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GENERAL INFORMATION ON NITRO COMPOUNDS^173

It is interesting to note that ranging from non-polar solvents to sulphuric acid

through polar solvents affects aromatic nitro compounds in the opposite direction,

i.e. it results in a bathochromic shift. This can be seen from Hammond and Mo-

die’s data [15] for nitrobenzene (Table 22).

TABLE 21

ABSORBTION SPECTRA OF NITROMETHANE

Solvent

Absorption maximum

mμ

Heptane 277.6

Ethanol 274

Water 268.8

Sulphuric acid 252.5

From the results of investigations into the absorption spectra of nitromethane

in non-polar solvents (cyclohexane and carbon tetrachloride) de Maine and his

co-workers [16] drew an important conclusion. They found that dilute solutions

of nitromethane do not obey Beer’s law, unlike more concentrated solutions in

TABLE 22

ABSORPTION SPECTRA OF NITROBENZENE

IN VARIOUS SOLVENTS

Solvent

Absorption maximum

mμ

Hexane 252

Water 267

69.6% H 2 SO 4 276

98.4% H 2 SO 4 288.5

carbon tetrachloride. From this the authors deduced that in more concentrated

solutions nitromethane was present as a dimer, while in dilute solutions it is a

monomer. Thus there is a state of equilibrium between the two forms:

2CH 3 NO 2 -> (CH 3 NO 2 ) 2 (3)

Brand and his co-workers [17] carried out extensive studies on the absorption

spectra of aromatic compounds in sulphuric acid solutions, i.e. in a strongly proto-

nizing solvent. They found that under the influence of the sulphuric acid the

maximum of the nitro group shifted. These shifts were most pronounced in the
case of mononitro compounds, and the least in the case of trinitro compounds.

They were smaller when sulphuric acid was used as a solvent, and larger when

oleum was used. The absorption curves for 2,4-dinitrotoluene are shown in Fig.