Organic compounds are conveniently classified as Aliphatic, meaning 'fatty', or Aromatic, meaning 'fragrant'. The literal meaning of these terms has little or no significance now as compounds are categorised much more precisely according to their molecular structure and properties.
Aliphatic compounds are those possessing open chains of carbon atoms or, alternatively cyclic compounds whose structure and properties resemble such open-chain compounds.
Aromatic compounds are those possessing the ring structure of benzene (or other molecular structures that resemble benzene in electronic configuration and chemical behaviour). There are many compounds that, at first appearance, bear little resemblance to benzene, but have a basic similarity in electronic configuration. However, at this stage, it is adequate to interpret character in terms of the benzene ring structure, since this definition incorporates most of the commonly encountered aromatic substances.
It would be impossible to study each organic compound individually; no one would live long enough. Rather, scientists strive to organise knowledge. The chemist classifies organic compounds by structural and chemical similarities. Simple organic compounds are categorised according to their Functional Groups. These are groups of atoms or bonds common to a series or family of compounds and which decide the principal chemical properties of the series. Such a series is called a Homologous Series, and can be represented by a general formula.
The Alkanes, for example, are a series of hydrocarbons with the general molecular formula CnH2n+2. Successive homologues differ in composition by the increment -CH2-. Removing an H atom from an alkane leaves an Alkyl group with the general formula CnH2n+1-, also represented by the letter R-. This group is common to most aliphatic homologous series. Each alkyl group is named by replacing the suffix ane with yl.