Nitrobenzene 1 General observations

There Is an Inverse correlation between the gas-phase relative rates of nitration and the generally observed trend of the relative rates of electrophilic aromatic siibstltutlons In solution. Toluene reacts 3 times slower and nitrobenzene 10 times faster than benzene. This suggests that nitration In the gas-phase Is nucleophilic In character, and thus does not correspond at all to solution-phase electrophilic nitration by the NO-" " Ion. We tentatively suggest as a possible rationalization of this unexpected result that the gas-phase reaction Involves primary electrostatic Interaction of the aromatic substrate with the nucleophilic terminal oj gen of the CH20N02" cation, followed by displacement by the aromatic ring on nitrogen, with simultaneous elimination of formaldehyde. 

Ort/io-selectivity is generally observed in the reactions of 2,4-dichloro- and 2,4-difluoro-nitrobenzene with alkoxide and thiophenoxide ions [199]. Also in less activated systems, nucleophiles generated from phenols and thiophenols with potassium fluoride-alumina and 18-crown-6-polyether will react in DMSO with cyano- or nitro-substituted fluoro- or chloro-benzenes [200]. Interestingly, the reaction of difluorobenzenes with two diffoent alcohols can occur sequentially. Introduction of the first etho" function deactivates the ring, and use of more forcing conditions allows substitution of the second fluorine [201]. Consecutive displacements of fluorine and nitro groups have been observed in the reaction of ort/io-fluoronitrobenzene with chiral acyl bicyclic lactones in a highly enantioselective synthesis of spirooxindoles [202]. 

The photoreduction of nitrobenzene had already been observed by Ciamician and Silber More recently, quantitative studies have been carried out by Testa and his group n-is) and other authors ts-is), Jt jg generally accepted n-is, 17,18) that the long wavelength ( -> 340 nm) absorption of nitrobenzene must be... 

About 800 aniline points or CST of hydrocarbons with aniline are listed. Since CST of hydrocarbons with nitrobenzene are about 50° C. lower than aniline points with the same hydrocarbons, nitrobenzene CST can be estimated easily for these 800 hydrocarbons. CST of many other aromatic solvents with the same hydrocarbons may be approximated similarly. Nonaromatic solvents are not so nearly parallel in this respect, but certain generalities are apparent among similar groups of components (139, 140). Moreover, many more aniline points (and from them other CST) of unknown isomeric hydrocarbons could be estimated with reasonable confidence by one of six equations for aniline points of hydrocarbon classes (that section of Table I). Altogether this table might furnish a basis for estimates of about a million CST. Even so, combinations are encountered frequently which are not predictable from this table, and new observations are required. 

In general, the spin-spin splittings observed between the five protons of a phenyl group can be extremely complex. An example is afforded by nitrobenzene (Figure 22-5), which has different chemical shifts for its ortho, meta, and para hydrogens and six different spin-spin interaction constants J23, J24, J2S, J26 Jst, Jsi (the subscripts correspond to position numbers of the protons) ... 

At high buffer concentrations, positive curvature may be observed in buffer dilution plots, indicating that the general acid and base are simultaneously participating in the rate-determining step.27 In such a case, the rate law must be expanded by third-order terms. Furthermore, plots of buffer slopes versus xHb may be nonlinear, when the unstable tautomer is a diprotic acid as, for example, the aci-nitro tautomer of nitrobenzene.28... 

The extent of the agreement between the experimental and calculated values is far better than observed for benzoylation in nitrobenzene but is less than that for mercuration or bromination. The steric requirements of acylation are much greater than for either bromination or mercuration. For toluene, mercuration yields 24.9% o- and bromination yields 32.9% o-, whereas acetylation produces only 1.2% o-substi-tution product. For acetylation, the observed rates of substitution are generally smaller than the calculated values for all cases involving... 

The association of nitrobenzene and nitronaphthalene in non-polar solvents, such as hexane and carbon tetrachloride, forms an exception. Here the association depends in the first place on interaction of the Keesom type with the very large moment (4.20 D). The stronger association of the last-mentioned compound points, however, in addition to an interaction due to complex resonance as observed between nitro compounds and aromatic hydrocarbons in general. It is plausible that when dissolved in benzene this association gives way to a solvation. This interaction between solute and solvent molecules is closely related to the association between like molecules. In benzene etc. no stoichiometric association is observed but, owing to the anisotropy of the polarizability, a more or less parallel... 

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