Introduction of the Nitro Group

The presence of an electron-withdrawing group in an aromatic ring makes attack by electrophiles more difficult. Electron-donor groups make this attack easier. Read Section 2.3 again if you are unsure about this. 

Nitric acid alone fails to nitrate benzene and sulfuric acid also does not readily react with it, yet the mixed acid is an efficient nitrating reagent. Solutions of nitric acid in sulfuric acid show an approximately four-fold molar freezing-point depression and this has been attributed to the generation of four ions, as shown in equation (1)  

The formation of the nitronium ion is critical to the success of nitration. The purpose of the sulfuric acid is to generate the nitronium ion other acids such as perchloric acid, hydrogen fluoride and boron trifluoride are also effective. Nitric acid alone contains only relatively small concentrations of nitronium ion and hence is not an effective nitrating agent, except with very reactive substrates. 

The evidence for the existence of the nitronium ion is compelling. Salts such as nitronium perchlorate, N02 C104 , and nitronium tetrafluoroborate, N02 Bp4, have been isolated and successfully used as nitrating agents. A line in the Raman spectrum at 1400 cm that originates from a linear triatomic species, which N02 is, provides spectroscopic confirmation. 

It is possible to nitrate highly activated aromatic compounds such as phenols using dilute nitric acid. The nitrating species is considered to be 

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Nitration can be carried out either directly by introduction of the nitro group in place of a hydrogen atom or by adding it to a double bond, or else indirectly, by introducing into a compd a group which can readily be replaced by or converted to the nitro group... 

The following nitrating agents are most frequently used in industry for the direct introduction of the nitro group ... 

W. M. Weaver, in The Chemistry oftheNitro andNitroso Groups, Parti, OrganicNitro Chemistry Series, Ed. H. Feuer, Wiley-Interscience, New York, Introduction of the Nitro Group into Aromatic Systems , Chapter 1, 1 8 (1970). 

The evolution of heat resulting from the introduction of the nitro group into methylaniline was calculated by Garner and Abemethy [14] as follows. On transition of ... 

Nitration of isatin,217,218 7-methylisatin,8 4,7-dimethylisatin,21 and 1-ethylisatin147 leads to the introduction of the nitro group into the 5-position. Nitration of 5-methylisatin gave 5-methyl-7-nitroisatin.8,21... 

Introduction of the nitro group as the first step is an unsatisfactory approach since Friedel-Crafts reactions cannot be carried out on nitro-substituted aromatic compounds. 

The introduction of the nitro group and fluorine atoms in the benzene ring considerably decreases the nucleophilic properties of the C-anion center compared to the N-anion, forming diazetine derivatives (00JFC(104)263). 

The introduction of the nitro group into position 7 would support the view already expressed (p. 48) that the N02+ ion is not the only nitrating agent. 

Alkylpyridines (picolines, and especially lutidines) can be nitrated much more readily, as Plazek [75] has shown. Introduction of the nitro group is greatly facilitated by the presence of the hydroxyl group in the pyridine ring. For example, 3-hydroxypyridine can be nitrated to 2-nitro-3-hydroxypyridine in good yield (Plazek and Rodewald [76]). It may be further nitrated to the dinitro derivative. From this, 3,5-dinitropyridine can be obtained (Plazek [77]). 

The introduction of the nitro group in azoles leads to a long-wave shift of the visible absorption maximum and an enhancement of the sensitizing properties of cyanine dyes. A long-wave shift of the sensitivity of photographic materials is... 

As with pyrazoles, the introduction of the nitro group into the imidazole ring position 4(5) leads to an approximately 30 ppm lowfield shift of the tpxo-carbon signal resonance (Tables 3.6 and 3.10) [24, 321-329], Thus, the chemical shift of the carbon atom bonded to the N02 group (C-ipso) is 149.2 ppm, whereas that of neighboring carbon (C-5) is 119.8 ppm. In 1-substituted 4-nitroimidazoles (Table 3.11) the shifts of the same carbons are 146 1 and 122 2 ppm, while in 1-substituted 5-nitroimida-zoles they are 138 1 and 132 1 ppm, respectively (Table 3.12). All this may be indicative of the possible existence of 4(5)-nitroimidazole as a 4-nitro tautomer. Moreover, another support for the structure may be provided by comparison of the proton spin-spin coupling constants - J, 2J and 1/ Cl I-111) [321, 322, 330],... 

The introduction of the nitro group does not affect much the tautomeric equilibrium in the series of benzimidazolones studied. However, the position and number of nitro groups in the compounds influence markedly the values of chemical shifts and coupling constants of 13C and 15N nuclei [562],... 

The substituent electron effects in indazole and other azoles essentially influence the nitro group vas (Table 3.60). The valence vibration bands of the C-nitro group in nitrated indazoles are in a frequency range of 1560-1510 (vj and 1380-1310 cm-1 (vs) [644, 657-664, 668, 669, 673, 679, 680], The introduction of the nitro group into position 1 in indazole displaces the asymmetric band to higher frequencies (-1620-1610 cm-1), and symmetric frequency, on the contrary, to lower frequencies (-1280-1240 cm-1) in comparison with C-nitroindazoles (Table 3.61) [657, 659, 663, 664], Similar changes are observed in 5-membered nitroazoles. 

At the same time in the IR spectra of 2-(2-oxyaryl)benzazoles there is a wide smearing band in the 3400-2400 cm-1 region, which is characteristic of intramolecular hydrogen bond 0-H...N. This distinction results from the introduction of the nitro group and is caused, in the author s opinion, by a decrease in the 7tN-orbital population owing to conjugation with the nitro group. 

Introduction of the nitro group into the azole cycle leads to a significant bathochro-mic shift of the absorption band maximum in the electron spectra. The absorption band of pyrazole is near 210 nm, whereas that of 4-nitropyrazole lies in the... 

Nitration is an electrophilic reaction of the addition of a nitro group to an organic compound with a nitrating agent. The introduction of the nitro group can take place on a carbon atom (C-nitration), oxygen atom (0-nitration), or nitrogen atom (N-nitration) [4, 8]. 

Gersdorff W, Effect of the introduction of the nitro group into the phenol molecule on toxicity to goldfish, J. Cell. Comp. Physiol., 14, 61, 1939. 

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