An Introduction to Environmental Chemistry

constituent of natural gas (Fig. 2.4a). When all the carbon atoms in a compound

are joined by single bonds (Section 2.3.1) the structure is called aliphatic or

saturated. Simple straight-chain aliphatic hydrocarbons are known as normal

alkanes (n-alkanes), for example ethane (Fig. 2.4b). Molecules that contain one

or more double-bonded (Section 2.3.1) carbon atoms are known as unsaturated

molecules. Ethene is an example (Fig. 2.4c), but you are probably more familiar

with the term unsaturated in relation to fats in foods. A particularly stable struc-

ture forms when double-bonded (Section 2.3.1) carbon atoms alternate with

single bonded carbon atoms, i.e.:

C—C==C—C==C—C==C

24 Chapter Two

(a) Methane

H – C – H

H

H

(b) Ethane

H – C – C – H

H

H

H

H

(c) Ethene

C = C

H

H

H

H

(d) Benzene and simplified

representation (benzene ring)

C = H

C C

H

H

=

C H

C =C

H

H

or or

(e) Benzo[a]pyrene

Fig. 2.4Organic molecules. (a) Methane. (b) Ethane. (c) Ethene—note the double bond.

(d) Benzene—the double bonds become delocalized so may be symbolized by a circle. It is

conventional to omit the H atoms from pictorial representations of benzene. Compounds

based on benzene are called aromatic compounds, for example (e) benzo[a]pyrene—a

polycyclic aromatic hydrocarbon (PAH). The benzene ring is particularly stable, which

enables it to be the building block of larger molecules such as this one. See also Figs 3.4, 4.28

and 4.34.