Aromatic Hydrocarbons

I. History

Benzene and all other substances whose structures resemble benzene and have similiar chemical properties are classified as aromatic hydrocarbons. In the past, the pleasant odors associated with these newly discovered substances resulted in theirbeing called aromatic compounds. However, this is not a valid concept today.

Some of the older aromatic hydrocarbon names have been so widely used that the IUPAC accepted the common names in place of the IUPAC rule based names..This can be confusing to a beginning organic chemistry student. I recommend that you memorize the common names of these historical exceptions with special names.

II. Arenes

A. Substituted benzene compounds which contain alkyl groups with fewer than six (6) carbon atoms.

B. Rules for naming arenes:

1. If there is only one substituent group on the ring, no number is needed since it will always be on the first carbon atom of the ring.

2. Some monosubstituted benzenes are named by adding the name of the substituted group as a prefix to the word, benzene. These follow the IUPAC rules for naming.

	Chlorobenzene			Nitrobenzene

3. Other substituted benzenes are exceptions to the above IUPAC rule since special names are used for them.

NOTE: MEMORIZE the names of these compounds!

Color codes:

Aniline				Benzaldehyde			Benzoic acid
Aminobenzene				Benzene carboxaldehyde			Benzene carboxylic acid

Phenol				Styrene			Toulene			ortho-Xylene
Hydroxybenzene				Vinylbenzene			Methylbenzene			ortho-dimethylbenzene

4. When the benzene ring has two substituted groups attached to it, three isomeric forms of the compound are possible. These positional isomers are given the prefixes: ortho- , para- and meta- respectively.

ortho-Xylene				meta-Xylene			para-Xylene
ortho-dimethylbenzene				meta-dimethylbenzene			para-dimethylbenzene

5.When the two substituted groups are the same and neither is part of a compound with a special name, the prefix, di-, is used after the positional isomer name and before the parent name benzene.

0-dichlorobenzene				m-dichlorobenzene			p-dichlorobenzene

6. When the two substituted groups are different and neither is part of a compound with a special name, the names of the two substituted groups are given in alphabetical order followed by the parent name benzene.

o-bromochlorobenzene					p-bromochlorobenzene		m-bromochlorobenzene

7. When one of the two substituted groups results in a monosubstituted benzene that requires a special name, the substituted compound is named as a derivative of the special named parent.

0-chlorophenol				p-chlorophenol			m-chlorophenol

8. When three or more substituent groups are on a benzene ring, the carbon atoms in the ring may be numbered clockwise or counterclockwise starting with one of the substituted groups. The numbering must be done in the direction which results in the lowest possible numbers for the substituted groups. If the compound is named as a derivative of a special named parent, the substituted group that makes it a special named compound is always located on the first carbon atom of the ring.

1,2,5-trichlorobenzene				2,4,6-trinitrotoluene (TNT)

9. Polycyclic aromatic hydrocarbons or "fused-ring" aromatics.

Naphthalene			Anthracene		Phenanthrene
C10H8			C14H10		C14H10

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