Nomenclature of Organic Compounds
Earlier, names were given to organic compounds on the basis of their history or the source from which they were obtained. For example, wood spirit was the name given to methyl alcohol as it was obtained by the destructive distillation of wood. Similarly, acetic acid derives its name from the Latin origin, acetum, meaning vinegar (which means containing acetic acid). Such names are called common names or trivial names. Common names generally do not indicate the structure of the compound. Moreover, the same compound may be called by different names by different chemists. With time, the number of known organic compounds increased to over a million. Therefore, there was a need for systematizing the nomenclature of organic compounds. An international conference was held in Geneva, in 1892, for evolving a systematic method for naming organic compounds. This method is referred to as the Geneva System. This was later improved by the International Union of Chemists (IUC) and International Union of Pure and Applied Chemists (IUPAC). The IUPAC laid down rules for naming organic compounds and also revised the rules from time to time, if the need arose.
Compounds of carbon and hydrogen are called hydrocarbons. They are classified as saturated and unsaturated hydrocarbons. Saturated hydrocarbons have only carbon-carbon single bonds. Saturated hydrocarbons are known as paraffins (latin word meaning little affinity) and their IUPAC name is alkanes. Unsaturated hydrocarbons are those which have at least one carbon-carbon double or a triple bond. Alkenes and alkynes are unsaturated hydrocarbons which are also called as olefins. Hydrocarbons are classified as straight chain, branched chain or cyclic hydrocarbons depending upon the nature of the skeleton of carbon atoms.
Nomenclature of Alkanes
Alkanes or paraffins are saturated hydrocarbons. The general formula of alkanes is CnH2n+2 where (n = 1,2,3,. . ., and so on). In alkanes the carbon-carbon linkage is through a single bond. In the homologous series of alkanes each member differs by a -CH2 group from its next or previous member. These have the same general formula (CnH2n+2). The chemical properties of alkanes are quite similar and their physical properties show a gradation. Each individual member of a homologous series is called a homologue.
Representation of a Molecule
A compound may be represented by different types of formulae. For example, molecular formula, structural formula, graphic formula, etc.
For example, propane can be represented as:
The Straight-Chain Alkanes
In straight-chain alkanes, carbon atoms are linked to each other in a straight fashion or head-to-tail linkage. Names of such alkanes end in "-ane". The prefix in the name of an alkane indicates the number of carbon atoms in the alkane. The prefixes of alkanes CH4 to C4H10 are derived from older common names (Table).
Table IUPAC names of some alkanes
Number of carbon atoms
In branched alkanes, all the carbon atoms are not present in a linear portion. Some may be present as a branch to the main (parent) chain. Such compounds are also alkanes as they have the same general formula CnH2n+2
In order to name such alkanes, names of suitable alkyl groups are added before the name of the straight chain alkane. An example of a branched-chain alkane is;
Alkyl groups are monovalent radicals. They are derived from alkanes by the removal of hydrogen atoms. In general, alkyl groups are represented as Râ( here RâH is the alkane). The formulae of some common alkyl groups and their names are given in Table 7.3