urbaf2

NITRATION WITH NITRATING MIXTURES 153

was set in motion by putting the belt in the right place, with the result that an

explosion followed (see also p. 446).

Stirring should be particularly vigorous at the beginning of nitration when

the reaction is most violent and the largest amount of heat is evolved. There is

a further reason which makes very fast stirring necessary at the beginning, namely

the fact that the two liquid phases-that of a light substance being nitrated and

that of the heavy nitrating acid-differ considerably in specific weight. As the

reaction goes on, and the organic phase is nitrated, its specific weight increases

while that of the acid decreases because of its dilution. Toward the end of nitration,

the difference between specific weights having been much reduced, mixing of the

two phases is greatly facilitated.

A few workers have investigated recently the problem of the influence of stir-

ring on the rate of nitration and the yield of the reaction. McKinly and White

[7] reported that the rate of nitration of toluene to nitrotoluene increased by 3%

only when the stirring speed was increased from 1327 to 2004 r.p.m.

However, much larger increases (4 to 5 times higher) were obtained in the same

reaction by Barduhn and Kobe [8] when they changed the stirring rate from 1000

to 2175 r.p.m.

The results of their experiments are given in Fig. 25.

HorsePower consumed in stirring

FIG. 25. Rate of nitration (Ra) as a function of stirring rate: XT and XN are the

concentrations of toluene and nitric acid respectively. (Barduhn and Kobe [8]).

SOLUBILITY OF NITRO COMPOUNDS

The solubility of a substance being nitrated in the acid used is an important

factor in the nitration process. The more easily the substance dissolves in the acid,

the higher the reaction rate, and hence the higher degree of nitration to be obtained

during a given time.

Since aromatic compounds are more ready soluble in sulphuric acid and in

mixtures in which its concentration is high, completion of full nitration is accele-

rated by using such mixtures. The solubility of dinitrobenzene in sulphuric acid
of various concentrations and at various temperatures is shown in Fig. 26.