Chemistry, Third edition

SUBSTITUTED BENZENE DERIVATIVES 345

The structures of proteins can be studied on a number of levels:

1.Theprimary structureof a protein is the order in which the amino acid residues

are linked together to form long chains.

2.Thesecondary structureof a protein is the way in which the chains are arranged

in space to form coils, sheets or almost spherical shapes. The chains are held

together by hydrogen bonds (see page 77). X-ray diffraction has proved to be a

valuable technique in determining the often complicated secondary structure of

proteins.

An example of a protein which has a well studied secondary structure is

keratin, found in hair. Each protein molecule in keratin is arranged in the shape of

a spiral, called a helix. Hydrogen bonding holds the helix together, by linking to

different sections of the same chain. The arrangement of the spiral in keratin is

called an alpha helix. This means that it is a helix with a right-handedturn, as in

Fig. 18.4.

3.Thetertiary structureof a protein is a description of further interactions that

occur, such as further folding of the secondary structure.

Substituted benzene derivatives

So far, we have discussed families of aliphatic compounds containing different func-

tional groups. Benzene derivatives containing the same functional groups often show

very different properties when the functional group is attached directly to the benzene

ring. They are also sometimes named differently from their aliphatic counterparts.

Phenols

Phenolsare compounds with hydroxy groups bonded to an aromatic ring (Ar–OH).

The simplest member of this family is phenol, C 6 H 5 OH.

Some phenols are obtained from coal tar, but nowadays most phenols are made by

oxidation of cumene C 9 H 12. Cumene is produced from benzene and propene.

Phenols have acidic properties. For example, they react with aqueous sodium

hydroxide to produce sodium phenoxides:

C 6 H 5 OH(s)NaOH(aq)C 6 H 5 O,Na(aq)H 2 O(l)

Phenol is therefore much more soluble in sodium hydroxide solution than it is in

water. Phenol is only weakly acidic, however, and it does not react with carbonates to

form carbon dioxide.

18.8

Fig. 18.4The arrangements

of the protein chains in

keratin.