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

Sometimes it is difficult to fulfill all the conditions a nitration mixture should satisfy, such as a low solubility of the nitro compound at an adequate strength of the acid. Sometimes water may be added, after the nitration has been completed, to reduce the solubility of the nitro compound so as to obtain quantitative separation of the product. This can be justified economically although the acid becomes diluted, thus necessitating the use of more oleum for fortifying the spent acid. 

Fig 27. Influence of the concentration of nitrating mixtures on the nitration of nitrobenzene to dinitrobenzene and phase conditions after cooling to 35°C (Hetherington and Masson [9]). 

The influence of sulphuric acid in O-nitration processes, as for example in the esterification of cellulose with a mixture of sulphuric and nitric acids, appears to be somewhat different. This will be dealt with in the chapter on esters (Vol. II). 

---

The overall low solubility of nitro compounds in the acids causes the distribution coefficient of dinitrotoluene being very low (0.3-0.4). The distribution coefficient of dinitrotoluene when a 50 50 mixture of 2,4-dinitrotoluene and a- trinitrotoluene was... 

---