Nitrobenzene inductive effects

In nitrobenzene the nitro group has a large electron affinity, and accordingly draws electrons away from the ring. The resonance effect works in the same direction, and, as a result, all positions have a deficiency of electrons. The meta positions are least affected, and the substitution takes place there with difficulty. In aniline, the inductive effect and the resonance effect oppose each other, but the latter wins out, and very easy o-p substitution takes place. 

Deprotonation provides the necessary electron push to kick out the electron pair joining C(6) with the nitrobenzene oxygen. If, however, N(l) is alkylated (as with the nucleosides and nucleotides), OH catalysis is much less efficient since it now proceeds by deprotonation from N(3) (with the uracils) or from the amino group at C(4) (with the cytosines). In these cases the area of deprotonation is separated from the reaction site by a (hydroxy)methylene group which means that the increase in electron density that results from deprotonation at N(3) is transferable to the reaction site only through the carbon skeleton (inductive effect), which is of course inefficient as compared to the electron-pair donation from N(l) (mesomeric effect) [26]. Reaction 15 is a 1 1 model for the catalytic effect of OH on the heterolysis of peroxyl radicals from pyrimidine-6-yl radicals (see Sect. 2.4). 

The nitro group has a considerable influence on the properties of the whole molecule of an aromatic compound. For example, owing to the presence of the nitro group, nitrobenzene does not take part in the Friedel-Crafts reaction. The reactivity of a chlorine atom, brought about by the presence of a nitro group in the ortho or para position, may be represented by a diagram based on the induction effect (la). 

A silicon atom might be expected to release electrons inductively, but because of empty 7-orbitals shows the overall character ( + 7 —717). Nitration of trimethylsilylbenzene with nitric acid in acetic anhydride at —10 to o °C gives 25-5,39-8,30-2 and 6-8 %, respectively, of 0-, m-, and /)-nitro-trimethylsilylbenzene and nitrobenzene, with a rate of reaction relative to that of benzene of about 1-5. The figures give no indication of an important conjugative effect. 

Subsequent DTA investigation showed that an exothermic reaction set in above 75°C after an induction period depending on the initial temperature and concentration of reactants, which attained nearly 300°C, well above the decomposition point of the cyclic ester component (170°C). The reaction conditions used could have permitted local over-concentration and overheating effects to occur, owing to slow dissolution of the clumped solid ester and aniline in the nitrobenzene solvent [ ] Crude carbyl sulfate contains excess sulfur trioxide [2]. 

The electrical conductivity isotherms of these systems are represented in Fig. 2, and the data are given in Tables 6-9. Tbe product x rf, which was used in our earlier papers [ 1] as a suitable measure of the extent of basic ionization in sulphuric acid solutions, can here be substituted simply by conductivity [6], because the differences in viscosities of these mixtures in the region of sulphuric acid are not considerable. It can be seen that the values of the electrical conductivities increase in the order nitrobenzene < m-nitrotoluene < o-nitrotoluene < p-nitrotoluene. This means that the basicities of these compounds increase in the same order, our results in that respect being in complete agreement with the conclusions of Gillespie and Solomons [6]. This increase of basicity is the result of the positive inductive influence of the methyl group in the benzene ring, but steric effects also play some part [6]. 

Therefore the electron substituent effects in radical dianions of 2-substituted 5(6)-nitrobenzimidazoles are transmitted with approximately equal contributions of inductive and resonance components. A similar picture is observed for para-substituted nitrobenzene radical anions also [689, 883], Actually, the aN(N02) correlation of 2-substituted 5(6)-nitrobenzimidazoles RDA (RDA BI) with the aN(N02) of para-substituted nitrobenzene RA (RA Bz) indicates the same mechanism of substituent effects transmission but with different intensity, as shown in equation (3.4) ... 

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