urbaf2

NITRATION AGENTS AND METHODS MORE RARELY USED 127

Owing to this course of the reaction the oxidation of phenols and the formation

of side-products can be largely avoided. This is discussed in more detail in the

chapter on the nitration of phenols (p. 502).

Another method, of great importance in laboratory practice, is the substitution

of a halogen by a nitro group.

SUBSTITUTION OF HALOGEN

In laboratory methods for the preparation of aliphatic nitro compounds, the

V. Meyer reaction [186] consisting in reacting alkyl iodides or bromides with sil-

ver nitrite, is widely used. As is well known, the reaction can proceed in two direc-

tions, resulting in the formation of a certain quantity of a less stable nitrous ester

besides a nitro compound. Instead of silver nitrite mercuric nitrite may be used

(Ray[187]).

Kornblum and his co-workers [188, 188a] have recently improved the method

used for the preparation of nitroparaffins with longer aliphatic chain (over C 8 ).

They reacted an alkyl chloride with sodium nitrite in the solvent (dimethylform-

amide) for several hours at low temperature, obtaining a homogeneous solution:

RCH 2 Cl + NaNO 2 -> RCH 2 NO 2 + NaCl (85)

The addition of urea to the reacting system prevents side reactions, e.g. the formation

of nitrous esters. The yield amounts to about 60%.

In aromatic compounds such a reaction is possible only in the case of polybro-

mo- or polyiodo-derivatives of phenol. Sodium nitrite and acetic acid may be used

for replacing one of the Br or I atoms by the nitro group (Zincke [189], Raiford

[190-193]):

Not only salts of nitrous acids but also nitrous acid itself can replace halogen

by the nitro group. This has been discovered by Wuster and Scheibe [194] when

they reacted sodium nitrite with bromodimethylaniniline in hydrochloric acid:

(87)

In addition, a certain quantity of bromonitrosodimethylaniline was formed.