Basic Organic Nomenclature
|Since carbon requires four bonds to other atoms,
it is possible for compounds to form in which more than two other
carbon atoms are bonded to a carbon: up to four other carbons, of
Note: It is sometimes convenient to refer to a carbon atom as primary (1�), secondary (2�), tertiary (3�) or quaternary (4�) according to the number of other carbon atoms directly bonded to it; further, this designation is extended to the non-carbon atoms (or groups) attached to that carbon. Thus a 3� carbon has three other carbons bonded to it, and, for an alkane, one hydrogen which by extension is termed a 3� hydrogen.
This possibility of having the same number and type of atoms bonded together in different ways is called isomerism: isomers are compounds having the same molecular formula but different atomic arrangements. In the alkane series, the number of possible isomers increases dramatically with number of carbons as shown:
The IUPAC system of nomenclature deals with the presence of isomers in a very easy way.
The smallest alkane to exhibit isomerism is butane with 2 isomers:
Isomer A has the four carbons in a chain and so is butane.
Isomer B has three carbons in a chain and the fourth carbon branching off the centre. To name compounds such as this one, with branches, the IUPAC system:
Work through the steps to come up with the following names for the three isomers of pentane:
Note that close inspection of the static line forms:
using the dynamic model:
This topic continues with the names of the 5 isomers of hexane and introduces the numbering of the main chain.
Next page: Branched Alkanes (ii)
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