17.1: Aromatic Compounds: Overview

In general, the term ‘aromatic’ indicates a pleasant smell or fragrance from fresh flowers, freshly prepared coffee, etc. In the early history of organic chemistry, many benzene derivatives were isolated from the pleasant odor oils of the plants. For example, vanillin was isolated from the oil of vanilla, methyl salicylate from the oil of wintergreen, and cinnamaldehyde from the oil of cinnamon. They all had a pleasant odor; hence the name aromatic was given.

In 1825, Faraday isolated benzene from compressed illuminating gas (the fuel burned in gaslights) and named it “bicarburet of hydrogen.” He also performed an elemental analysis of the compound and observed a 1:1 ratio of hydrogen to carbon.

Later, Eilhardt Mitscherlich synthesized benzene from benzoic acid (isolated from gum benzoin) and calcium oxide in 1834. By conducting vapor density measurements, he showed the molecular formula of benzene to be C₆H₆. Since the new compound was derived from gum benzoin, he named it Benzin, now known as benzene. It is used as a laboratory solvent.

The term ‘aromatic’ is still used to describe all benzene derivatives, whether fragrant or not. Many of the natural as well as pharmaceutical compounds are aromatic. For example, estrone is a steroid, atorvastatin (Lipitor) is a cholesterol-lowering drug, Zoloft is an antidepressant, and Serevent used to treat asthma, are aromatic compounds.

Aromatic compounds are primarily obtained from coal and petroleum. Coal produces coal tar when heated to 1000 °C in the absence of air. Coal tar, upon fractional distillation, produces many aromatic hydrocarbons such as benzene, toluene, dimethyl benzene (xylene), naphthalene, biphenyl, indene, anthracene, etc.

However, aromatic compounds can not be obtained directly from petroleum. During petroleum refining, the alkanes are converted into aromatic compounds in the presence of a catalyst at 500 °C under high pressure.

Aromatic compounds are hydrocarbons with conjugated π bonds enclosed in a ring. Benzene and its derivatives are the best-known examples of aromatic compounds.

Originally, benzene derivatives were isolated from natural oils produced by plants and trees and were called 'aromatic' due to their distinctive odor.

For example, benzaldehyde from almonds and cherries, toluene from Tolu balsam, and eugenol from clove have distinct odors.

Over time, the term aromatic has continued to describe all benzene derivatives, irrespective of whether they are fragrant or not. For example, the pain reliever aspirin contains acetylsalicylic acid, an odorless aromatic compound.

Structurally, aromatic compounds are highly unsaturated hydrocarbons and exhibit different chemical behavior from alkenes, alkynes, and open-chain polyenes.

The primary sources of simple aromatic compounds are coal tar and petroleum.

Fractional distillation of coal tar yields many aromatic compounds such as benzene, toluene, xylene, naphthalene, and others.

Similarly, during petroleum refining, alkanes are converted to aromatic compounds.