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NITRATION AND NITRATING AGENTS 5

certain difficult problems have been elucidated in the course of extensive research

so that now what might be defined as a modern theory of nitration can be outlined,
although it is still far from being perfect.
Nitration can be carried out either directly by introduction of the nitro
group in place of hydrogen atom or by adding it to a double bond, or else indi-
rectly, by introducing into a compound a group which can readily be substi-

tuted by the nitro group.

The following nitrating agents are most frequently used for the direct intro-

duction of the nitro group:

(1) concentrated nitric acid

(2) mixtures of concentrated nitric acid and concentrated sulphuric acid (or

oleum) in different proportions-these are usually known as nitrating mixtures

(3) alkali nitrates in the presence of sulphuric acid

(4) dilute nitric acid

(5) nitrogen dioxide

(6) a solution of nitrogen dioxide in sulphuric acid

(7) nitrogen dioxide in the presence of catalysts.

For laboratory experiments and sometimes in industry more expensive nitrating

agents may be used, as for example solutions of nitric acid in inert organic solvents

(chloroform, carbon tetrachloride, ether, nitromethane, etc.), or a solution of nitric

acid in phosphoric or acetic acids or in acetic anhydride. The use of these nitrating

agents may be of some practical value and will be discussed later on in detail.

For nitrating on the laboratory scale, mixtures of nitric acid esters or acyl

nitrates, e.g. acetyl nitrate CH 3 CONO 3 , and sulphuric acid may also be used.

Several lesser known nitrating agents, which can find practical use on a labo-

ratory scale are metal nitrates in the presence of acetic acid or acetic anhydride,

described by Menke [2], tetranitromethane and hexanitroethane in an alkaline

medium, used by Schmidt [3], and nitroguanidine in solution in sulphuric acid, used

for the nitration of aromatic amines and phenols.

Besides these direct methods of introducing nitro groups, several indirect methods

are known that consist in the introduction of a group which can readily be substitu-

ted by a nitro group. In one of these which is widely used in the nitration of phenols,

a compound is sulphonated and subsequently, by reaction with nitric acid, the

sulpho group is replaced by the nitro group.

Other indirect nitration methods applied on an industrial scale, for nitrating

phenols, comprise: introducing a nitroso groups into the phenol and then oxi-

dizing it to the nitro group, and a method involving oxidation of a primary amino

group to the nitro group.

In experimental work indirect methods of introducing nitro groups find wide

application as, for example, the substitution of a halogen (iodine or bromine in

an alkyl iodide or bromide) by the Nitro group, by means of silver nitrite (the
Victor Meyer reaction), and the new modification of this method described recently

by Kornblum et al. [4, 4a], in which alkyl halides are reacted with sodium nitrite.