Figure 44. Three distinctly different position isomers of dichlorobenzene.

These compounds, called position isomers, have the same molecular formula (C6H4CI2) but different physical and chemical properties.

More Complex Benzenoid Compounds. Heterobenzenoid Ring Systems.

A closed circuit of six 7t-electrons confers stabilization of the ring systems of many compounds other than benzene. The rings can be a different size or contain non-carbon atoms, especially O, N and S. One or more such rings can be connected to one another. Some examples of important ring systems are shown in Figures 45a and 45b.



Figure 45a. Important benzenoid and heterobenzenoid ring systems.

quinoline indole

0 pyrimidine o pyrrole o thiophene n


thiazole co o>

Sx v

benzoxazole anthracene

Figure 45b. Important benzenoid and hetero-benzenold ring systems.

Benzene-like ring systems that contain embedded non-carbon atoms (heteroatoms) are termed heterobenzenoid or hetero-

aromatic compounds. Many therapeutically useful compounds contain such ring systems at the core of the molecular structure.

Non-Benzenoid Heterocyclic Systems

There are many kinds of cyclic structures in organic chemistry. They vary with regard to the number and type of atoms in the ring and also in the number of 71-bonds. Some examples of this variety are given in Figure 46.

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