Naming and drawing benzene derivatives
Substituted benzenes, ring numbering, ortho/meta/para, and parent names such as phenol, aniline, benzoic acid and toluene.
Benzene, C6H6, is a six-membered ring usually drawn with three alternating double bonds. Both ways of placing those double bonds describe the same molecule. They are two resonance contributors of one structure in which all six C–C bonds are equivalent. The practice checker accepts either Kekulé form or aromatic bonds.
One substituent
Name the substituent as a prefix to "benzene": chlorobenzene, nitrobenzene, ethylbenzene. All six ring positions are equivalent, so no locant is needed. Several monosubstituted benzenes have retained names that are used almost universally:
| Retained name | Substituent | Systematic description |
|---|---|---|
| toluene | –CH3 | methylbenzene |
| phenol | –OH | hydroxybenzene |
| aniline | –NH2 | benzenamine |
| benzoic acid | –COOH | benzenecarboxylic acid |
| benzaldehyde | –CHO | benzenecarbaldehyde |
Two or more substituents
Number the ring to give the substituents the lowest locants; if two numberings tie, the substituent first in alphabetical order gets C1. Two substituents can be 1,2, 1,3 or 1,4 to each other, also described as ortho (o-), meta (m-) andpara (p-).
Worked example: 1-bromo-4-chlorobenzene
The locant set is {1,4} whichever substituent starts the numbering, so the alphabetical rule decides: bromo gets C1 and chloro C4. Draw them on opposite corners of the hexagon.
When the parent is phenol, aniline or benzoic acid
If the name is built on a parent like phenol, aniline or benzoic acid, the group that defines the parent is always on C1, and other substituents are numbered from it. 4-Methylphenol has the CH3 opposite the OH; 3-nitroaniline has the NO2 meta to the NH2.
A nitro group is usually drawn with a positively charged nitrogen, one N=O and one N–O−. The shorthand with two N=O bonds describes the same group, and the checker accepts both.
Phenyl and benzyl
As a substituent, C6H5– is a phenyl group. C6H5CH2–, with an extra CH2, is a benzyl group. Phenylmethanol (benzyl alcohol) is methanol carrying a phenyl group.
Common mistakes
- Drawing cyclohexane. A benzene ring needs its three double bonds (or aromatic bonds); without them the formula has six extra hydrogens.
- Miscounting around the ring. Para is directly opposite C1; meta skips one carbon.
- Counting the substituent carbon in the ring. In benzoic acid the COOH carbon is outside the ring: seven carbons in total.
- Confusing phenyl and benzyl. Benzyl has one extra CH2 between the ring and the rest of the molecule.