Ortho-Dinitrobenzene

The reaction leads to a para-substituted product, entirely in accordance with the calculated distribution of spin density in the anion radical of ortho-dinitrobenzene (Todres 1990). It was established, by means of labeled-atom experiments and analysis of the gas produced, that azo-coupling is accompanied by conversion of one of the nitro groups into the hydroxy group and liberation of nitric oxide. In other words, the initial radical product of azo-coupling is stabilizing by elimination of the small nitrogen monoxide radical to give the stable nonradical final product (Todres, Hovsepyan et al. 1988), Scheme 1-5. 

Explain why nitrobenzene, although it reacts predominantly in the meta position, gives more ortho-dinitrobenzene than para. 

Figure 9.13. FT-Raman spectra of a mildly fluorescent, impure sample of ortho dinitrobenzene before (A) and after (B) correction for instrumental response. Modulation at A is caused by the laser rejection filter. (Adapted from Reference 4, p. 104.)...

Synonyms/Trade Names ortho-Dinitrobenzene 1,2-Dinitrobenzene ... 

Normal day vernalized Optical density Ortho-dinitrobenzene Ortho-phenylene diamine High energy phosphate bond p- Chloromercurihenzolc acid Phosphoglyceric acid Phenazine methosulphate Parts per million rad... 

A brief account of aromatic substitution may be usefully given here as it will assist the student in predicting the orientation of disubstituted benzene derivatives produced in the different substitution reactions. For the nitration of nitrobenzene the substance must be heated with a mixture of fuming nitric acid and concentrated sulphuric acid the product is largely ni-dinitrobenzene (about 90 per cent.), accompanied by a little o-dinitrobenzene (about 5 per cent.) which is eliminated in the recrystallisation process. On the other hand phenol can be easily nitrated with dilute nitric acid to yield a mixture of ortho and para nitrophenols. It may be said, therefore, that orientation is meta with the... 

The apparent relationship between torsion angle and 170 chemical shift has been explored further by examination of the data for several ortho alkyl-substituted nitrobenzenes and dinitrobenzenes (Table 23). The deshielding values of the shifts reflect increasing nitrogen-oxygen double bond character, which is expected as the nitro group is rotated from the plane of the aromatic ring. 

As was the case for dinitrobenzene, the meta and para nitroaniline isomers have essentially the same gaseous enthalpy of formation. In the gaseous phase, it is surprising to find that despite the more attractive quinonoid resonance structures92 for the para isomer (58) than for the meta (59) the met a and para nitroaniline have essentially the same gas-phase enthalpy of formation. In the solid and liquid states the intermolecular stabilization lowers the enthalpy of formation of the para isomer relative to the meta. Interestingly, the gas-phase intramolecularly hydrogen-bonded ortho isomer is of comparable stability to its isomers. In contrast, it is considerably less stable than its isomers in the solid state because it can form fewer intermolecular hydrogen bonds. All isomers of nitroaniline are more stable than calculated by additivity. 

Table 37. Solvent dependence of ortho and meta H—F coupling constants of 1,3-difluoro-4,6-dinitrobenzene...

Dinitrobenzene is unreactive under these conditions. Replacement with fluorine appears to take place only when a halogen atom is present in the ortho- or meta-position relative to the nitro group. 4-Bromo-l,2-dinitrobenzene (3) has been converted to 4-bromo-2-fluoro-l-nitrobenzene (4) by heating with potassium fluoride in dimethylformamide at 100 C for 2 hours.138... 

Although the simple aryl halides are inert to the usual nucleophilic reagents, considerable activation is produced by strongly electron-attracting substituents provided these are located in either the ortho or para positions, or both. For example, the displacement of chloride ion from l-chloro-2,4-dinitrobenzene by dimethylamine occurs readily in ethanol solution at room temperature. Under the same conditions chlorobenzene completely fails to react thus the activating influence of the two nitro groups amounts to a factor of at least... 

Nitronium tetrafluoroborate used in large excess (>6 equiv.) is able to transform hexamethylbenzene and its derivatives to dinitroprehnitene (1,2,3,4-tetramethyl-5,6-dinitrobenzene) in a highly selective nitration process486 [Eq. (5.180)]. Scheme 5.47 summarizes the key steps of the mechanistic proposal, including the ipso - n i tro are n iu m ion 123, the formation of benzyl nitrite 124, and the complexation of the N02+ ion to form the mononitro intermediate (125) facilitating the attack to the ortho position resulting in the formation of the 1,2-dinitro product. 

The presence of electron-withdrawing groups (notably the nitro group) sited in the ortho or para positions of an aryl halide facilitates the halogen replacement. Here the mechanism proceeds by an addition-elimination sequence and it is discussed and illustrated in the formation of 2,4-dinitroaniline and 2,4-dinitrophenylhydrazine (Section 6.8.2, p. 959), and in the use of 1-fluoro-2,4-dinitrobenzene in the derivatisation of an amino group in an amino acid (Section 9.6.23, p. 1279). 

This method cannot be applied to polynitro amines, since these are so weakly basic that they can be diazotised only under special conditions in strongly acidic media (Section 6.7.1, p. 922). In such cases use may be made of the susceptibility to nucleophilic displacement of halogen when activated by ortho and para nitro groups. Thus the valuable reagent 2,4-dinitrophenylhydrazine (Expt 6.93) is readily prepared by reacting l-chloro-2,4-dinitrobenzene with hydrazine. Reaction with ammonia similarly gives 2,4-dinitroaniline (cognate preparation in Expt 6.93). 

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