In the following compound, according to the rules of IUPAC nomenclature, the groups designated as A, B, C will be numbered respectively as:
5, 1, 2
The given compound has a cyclohexane ring with three substituents.
Where
A = - CH3 (methylgroup)
B = - OH (hydroxyl or alcohol group)
|C=CH3−CH−CH3(isopropyl group)
Clearly -OH is the principal functional group. The substituent C could be named in 2 ways
1. isopropyl
2. (1-methyl ethyl)
Either way, the carbon attached to -OH gets numbered 1. Substituents C and A will be numbered 2 and 5 respectively according to lowest set of locants rule. After numbering the carbon attached to -OH as '1', the numbering proceeds in a way to assign the lowest number to the carbon at which the first point of difference occurs.