Some functional groups such as alcohols, ketones, amines,
alkenes, and alkynes may be bonded to any of several carbon atoms
in a carbon chain. These compounds are positional isomers. In
order to be able to precisely define the position of these functional
groups and also the carbon branches, a number is assigned to
The numbers should be assigned so that the carbon with
the functional group or branch has the LOWEST number.
The numbers may go from either left to right or vica versa.