive because the two effects cancel out. Such a mixture is called a racemic mixture or a racemate.