By Arthur Winter

Aromatics are a class of ring compounds that contain double bonds. The name aromatic comes from the fact that many of the simple aromatic compounds that were first isolated were highly fragrant; the lovely odors of such substances as vanilla, almond, and wintergreen are due to the presence of aromatic compounds in these products. But many aromatic compounds are unpleasant smelling, or are odorless. Most simple aromatics, in fact, are obtained commercially from coal tar.

Aromatic compounds contain double bonds, but they don’t react like alkenes, so they’re classified as a separate functional group. Benzene, for example, doesn’t behave as if it were 1,3,5-cyclohexatriene. Although bromine reacts rapidly with alkenes to make dibromides (as shown in the figure), bromine is completely unreactive with benzene. This lack of reactivity results from aromatics being much more stable than alkenes.

The relative stability of an alkene and benzene in the presence of bromine.
The relative stability of an alkene and benzene in the presence of bromine.

Because of this greater stability, aromatic compounds require significantly more vigorous reaction conditions to make them react compared to the conditions required to react simple alkenes. Aromatic rings scoff when exposed to the relatively mild reagents that react with alkenes (reagents such as those used in hydroboration, oxymercuration, and HBr addition). Trying to react aromatics with these reagents is like shooting popguns at a well-defended castle. These reagents simply glance off the rings and remain impotent in the reaction flask.

The parent aromatic compound is benzene. Its actual structure is the hybrid of its two resonance structures, shown in the next figure. Every bond in benzene is exactly the same length — neither as long as a single bond nor as short as a double bond — and its bond character can best be described as being 1.5, the bond length between that of a single bond and a double bond.

Benzene resonance structures.
Benzene resonance structures.

Many other aromatic compounds that have the same exceptional stability as benzene are known. Some of these aromatics consist of benzene rings smooshed together to make fused rings, such as benzopyrene (shown in the next figure). These fused aromatic compounds are found in coal.

Aromatic compounds come in rings of all sizes, and many include heteroatoms (non-carbon atoms) like oxygen, nitrogen, and sulfur; aromatics that contain such heteroatoms include furan and pyridine, shown in the figure. All the DNA bases, including adenine, also shown here, have aromatic character. Because they’re so stable, aromatic compounds are ubiquitous in nature.

Some aromatic rings found in nature.
Some aromatic rings found in nature.