P- Dinitrobenzene

An interesting application of the diazo reaction is to the preparation of the otherwise difficultly accessible o- and />-dinitrobenzenes o- or p-rutrophenyl-diazonium fluoborates react with sodium nitrite in the presence of copper powder to yield o- or p-dinitrobenzene ... 

These additives can have essentially a reductive character such as the Zn powder or a radical trapping character such as p-dinitrobenzene, triphenylphosphine, p-diaminobenzene, di-tertiobutylphenol or both characters in the case of hydroquinone. 

Some of the reactions in this chapter operate by still other mechanisms, among them an addition-elimination mechanism (see 13-15). A new mechanism has been reported in aromatic chemistry, a reductively activated polar nucleophilic aromatic substitution. The reaction of phenoxide with p-dinitrobenzene in DMF shows radical features that cannot be attributed to a radical anion, and it is not Srn2. The new designation was proposed to account for these results. 

A kinetic study of the previously reported substitution of aromatic nitro groups by tervalent phosphorus has established an aromatic 5n2 mechanism. Similarities in values of activation energies, and in relative reactivities of phosphite and phosphonite esters, between this displacement and the Arbusov reaction suggest a related mechanism (31), while the lack of reactivity of p-dinitrobenzene is attributed to the need for intramolecular solvation (32). The exclusive formation of ethyl nitrite, rather than other isomers, is confirmed from the decomposition of triethoxy-(ethyl)phosphonium fluoroborate (33) in the presence of silver nitrite. A mechanism involving quinquevalent phosphorus (34) still seems applicable, particularly in view of the recent mechanistic work on the Arbusov reaction. ... 

The kinetics of formation of phosphonates by reaction of o-dinitrobenzene with phosphites have been examined. The energy of activation for the reaction increases as the nucleophilicity of the phosphite decreases, e.g. ethyl diphenylphosphinite 14kcalmol"S diethyl phenylphosphonite 16 kcalmol S and triethyl phosphite 21kcalmol . An intermediate of the type (61), formed by nucleophilic attack of the phosphite, was proposed. In (61) there is a particularly favourable electrostatic interaction. That p-dinitrobenzene is unreactive, is thought to stem from the fact that this compound cannot form an intermediate with such a stabilizing factor. 

Intermolecular displacements of a nitro group from p-dinitrobenzene proceed very rapidly to give various substitution products. o-Dinitrobenzene is as reactive as p-dinitrobenzene, but m-dinitrobenzene is less reactive. 

SNAr reactions also provide an important strategy for the preparation of various kinds of diaryl ethers. p-Dinitrobenzene reacts with even sterically hindered phenols to give the corre-... 

Recently, an interesting reaction of p-dinitrobenzene with trialkylborane has been reported, in which the nitro group is replaced by an alkyl group in good yield (Eq. 9.12).21 The reaction is not a simple ionic reaction, but proceeds via free radical intermediates. 

The ordering of the two values is hard to rationalize because there are two opposing influences. Because of its shape, p-dinitrobenzene may be expected to pack better than the m-isomer, thereby increasing its relative enthalpy of fusion. However, the latter has a nonzero dipole moment while the former does not. Intermolecular, electrostatic attraction is expected to be stronger for the crystalline meta compound. 

Fig. 3 Electrochemical and homogeneous standard free energies of activation for self-exchange in the reduction of aromatic hydrocarbons in iV.A -dimethylformamide as a function of their equivalent hard sphere radius, a. 1, Benzonitrile 2, 4-cyanopyridine 3, o-toluonitrile 4, w-toluonitrile 5, p-toluonitrile 6, phthalonitrile 7, terephthalonitrile 8, nitrobenzene 9, w-dinitrobenzene 10, p-dinitrobenzene 11, w-nitrobenzonitrile 12, dibenzofuran 13, dibenzothiophene 14, p-naphthoquinone 15, anthracene 16, perylene 17, naphthalene 18, tra 5-stilbene. Solid lines denote theoretical predictions. (Adapted from Kojima and Bard, 1975.)...

TRINITROBENZENE m-DIBROMOBENZENE m-CHLORONITROBENZENE o-CHLORONITROBENZENE p-CHLORONITROBENZENE tn-D I CHLOROBENZENE o-DICHLOROBENZENE p-DI CHLOROBENZENE m-DIFLUOROBENZENE o-DIFLUOROBENZENE p-DIFLUOROBENZENE m-DI NITROBENZENE o-DINITROBENZENE p-DINITROBENZENE BROMOBENZENE MONOCHLOROBENZENE m-CHLOROPHENOL o-CHLOROPHENOL p-CHLOROPHENOL... 

Dinitrobenzene, see 1,3-Dinitrobenzene m-Dinitrobenzene, see 1,3-Dinitrobenzene o-Dinitrobenzene, see 1,2-Dinitrobenzene p-Dinitrobenzene, see 1,4-Dinitrobenzene... 

The yields of reaction products from thermal nucleophilic substitution reactions in DMSO of 0- and p-nitrohalobenzenes (Zhang et al. 1993) or p-dinitrobenzene (Liu et al. 2002) with the sodium salt of ethyl a-cyanoacetate were found to be markedly diminished from the addition of small amounts of strong electron acceptors such as nitrobenzenes. At the same time, little or no diminution effects on the yields of the reaction products were observed from the addition of radical traps such as nitroxyls. These results are consistent with the conclusion that such reactions proceed via a nonchain radical nucleophilic substitution mechanism (Scheme 4.26). 

Both routes, a and b, are nonchain processes because the yields of the final products (54 and 35%, respectively) do not change on addition of p-dinitrobenzene and tert-butylnitrone as well as in the... 

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