The nitrodecarboxylation of 2-acetamidothiophene-3-carboxylic acid was
investigated in an attempt to improve the yield of 2-acetamido-3,Sdinitrothiophene obtained as product. The latter compound is valuable
as an intermediate in the dyestuffs industry. It was found that both
2-acetamido-5-nitrothiophene-3-carboxylic acid and 2-acetamido-3,5-
dinitrothiophene are susceptible to hydrolysis to unstable amines in the
nitrating medium, a mixture of sulphuric and nitric acid.
The rates of nitrodecarboxylation of a variety of substituted
thiophenes were determined as a function of sulphuric acid concentration •.
The characteristics of the reaction were found to be similar to those of
conventional nitrodeprotonation. although the latter reaction is
considerably faster, (the ipso factor, if C02H for nitrodecarboxylation
was found to be S.6x10- 4). Ring systems in which the carbon atom
containing the carboxyl group is activated towards electrophi1ic attack
are particularly susceptible to nitrodecarboxy1ation. A Hammett reaction
constant. p+, of -4.6 was obtained for the nitrodecarboxy1ation reaction,
the reaction constant for the corresponding nitrodeprotonation reaction
being -3.4. A concerted mechanism, similar to that for conventional
nitration, is favoured for the nitrodecarboxy1ation reaction.
The rates of hydrolysis of a variety of amido-thiophenes were also
determined as p function of sulphuric acid concentration. In concentrated
acid most amides were hydrolysed by the unimolecu1ar A1 mechanism whereas
in more dilute acid the bimolecular A2 mechanism of hydrolysis predominated.
The amino-thiophenes produced were particularly unstable in presence of
nitric acid. This hydrolysis to an unstable amine partially accounts for
the unsatisfactory yield of 2-acetamido-3,S-dinitrothiophene-3-carboxy1ic
acid in sulphuric-nitric acid. Indeed, the stability of the corresponding
formamido derivatives to hydrolysis is reflected in the improved yield
of 2-formamido-3.5-dinitrothiophene obtained by a similar nitration
process (70.3% compared with 46.2% for the acetamido derivative).
Unsuccessful attempts were made to isolate oxidation products of
unstable thiophene derivatives and the rates of oxidation were determined
in sulphuric-nitric acid mixtures.