In the following compound, according to the rules of IUPAC nomenclature, the groups designated as A, B, C will be numbered as:
5, 1, 2
The given compound has a cyclohexane ring with three substituents.
A =− CH3 (methyl group)
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
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.