urbaf2

AROMATIC NITRO COMPOUNDS 217

in the vulcanization of rubber is limited to butyl rubber, as may be seen from the

patent literature [94a].

INFLUENCE OF NITRO GROUPS ON SOME PHYSICO-CHEMICAL

PROPERTIES OF PHENOLS AND AMINES

The nitro group enhances the acidic properties of phenols, possibly due to

the influence of the induction effect (p. 197):

The comparison of dissociation constants (K) of several phenols in aqueous

solution with those of their nitro derivatives (Table 28) illustrates this effect of

the nitro group.

Phenol

Phenol

o- Nitrophenol

m- Nitropheno1

p- Nitrophenol

2,4-Dinitrophenol

2,6-Dinitrophenol

Catechol

3-Nitrocatechol

Hydroquinone

Nitrohydroquinone

TABLE 28

K

1.2 x 10-10

6.8 x 10-8

5 x 10-9

7 x 10-8

5.6 x 10-4

1 x 10-4

3.5 x 10-10

1.88 x 10-6

4.5 x 10-11

1.01 x 10-6

Reference

[82]

[82]

[82]

[82]

[83]

[84]

[82]

[84]

[82]

[84]

A nitro group ortho to the phenol group can form a chelate through an

internal hydrogen bond (p. 177, see also p. 171).

This structure accounts for certain properties of o- nitrophenol, and namely those

by which it differs from its m- and p- isomers. For example, o- nitrophenol is vola-

tile in steam. This may be explained by the fact that owing to the presence of an

internal hydrogen bond, the intermolecular links which are responsible for mole-

cular association are either weakened or non-existent. This is the reason why the