Nomenclature of Substituted Benzene Compounds

Naming Substituted Benzene Compounds

When a single substituent is attached to a benzene ring, the IUPAC name is formed by placing the substituent name as a prefix before the word "benzene". So chlorobenzene, bromobenzene, nitrobenzene, methylbenzene, methoxybenzene, and aminobenzene are all straightforward IUPAC names.

However, many of these compounds have common names so deeply rooted in chemical tradition that they are universally accepted and used. Methylbenzene is almost always called toluene, methoxybenzene is anisole, and aminobenzene is aniline. These common names are so standard that they often appear in exam questions and in the chemical literature as if they were official IUPAC names.

Disubstituted Benzene: Numbering and the Lowest Locant Rule

When two substituents are present on the benzene ring, you must number the carbon atoms of the ring so that the substituents receive the lowest possible set of locant numbers. Consider the three isomers of dibromobenzene. The compound with bromine atoms on adjacent carbons is numbered as 1,2-dibromobenzene. The isomer with one carbon between the bromines is 1,3-dibromobenzene — not 1,5-dibromobenzene, because 1,3 gives a lower sum of locants (4) than 1,5 (6). The isomer with bromines opposite each other is 1,4-dibromobenzene.

The lowest locant rule is simple: you number the ring in the direction that gives the smaller number to the second substituent. For a disubstituted ring, the first substituent is always at position 1, and you then number clockwise or anticlockwise to give the next substituent the smallest possible number.

The Ortho-Meta-Para System

In the trivial (common) system of nomenclature, the relative positions 1,2; 1,3; and 1,4 are indicated by the prefixes ortho, meta, and para respectively. These are abbreviated as o-, m-, and p-.

So 1,2-dibromobenzene is o-dibromobenzene, 1,3-dibromobenzene is m-dibromobenzene, and 1,4-dibromobenzene is p-dibromobenzene. This system is only used for disubstituted benzene rings. Once you have three or more substituents, the ortho-meta-para prefixes become ambiguous and cannot be used.

A common mistake is to use ortho, meta, para for trisubstituted benzenes. This is incorrect — these prefixes only describe the relationship between two substituents, and with three or more substituents the relative positions are no longer uniquely defined by these three terms.

Tri- and Higher Substituted Benzene Derivatives

For compounds with three or more substituents on the benzene ring, you must use the lowest locant rule systematically. The procedure is:

1. Identify the parent compound. In many cases, the common name of a monosubstituted benzene derivative (toluene, anisole, aniline, phenol) is taken as the base compound.
2. The substituent that defines the parent compound is assigned number 1.
3. Number the ring in the direction that gives the next substituent the lowest possible number.
4. List all substituents in alphabetical order in the name, ignoring any numerical prefixes like di-, tri-, etc.

Consider the compound 1-chloro-2,4-dinitrobenzene. The parent is benzene. The numbering starts at the chlorine atom (position 1) and proceeds so that the two nitro groups are at positions 2 and 4. If you numbered in the opposite direction, the nitro groups would be at positions 3 and 5, which gives higher locants. The name is not 4-chloro-1,3-dinitrobenzene because that would give a higher set of locants for the nitro groups.

Now take 2-chloro-1-methyl-4-nitrobenzene. Here the parent is toluene (methylbenzene). The methyl group is at position 1. The ring is numbered so that the chlorine gets position 2 and the nitro group gets position 4. The name 4-methyl-5-chloronitrobenzene would be incorrect because it does not use the parent name toluene and the numbering is not based on the lowest locant rule applied to the substituents in alphabetical order.

For 2-chloro-4-methylanisole, the parent is anisole (methoxybenzene). The methoxy group is at position 1. The chlorine is at position 2 and the methyl group at position 4.

For 4-ethyl-2-methylaniline, the parent is aniline (aminobenzene). The amino group is at position 1. The ethyl group is at position 4 and the methyl group at position 2. Note that ethyl comes before methyl alphabetically, so ethyl is listed first even though it has a higher locant number.

For 3,4-dimethylphenol, the parent is phenol. The hydroxyl group is at position 1. The two methyl groups are at positions 3 and 4.

Benzene as a Substituent: The Phenyl Group

When a benzene ring is attached to an alkane that already contains a functional group, the benzene ring is treated as a substituent rather than the parent. The name for the benzene ring as a substituent is phenyl, abbreviated as Ph, with the formula $C_6H_5-$. …