Basic Organic Nomenclature
There are five isomers of hexane, C6H14:
Inspection of these isomers shows that the two methylpentanes (#2 and #3) and the two dimethyl pentanes (#4 and #5) differ only in the positions on the main chain of the methyl groups. Such isomers are termed positional or constitutional isomers and as illustrated by the names of these pairs of isomers, different constitutional isomers are distinguished by numbering the main chain.
The carbon atoms in the main chain are numbered and the branches are prefixed with the appropriate number (the locant) as a position identifier.
The numbering of the main chain is done in the direction which gives the lowest number to the first branching point.
The position identifier (locant) is prefixed to the name of the branch using hyphens to separate the position identifiers from the name. If there are two or more of the same branch in a given molecule the prefixes di, tri, tetra, etc., are used together with a set of position identifiers, one for each of the branches, with the position identifiers separated by commas.
Note that each and every branch must have a position identifier, even if it requires repeating the identifier as in 2,2-dimethylbutane.
Applying these rules:
#2 is 2-methylpentane and not 4-methylpentane,
#4 is 2,2-dimethylbutane and not 3,3-dimethylbutane.
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