15.1 Naming Aromatic Compounds

Naming Conventions for Aromatic Compounds

Naming conventions for benzene compounds

For monosubstituted benzene, you simply prefix the substituent name to "benzene." Chlorobenzene, nitrobenzene, and methylbenzene all follow this pattern. Some monosubstituted benzenes have common names that are accepted by IUPAC: methylbenzene is almost always called toluene, hydroxybenzene is phenol, aminobenzene is aniline, and so on. You'll need to memorize these common names because they show up constantly as parent names.

Disubstituted benzene compounds use positional prefixes to show where the two groups sit relative to each other:

• ortho- (o-) = 1,2-positions, substituents on adjacent carbons (e.g., ortho-dichlorobenzene or 1,2-dichlorobenzene)
• meta- (m-) = 1,3-positions, substituents separated by one carbon (e.g., meta-dinitrobenzene or 1,3-dinitrobenzene)
• para- (p-) = 1,4-positions, substituents on opposite carbons (e.g., para-chlorotoluene or 1-chloro-4-methylbenzene)

When the two substituents are different, alphabetize them in the systematic name. Write 1-chloro-4-methylbenzene, not 4-methyl-1-chlorobenzene. Also keep in mind that three positional isomers (ortho, meta, para) exist for any disubstituted benzene, so specifying the relationship is never optional.

IUPAC rules for complex aromatics

When a benzene ring has three or more substituents, the ortho/meta/para system no longer works and you must use numbered positions. Here's the general procedure:

1. Identify the parent aromatic structure (benzene, toluene, phenol, naphthalene, etc.). If a common parent name applies, use it.

2. Number the ring carbons so that the substituents receive the lowest possible set of locants. If a common parent name fixes one substituent's position (e.g., the $-OH$ in phenol is always C1), start numbering there.

3. List substituents alphabetically in the name, ignoring multiplying prefixes like di- and tri- when alphabetizing.

4. Use multiplying prefixes for identical substituents:

   • di- for two (1,4-dichlorobenzene)
   • tri- for three (1,3,5-trihydroxybenzene, commonly called phloroglucinol)
   • tetra- for four (1,2,4,5-tetramethylbenzene, commonly called durene)

A few worked examples to make this concrete:

• 2-bromo-1,4-dimethylbenzene: Two methyl groups at positions 1 and 4, one bromine at position 2. "Bromo" comes before "dimethyl" alphabetically (ignore the "di-").
• 3-ethyl-1-fluoro-2-methylbenzene: Three different substituents, listed as ethyl → fluoro → methyl, with numbering chosen to give the lowest set of locants.

Structures from systematic names

Going from a name to a drawn structure is a skill you'll use on every exam. Follow these steps:

1. Find the parent name at the end of the word (benzene, naphthalene, pyridine, phenol, aniline, etc.) and draw that ring skeleton.
2. Read off the substituents and their locants from the prefixes and numbers.
3. Place each substituent on the correct carbon of the ring.

Three examples, worked step by step:

para-Ethylphenol (4-ethylphenol)

1. Draw a benzene ring. The parent is phenol, so place $-OH$ at C1.
2. "para" means position 4. Attach $-CH_2CH_3$ at C4.

2,4,6-Tribromoaniline

1. Draw a benzene ring. Aniline means $-NH_2$ is at C1.
2. Place $-Br$ at C2, C4, and C6.

3-Chloropyridine

1. Draw a pyridine ring (six-membered ring with nitrogen at position 1).
2. Place $-Cl$ at C3.

Polycyclic Aromatic Compounds

Polycyclic aromatic hydrocarbons (PAHs) contain two or more fused benzene rings sharing edges. The most common ones to know:

• Naphthalene: two fused rings (10 carbons, numbered 1–8 around the perimeter; the shared carbons are labeled 4a and 8a)
• Anthracene: three rings fused in a straight line
• Phenanthrene: three rings fused in an angled arrangement

Numbering in polycyclic systems follows IUPAC conventions where shared (fusion) carbons do not get their own primary numbers. The orientation of the fused rings matters because it changes which carbons are available for substitution and how they're numbered.

More complex PAHs use bracket notation in their names. For example, benzo[a]pyrene tells you a benzo ring is fused at the "a" bond of pyrene. You'll encounter these in discussions of carcinogenicity and environmental chemistry, but for naming purposes, the key idea is that the bracket specifies where the extra ring is fused onto the base structure.