- There are only 3 isomers for any particular disubstituted benzene because of the symmetry of
the planar ring. To identify these, the IUPAC system prefers the use of numbers: 1,2; 1,3; 1,4.
However, there is in very common use another system which uses the prefixes:
ortho-, or o- for the 1,2 isomer;
meta-, or m- for the 1,3 isomer;
para-, or p- for the 1,4 isomer;
- Unique common names are used for some disubstituted benzenes...they need to be learned!
Also: meta- and para-xylene
Also: ortho- and para-cymene
Also: ortho- and meta-toluidine
Also: ortho- and meta-cresol
The 1,3 derivative is isophthalic acid, and the 1,4 derivative is terephthalic acid
Only used with this isomer
The 1,2 derivative is pyrocatechol, and the 1,3 derivative is resorcinol
- If no common name is used for the disubstituted benzene (as above),
when a common name is used with one or both of the substituents,
the substituent of higher preference
is used to give the root name and the other substituent is named in a prefix.
2-nitrobenzoic acid, o-nitrobenzoic acid