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AROMATIC COMPOUNDS

Aromatic compounds are a special subset of carbon compounds with a unique mode of bonding. The most commonly encountered aromatic compounds are the ones based on a six-member carbon ring, benzene, as shown in Figure 3.9.

In the depiction of benzene, the simplest aromatic compound, it is noted that there are three double bonds and three single bonds shown between the carbons. The three double bonds could be arranged to be between two different sets of carbons and retain the same general structure. In reality, neither structure is true, but rather all bonds in the molecule are about 1.5 bonds (strange as the nonintegral bond count may be). This effect is called resonance, and gives the molecule extra stability.

A benzene ring. This six-carbon ring is the basis of many common aromatic compounds

Figure 3.9. A benzene ring. This six-carbon ring is the basis of many common aromatic compounds

The rules for naming aromatic compounds are generally the same as for aliphatic compounds. The locations of substituents are identified with a number. If there are multiple options, the lowest possible numbers to be used is the preferred option. If different types of substituents exit, alphabetical order is preferred.

Example 3.2

Name the following aromatic compounds.

Aromatic substitutions

Figure 3.10. Aromatic substitutions

Solution

The first molecule shown in Figure 3.10 would be chlorobenzene. No distinction is necessary beyond that, as the chlorine could be on any of the carbons, and the molecule would still be the same. The second is 1,3-dichlorobenzene. The third is 1,4-bromochlorobenzene.

In some cases older nomenclature is used, with the words meta, para, and ortho indicating the locations of substituents. Table 3.5 lists the correlation between the different types of nomenclature.

Table 3.5. Substitution patterns in aromatic compounds

Name

Pattern

Use

Structure

ortho

1,2 substitution

o-Dibromobenzene

Name

Pattern

Use

Structure

meta

1,3 substitution

те-Dichlorobenzene

para

1,4 sub- stitu- tion

j^-Dichlorobenzene

Other substituents are also named as in the preceding cases for aliphatics.

 
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