- 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;
Examples:
1,2-dichlorobenzene, o-dichlorobenzene
1,3-dibromobenzene, m-dibromobenzene
1-bromo-4-chlorobenzene, p-bromochlorobenzene
1,3-dinitrobenzene, m-dinitrobenzene
- Unique common names are used for some disubstituted benzenes...they need to be learned!
o-xylene
Also: meta- and para-xylene
m-cymene
Also: ortho- and para-cymene
p-toluidine
Also: ortho- and meta-toluidine
p-cresol
Also: ortho- and meta-cresol
phthalic acid
The 1,3 derivative is isophthalic acid, and the 1,4 derivative is terephthalic acid
anthranilic acid
Only used with this isomer
hydroquinone
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
3-aminophenol, m-aminophenol
4-methoxyaniline, p-methoxyaniline
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