An Introduction to Environmental Chemistry

–COOH functional groups, for example, increase the polarity of the molecule

(see Box 4.14), increasing its aqueous solubility. Organic molecules are often

described collectively by the functional groups they contain, for example the car-

boxylic acids all contain –COOH, an example being ethanoic (acetic) acid:

The word acid indicates that these substances act as a weak acid (see Box 3.3) in

water, releasing H+by dissociation from the carboxyl group (see Box 4.5). Some

molecules contain more than one functional group, for example the amino acids

contain both –COOH and –NH 2 :

Amino acids also behave as weak acids due to the –COOH group, so they can

release H+ions by dissociation, for example in the simple amino acid glycine:

eqn. 2.10

However, the amino functional group (–NH 2 ) is also able to accept an H+ion and

act as a base (see Box 3.3), i.e.:

eqn. 2.11

The amino acids thus have the unusual ability to form dipolar ions, denoted by

the +sign on the amino group and the—sign on the COO. This property makes

amino acids highly soluble in polar solvents like water or ethanol, each polar end

attracted to the suitable solvent (see Box 4.14).

2.7.2 Representing organic matter in simple equations

Organic matter is implicated in many chemical reactions in natural environments.

Organic matter is typically a mixture of materials derived from a number of dif-

ferent plants, which are themselves composed from a variety of complex biopoly-

mers (see Box 4.10). The precise chemistry of organic matter in a soil or sediment

is thus not usually known. This problem is usually avoided by using simple com-

pounds such as carbohydrates to represent organic matter. Carbohydrates have

the general formula Cn(H 2 O)n(where n is an integer), which shows they contain

only carbon, hydrogen and oxygen, the latter two elements in the same ratio as

in water. In this book organic matter is often represented by the generic formula

CH 2 O. A specific example might be the sugar glucose, i.e.:

H++NH 2 CH 2 COO-Æ+NH 3 CH 2 COO-

NH 2 CH 2 COOH

glycine

ÆH++NH 2 CH 2 COO-

C

H

C

H O

OH

carboxyl group

N

H

H

amino group

HC

H

C

H O

OH

carboxyl group

26 Chapter Two