Disubstituted benzenes

Basic IUPAC Organic Nomenclature

When there are two (or more) substituents, the relative position of the subsituents must be defined.
There are two methods used based on either numerical locants or specific words for the three possible forms:
The words can also be shortened to their first letter, i.e. o-, m- and p-
The terms ortho-, meta- or para- (or their singel letter equivalents) are used as prefixes.
These terms are ONLY used for benzene systems.
When using numerical locants, the principal functional group is defined to be at C1.
The numerical locant method is also applicable to other aromatic systems.

Numerical locants method:

Principle functional group is the benzne therefore root = benzene There are two chlorine substituents therefore dichloro The substituent lowest locants = 1 and 2 1,2-dichlorobenzene
Principle functional group is the benzne therefore root = benzene There is a chlorine substituent therefore chloro There is a bromine substituent therefore bromo Numbering from the -Br (due to alphabet priority) clockwise to give the chlorine substituent the lowest locant = 3 1-bromo-3-chlorobenzene
Principle functional group is the aromatic alcohol therefore = phenol The substituent is a chlorine therefore = chloro Numbering from the -OH, counterclockwise to give the substituent the lowest locant = 2 2-chlorophenol

Ortho / meta / para method:

Principle functional group is the benzne therefore root = benzene There are two chlorine substituents therefore dichloro The substituent locants are 1 and 2 therefore ortho ortho-dichlorobenzene or o-dichlorobenzene
Principle functional group is the benzne therefore root = benzene There is a chlorine substituent therefore chloro There is a bromine substituent therefore bromo The substituent locants are 1 and 3 therefore meta meta-bromochlorobenzene or m-bromochlorobenzene
Principle functional group is the aromatic alcohol therefore = phenol The substituent is a chlorine therefore = chloro The substituent locants are 1 and 2 therefore ortho ortho-chlorophenol or o-chlorophenol