Nitration 5-nitro-8-hydroxyquinoline

Little is known quantitatively about substituent effects in the nitration of derivatives of azanaphthalenes. In preparative experiments 4-hydroxy-quinoline, -cinnoline, and -quinazoline give the 6- and 8-nitro compounds, but with nitric acid alone 4-hydroxyquinoline and 2,4-di-hydroxyquinoline react at With nitric acid, 4-hydroxycinnoline... 

The case of i-methyl-4-quinolone is puzzling. The large proportion of the 3-nitro isomer formed in the nitration (table 10.3 cf. 4-hydroxyquinoline) might be a result of nitration via the free base but this is not substantiated by the acidity dependence of the rate of nitration or by the Arrhenius parameters. From r-methyl-4-quinolone the total yield of nitro-compounds was not high (table ro.3). 

Little is known quantitatively about substituent effects in the nitration of derivatives of azanaphthalenes. In preparative experiments 4-hydroxy-quinoline, -cinnoline, and -quinazoline give the 6- and 8-nitro compounds, but with nitric acid alone 4-hydroxyquinoline and 2,4-di-hydroxyquinoline react at C(3).31 With nitric acid, 4-hydroxycinnoline still gives mainly 4-hydroxy-6-nitrocinnoline, but some of the 3-nitro compound can also be isolated.81 51 The change of orientation with reagent could be due to a change to free-base nitration in the more weakly acidic medium, or to the occurrence in nitric acid of nitration via nitrosation.31... 

The rate of nitration is greatly increased under microwave irradiation. While 8-hydroxyquinoline is nitrated in 3.5 h using copper(ll) nitrate and glacial acetic acid, the same reaction is effected in 1 min under microwave irradiation in a conventional microwave oven to give the 5-nitro-8-hydroxyquinoline in 70% yield < 1997IJB1071 >. 

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 compound. 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 precedes 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 ... 

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