Nitro groups, detection

Nitro compounds, when hquid, have characteristic odours, are insoluble in water, highly refractive and with a density greater than unity. Many are crystalline sohds. Most nitro compounds are slightly coloured, generally yellow the intensity of the colour increases with the number of nitro groups. The following reactions will assist in their detection. 

The addition intermediates ean frequently be detected spectroseopically and sometimes ean be isolated. They are ealled Meisenheimer complexes. Espeeially in the case of adducts stabilized by nitro groups, the intermediates are often strongly eolored. 

Secondary steric effects of nitro groups are more easily detected by comparing the reactivities with those of aza derivatives. For example, in structure 20 the rate depression on passing from methyl to -butyl is only 2.5-fold and can be attributed to an inductive effect, whereas in structure 21 a similar change involves the factor 16, which can be attributed in part to steric inhibition of resonance (S.I.R.) of thep-N02 group (reaction with piperidine). 

The molecular ions are detectable for all isomers except the 2,3-isomer, with the 3,4- and 3,5-isomers being the most abundant and the 2,6-isomer being the least abundant (2%). Because in the 2,3- and 2,6-isomers nitro groups are ortho to the methyl group, OH is readily lost at the expense of the molecular ion. If the molecular ion (m/z 182) is intense, the unknown mass spectrum is either the 3,4- or the 3,5-isomer. If the m/z 165 peak is very abundant then the mass spectrum represents the 2,3-, 2,4-, 2,5-, or a 2,6-isomer. 

In freshwater systems, the only biodegradation product detected was 4-nitrophenol, which was rapidly utilized and transformed to undetectable metabolites by the microorganisms present. In seawater, the main initial product was methyl aminoparathion, formed by reduction of the nitro group (Badawy and El-Dib 1984). Studies in raw river water showed that 4-nitrophenol and dimethyl thiophosphoric acid are the main degradation products (Eichelberger and Lichtenberg 1971). 

Nitrophenols are phytotoxic, and dinoseb (6-iec-butyl-2,4-dinitrophenol) has been used as a herbicide, while nitrophenols have been detected in rainwater and plausible mechanisms for their abiotic formation have been proposed (Kohler and Heeb 2003 Vione et al. 2005). The pathway for the degradation of phenols with a single nitro group depends on the position of the substituents, while... 

The oxygen transfer from an ortho nitro group to a carbon-carbon triple bond has been studied by high-resolution mass spectrometry, linked scan techniques and chemical substitution81. Oxygen transfers to both acetylenic carbons were detected as parallel fragmentation pathways81. 

Confirmation of the radical mechanism was provided by Ridd and Sandall35, who detected radical cations by nitrogen-15 NMR of the reaction mixture for the nitramine rearrangement of 2.6-dibromo-jV-nitroaniline and of A -methyl/V-nitroaniline labelled with nitrogen-15 in the nitro group. The results did not allow the authors to indicate whether the products were formed within the solvent cage or from separated radicals. 

Fluorescent cellulose triacetate membranes were prepared by incorporation of pyrene-butyric acid (219), and were applied to in situ detection of ground water contamination by explosives, based on fluorescence quenching by the nitro groups LOD 2 mg/L of DNT (220) and TNT (221) and 10 mg/L for RDX (276) the response follows the Stern-Volmer law for DNT and TNT442. 

This behavior is certainly very different from that found for 2-bromopyridine (see earlier discussion), where addition of the amide ion takes place exclusively at position C-2. Apparently, the nitro group at C-5 activates mainly its neighboring C-6 position (C-6 > C-2 > C-4). It remains remarkable that none of the intermediary a-adducts are detected in the reaction mixture. 

The reduction of aliphatic nitrocompounds in acid solution proceeds in two steps. First the nitrosocompound is formed. A low steady state concen ation of 2-methyl-2-nitrosopropane has been detected during the reduction of 2-methyl-2-nitropropane [13]. At the cathode potential necessary to attach the first electron to a nitro group, the nitroso intermediate undergoes further reduction to the hydroxyla-mine. When the nitrocompound has one a-hydrogen substituent, tautomerism of the nitroso intermediate to an oxime is in competition with further reduction. Both temperature and proton availability affect the rate of this isomerisation. Reduction of aliphatic nitrocompounds to the hydroxylamine is usually carried out in acid solution at 0-5° C to minimise oxime formation [14, 15], The hydroxylamine is stable towards further reduction in acid solution. Oximes in acid solution are reduced... 

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