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AROMATIC NITRO COMPOUNDS 219

haved abnormally, as its molecular weight increased with the increase of the con-

centration of solution. Hantzsch suggested that an addition compound, (ArNO 2.

.H 2 SO 4 ), was formed by adding a sulphuric acid molecule on to the nitro group,

as if it had a basic character.

The investigations were continued by Hammett [102] and by Gillespie [103,

103a]. This work established that if oleum of high concentration was used as a

solvent, higher nitrated aromatic compounds also exhibited basic properties.

These results were not quite in agreement with those of spectrophotometric

investigations reported by Brand and his co-workers [104]. Brand suggested that

the difference was due to insufficient knowledge of the true form of nitro com-

pounds in the solutions under consideration. He assumed that nitro compounds

in sulphuric acid solutions became cations (Ar ).That was in agreement

with an earlier hypothesis (Masson [105], Hetherington and Masson [106]) on the

mechanism of aromatic nitration.

In support of Hantzsch’s view reference should be made to the fact that Metro

compounds can form addition products with sulphuric acid. Thus, Cherbuliez [107]

isolated C 6 H 5 NO 2 .H 2 SO 4 (m. p. 11.6°C) as the product of addition of nitrobenzene

to sulphuric acid. This was later supported by Masson [105] who assigned a salt

structure to the product [C 6 H 5 NO 2 H+] [HSO 4 - ].

Gillespie and Millen [108] assumed that in such a substance both molecules

were linked by a hydrogen bond:

Confirmation of that view has been provided by Hammett and Chapman’s

work [109]. They inferred from their examination of the solubilities of various

organic compounds, among others nitrobenzene, in mixtures of sulphuric acid

and water that this form of hydrogen bonding could exist. It might be that similar

hydrogen bonds were also formed between m- dinitro- or sym- trinitro-benzene

and sulphuric acid. Gillespie and Millen [108] had drawn attention to that, basing

their supposition on the observation by Klough, Savage and van Marl [110], that

the solubilities of higher nitro compounds increased rapidly with increase in the

concentration of sulphuric acid.

Also Hammond and Medic [111] on the basis of analysis of the ultra-violet

spectrum, advanced the hypothesis that sulphuric acid combined with the nitro

group through a hydrogen bond. Liler and Kosanovic [150] came to the same

conclusion on the basis of their experiments on viscosities, electrical conduct-

ivities and refractivities of solution of nitro compounds in sulphuric acid.

On the other hand it should be emphasized that solutions of aromatic nitro

compounds, especially mononitro derivatives are distinctly coloured, as already
observed by Hantzsch [101]. The yellowish colour of nitrobenzene and p- nitro-