It is easy to imagine this structure bent around upon itself into a ring or cycle,
such that the first carbon atom in the chain also bonds with the sixth carbon atom
(Fig. 2.4d). These six-carbon cyclic structures are very common and are known
as aromatic compounds as many of them have a distinctive odour. Benzene is
the simple hydrocarbon with this ring structure (Fig. 2.4d); it can be formally
described as an unsaturated aromatic hydrocarbon. It is possible for stable aro-
matic rings of this type to be fused together into larger multiple ring structures.
These complex but stable structures are known as polycyclic. The polycyclic aro-
matic hydrocarbons or PAHs (e.g. benzo[a]pyrene, Fig. 2.4e) are becoming quite
familiar in well-publicized cases of urban air pollution (see Section 3.7). The pres-
ence of aliphatic or aromatic hydrocarbon groups in a molecule are often denoted
by the symbol, R, when the specific identity of the molecule is unimportant (e.g.
see eqns. 4.9 & 4.10).
To simplify the depiction of organic molecules it is usual not to label each
carbon and hydrogen atom of the basic structure. Hydrogen atoms are usually
omitted, while the position of each carbon atom in the ‘skeleton’ is indicated by
the change in angle of the line drawing (Fig. 2.4d). Also, quite often, the bond
system of aromatic rings is represented as a circle (Fig. 2.4d). This shows that the
bonds are delocalized, i.e. the bonds between all the six carbon atoms are iden-
tical and intermediate in strength between single and double bonds.
2.7.1 Functional groups
Atoms of other elements, typically oxygen, nitrogen and sulphur, are incorpo-
rated into the basic hydrocarbon structures, usually as peripheral components
known as functional groups (Table 2.1). Each functional group confers specific
properties on the compound, and can be a major factor in determining the chemi-
cal behaviour of the compound. Functional groups include the hydroxyl (–OH),
carboxyl (–COOH), amino (–NH 2 ) and nitro groups (–NO 2 ). The –OH and
Environmental Chemist’s Toolbox 25Table 2.1Important functional groups in environmental chemistry. Modifed from Killops
and Killops (1993). Reproduced with kind permission of the authors.
Symbol Group name Resulting compound name
R—OH Hydroxyl Alcohol (R=aliphatic group)
Phenol (R=aromatic group)
—C =O Carbonyl Aldehyde (R =H)
|
R Ketone (R=aliphatic or aromatic group)
—C =O Carboxyl Carboxylic acid
|
OH
—NH 2 Amino Amine
—NO 2 Nitro
—O— Oxo Ether*- Ethers make bonds in biopolymers such as cellulose (see Box 4.11).