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NITRATION AND NITRATING AGENTS 41

action may be reversible. Their assumption has been based on the observation

that the heating of 2,3-dinitroacetanilide with sulphuric acid yields mainly 2,5-di

nitroaniline (46% yield), 3,4-dinitroaniline (23% yield) and a small quantity of

2,3-dinitroaniline (5%).

However, more recent studies by these workers [97] show that the mechanism of

these reactions consists in the reverse of the Bamberger rearrangement. It is known that

the Bamberger rearrangement in aromatic nitramines consists in the nitro group wan-

dering from nitrogen to carbon. The compounds studied by the authors would

undergo reverse rearrangement, followed by the Bamberger rearrangement:

It has been found by the same authors that heating 2,3-dinitrophenol with

sulphuric acid leads to partial isomerization to 2,5-dinitrophenol.

Other dinitro compounds, viz. those substituted in the 2,5- and 3,4-positions,

do not undergo such rearrangement. This fact is evidence that only that group

can migrate which meets with steric hindrance (i.e. the nitro group in the ortho

position to the adjacent group or groups).

No evidence of reversibility of aromatic C-nitration has so far been found.

Thus T. Urbanski and Ostrowski [98] have kept solutions of various nitro

derivatives of toluene in cont. sulphuric acid at 90-95°C for ca. 60 hr. o- Nitro-

toluene (I), m- nitrotoluene (II), p- nitrotoluene (III), 2,4,6-trinitrotoluene (IV)

and 2,4,5-trinitrotoluene (V) were examined. No appreciable change was found

of the boiling points of (I) and (II), and of the melting points of (III)-(V). Only

in the case of m- nitrotoluene and 2,4,5-trinitrotoluene the solution gave a very

slight blue colour with diphenylamine. This might have been produced by trace

splitting of the mobile m- nitro group and could not be considered as any evidence

of an equilibrium in the systems examined.

Nitric acid and sulphur dioxide

Varma and Kulkarni [99] studied the nitrating action of nitric acid saturated

with SO 2. This solution acts much more vigorously than the usual nitrating mixture