NITRATION AGENTS AND METHODS MORE RARELY USED 85
3% nitric acid, while 4% acid converts methyl- and ethyl-acetanilide into
the corresponding dinitro derivatives. In 1958 T. Urbanski [19] found that
8-hydroxyquinoline can be converted to the 5,7-dinitro compound by boiling with
8% nitric acid and more recently he and Kutkiewicz [20] have found that
the same result can be obtained by prolonged boiling with nitric acid of concen-
trations as low as 0.5%. However, the reaction occurred only after a certain induction
period which ended by evolution of nitrous fumes. When NaNO 2 was added to
the nitric acid, the reaction started almost immediately. The nitration of 8-hydroxy-
5-nitrosoquinoline leads readily to formation of the same nitration product
without the induction period.
The introduction of the 5-nitro group can easily be explained by nitrosation
of 8-hydroxyquinoline in position 5 and subsequent oxidation of the nitroso com-
pound. No similar explanation can be given with regard to the mechanism of the
introduction of the second nitro group, in position 7, as no nitrosation of nitro-
phenols is known and the formation of “8-hydroxy-5-nitro-7-nitrosoquinoline”,
does not seem to be possible. The formation of “5,7-dimtroso-8-hydroxyquinoline”
as an intermediate is also improbable because no instance of the introduction of
two nitroso groups into a monophenol is known. Thus nitration most likely pro-
cedes through the formation of the 5-nitroso derivative only.
It has also been found that 5-nitro-8-hydroxyquinoline can be nitrated with
1% nitric acid to yield 5,7-dinitro-8-hydroxyquinoline. This excludes nitrosation
as an intermediate step for the reason given above:
The introduction of the nitro group into position 7 would support the view
already expressed (p. 48) that the NO 2
+
ion is not the only nitrating agent.
Slavinskaya [21] in 1957, found that phenol can be nitrated to o-, p- and 2,4-
dinitrophenol using nitric acid dissolved in ethyl nitrate at a concentration as low
as 0.5% HNO 3. Phenetole and naphthalene can also be nitrated with this solution
to yield mononitro products. The presence of NO 2 was essential for successful
nitration at such a low concentration of HNO 3.