How can we decide principal functional group in nomenclature of benzene ? please guide with help of an example.

In these structures, it is immaterial whether the single substituent is written at the top, side, or bottom of the ring: a hexagon is symmetrical, and therefore all positions are equivalent.

When there is more than one substituent, the corners of the hexagon are no longer equivalent, so we must designate the relative positions. There are three possible disubstituted benzenes, and we can use numbers to distinguish them. We start numbering at the carbon atom to which one of the groups is attached and count toward the carbon atom that bears the other substituent group by the shortest path.

The prefixortho(o-) for 1,2-disubstitution,meta(m-) for 1,3-disubstitution, andpara(p-) for 1,4-disubstitution. The substituent names are listed in alphabetical order. The first substituent is given the lowest number.