Chemistry, Third edition

AROMATIC HYDROCARBONS 327

Fused ring aromatic compounds

Aromatic rings that share a pair of carbon atoms are said to be fused. Some

examples are

Napthalene has been used in mothballs. All these compounds are obtained from

coal tar. They are also called polyaromatic hydrocarbons(PAHs). Polyaromatic

hydrocarbons are found abundantly in the environment. They originate from many

sources including cigarette smoke and car exhaust gases. There is cause for concern

because some PAHs are considered to be toxic.

napthalene anthracene phenanthrene

BOX 17.9

Friedrich Kekulé

German chemist Friedrich Kekulé

(1829–1896) was Professor of Chemistry at

the universities of Ghent and Bonn. He

proposed the concept of the ‘carbon chain’,

or the catenation of carbon, as the basis of

many organic compounds. In the 1860s, he

turned his attention to aromatic compounds.

Benzene had been identified by Faraday in

1825. It was known that the empirical
      formula of benzene was C 6 H 6 , but its
      structure remained a mystery until Kekulé
      proposed his cyclic structure in 1865.

Some possible structures that have the same

molecular formula are shown alongside. None

of these was acceptable. One reason is

because of the reluctance of benzene to

undergo addition reactions. See if you can

think of any more possibilities for the

structure of C 6 H 6.

Kekulé claimed that the answer to the puzzle

of the structure of benzene came to him in a

dream. In his dream, groups of six carbon

atoms turned into snakes that swirled around

his laboratory. Suddenly, the snakes formed

circles. In this way the ‘benzene rings’ were

revealed to the scientist.

A postage stamp showing Kekulé and his

proposed structure of benzene. He claimed

that the structure came to him in a dream.