Dinitrobenzenes selective

It is convenient to include under Aromatic Amines the preparation of m-nitroaniline as an example of the selective reduction of one group in a polynitro compound. When wt-dinitrobenzene is allowed to react with sodium polysulphide (or ammonium sulphide) solution, only one of the nitro groups is reduced and m-nitroanUine results. Some sulphur separates, but the main reaction is represented by ... 

Rapid, simple, quaUtative methods suitable for determining the presence of benzene in the workplace or surroundings have been utilized since the 1930s. Many early tests offered methods for detection of aromatics but were not specific for benzene. A straightforward test allowing selective detection of benzene involves nitration of a sample to y -dinitrobenzene and reaction of the resultant ether extract with an ethanoHc solution of sodium hydroxide and methyl ethyl ketone (2-butanone), followed by the addition of acetic acid to eliminate interferences from toluene and xylenes. Benzene imparts a persistent red color to the solution (87). The method is claimed to be sensitive to concentrations as low as 0.27 ppm benzene from 10 mL air samples. 

Furthermore, in many cases, changes in the mechanism have also been observed and they will be discussed in a later section. Nevertheless, by selecting a system that exhibited the same rate-determining step in a variety of solvents it would be possible to assess how the rate of a given process may be affected by a solvent transfer. Such is the case of the reaction of l-chloro-2,4-dinitrobenzene with piperidine, where the rate dependence with amine concentration has been studied in 12 aprotic solvents483 as well as in 10 protic solvents4815. It was found that the reaction does not exhibit base catalysis in any of the solvents studied that is, addition of piperidine is rate-limiting in all the... 

Selected examples of the reaction of 1,2,3,4-tetrachIoro-5,6-dinitrobenzene with amines... 

Vinylic halides are virtually unreactive and a high selectivity is to be found in the preferential cleavage of aliphatic carbon-halogen bonds of haloalkanoic amides and esters, and of nitro- and cyanoaryl derivatives. Activated haloarenes, e.g. 1-chloro-2,4-dinitrobenzene, however, give a complex mixture of products [7]. 

The catalyst surface contains both reducing and basic sites and experiments involving selective poisoning of the former by m-dinitrobenzene demonstrated the high probability of a SET mechanism. The same group also concluded that an SET mechanism was probably involved for Michael, Wittig-Horner, and Claisen-Schmidt condensations in the presence of the same catalyst [139,140]. 

A kinetic flow injection analytical method was also described for isoxsuprine, which was based on the use of a fluoride-selective electrode for detection after reaction with l-fluoro-2,4-dinitrobenzene [38]. 

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. 

Aromatic o-dinitro compounds 152-> can be selectively reduced in acidic medium to either an o-nitrophenylhydroxylamine (cpe at less negative potentials) or an o-phenylenediamine (cpe at more negative potentials) as for example in the reduction of dinitrobenzene itself ... 

HMordenite, HFaujasite-780, HFaujasite 720 and Na-Faujasite zeolites. Among the different catalysts, HFaujasite-720 was the most active and selective catalyst towards 2,4-dinitrotoluene, achieving a yield of dinitrotoluenes of 92 % with a ratio of 2,4- to 2,6- isomers of 4.3 1 in 3 min reaction time. Using this zeolite, l-chloro-2-nitrobenzene and pyrazole were also nitrated regioselectively to obtain l-chloro-2,4-dinitrobenzene in a l-chloro-2,4-dinitro l-chloro-2,6-dinitro ratio of 30 1, and 1,4-dinitropyrazole in 80% yield, respectively. The authors proposed a nitration mechanism in which the protons in the zeolite are replaced by nitronium ions derived from N2Os in a fast pre-equilibrium process. This produces active sites for transfer of nitronium ion from faujasite to aromatic in the rate-controlling step. 

Fig. 5. Headspace SPME-GC/MS of selected cast explosives (1 = 1,3-dinitrobenzene, 2 = 2, 4-dinitrotoluene,...

Pd-C in both ethanol and DMF, little, if any, selectivity was obtained product ratios were 50 50 in ethanol and 60 40 in DMF. Palladium black in ethanol, which afforded 4-amino-5-nitroveratrole in 70-75% yield from 4,5-dinitroveratrole, provided no selectivity with 2,4-dinitroaniline, as well as with o- and m-dinitrobenzenes both nitro groups were hydrogenated.140 Reduction by sodium borohydride and Pd-C in methanol also does not appear to be selective, as 1,2,4-triaminobenzene is formed.141 It is of interest that over Pt-C in acidic alcohol 2,4-dinitroaniline is selectively hydrogenated at the 4-nitro group to give 4-amino-2-nitroaniline in 70% yield.142... 

Columns in which 6-cyclodextrin was immobilized on polyacrylamide or agarose gel were shown to be very useful in the separation of disubstituted benzene isomers (60,61). Acetylating the immobilized 6-cyclodextrin further improves the selectivity, i.e. it can completely separate o-, m- and p-toluidine, and dinitrobenzenes (64) which cannot be done on unmodified stationary phase. 

Structural Isomers. Chromatograms illustrating the separation of ortho, meta and para isomers of cresol (22) and and xylene ( O)on RP columns are shown in Figures 4 and 5. They enable a comparison of the chromatographic properties and selectivities due to <. - and -CD complexation between positional isomers of the above compounds.Similar behaviour was observed for ortho,meta and para isomers of fluoronitrobenzene, chloronitrobenzene, iodoni-trobenzene, nitrophenol, nitroaniline, dinitrobenzene (22), nitrocinnamic acid (22) some mandelic acid derivatives (19,21,34) and ethyltoluene (28). Both [Pg.225]

Vannikov et al. (1989, 1989a) measured hole mobilities of TPA doped PS and a PC containing varying concentrations of />-, m-, and o-dinitrobenzene (DNB). The DNB compounds were selected for differences in dipole moment. 

Other reducing agents such as sodium hydrosulfite (sodium dithion-ite, Na,S204) and sodium sulfide have also been used. For example, the latter reduces one of the nitro groups in polynitro compounds selectively 1,3-dinitrobenzene gives 3-nitroaniline in this way (Scheme 7.7). Tin(II) chloride reduces nitro groups selectively in the presence of carbonyl groups under acidic conditions. 

Aromatic amines are prepared by the reduction of nitro compounds under acidic conditions this leads to the protection of the resultant amine as the salt. The selective reduction of one nitro group in 1,3-dinitrobenzene can be achieved with sodium polysulfide. 

Substituted / -dinitrobenzenes may in acid solution be reduced selectively to a nitro-phenylhydroxylamine [151,155,156] further reduction leads to the / -phenylenediamine /7,y-dinitroazoxybenzene has been shown to be an intermediate [94]. l-X-2,5-Dinitrobenzenes are reduced first in the 2-nitro group when X = COOH, CONHo, or Br, whereas the 5-nitro group is first attacked when X = CH3 or OCH3 [156],... 

Raney copper is another Raney type catalyst that is prepared from a copper-aluminum alloy. This catalyst has been used infrequently but does show some reaction selectivity not possible with other catalysts. Of particular interest is its use for the selective hydrogenation of substituted dinitrobenzenes (Eqn. 11.6).2 This catalyst, as well as Raney cobalt, generally promotes fewer side reactions than does Raney nickel. 25... 

Fig. 14.1. Steric effects in the selective hydrogenation of 1 -substituted-2,4-dinitrobenzenes.

The selective hydrogenation of one nitro group of a dinitrobenzene represents yet another type of reaction selectivity observed with aromatic nitro groups. This reaction, which follows a combination of Type 11 and Type 111 selectivity (Chapter 5) (Eqn. 19.7) is influenced by a number of factors. It was found that in the hydrogenation of 2,6-dinitrotoluene to 2-amino-6-nitrotoluene (Eqn. 19.8), platinum and rhodium were more selective than palladium. With Pt/Al203... 

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