Nitro-benzene coefficient

One might expect this to be more inffuential at lower acidities where there is a good supply of free water and be especially hard to detect by non-unit slope Ho plots at higher acidities. This is in agreement with the fact that the amides and pyrrole-indole bases which are protonated in relatively weak acid show serious deviations from Hammett indicator behavior while carboxylic acids (327) and nitro-benzenes (36) appear to be good indicators since they are protonated in the region where activity coefficient behavior apparently becomes more ideal. 

Robertson et al.261 measured rates of bromination of some aromatic hydrocarbons in acetic acid containing sodium acetate (to eliminate protonation of the aromatic by liberated hydrogen bromide) and lithium bromide (to reduce the rate to a measurable velocity ) at 25 °C, the second-order rate coefficients for 3-nitro-N,N-dimethylaniline and anisole being 14.2 and 0.016 respectively the former compound was thus stated to be about 1012 times as reactive as benzene (though no measurement of the latter rate coefficient, inferred to be 1.33 xlO-11, could be found in the literature) and this large rate spread gives one further indication of the unreactive nature of the electrophile. Other rates relative to benzene were ... 

The most valuable and comprehensive kinetic studies of alkylation have been carried out by Brown et al. The first of these studies concerned benzylation of aromatics with 3,4-dichloro- and 4-nitro-benzyl chlorides (these being chosen to give convenient reaction rates) with catalysis by aluminium chloride in nitrobenzene solvent340. Reactions were complicated by dialkylation which was especially troublesome at low aromatic concentrations, but it proved possible to obtain approximately third-order kinetics, the process being first-order in halide and catalyst and roughly first-order in aromatic this is shown by the data relating to alkylation of benzene given in Table 77, where the first-order rate coefficients k1 are calculated with respect to the concentration of alkyl chloride and the second-order coefficients k2 are calculated with respect to the products of the... 

It was expected that the nitro and cyano groups in 85 d and e would have lowered the benzene LUMO sufficiently that reaction could occur there. However, these LUMOS are localized essentially on the substituent groups, and the LUMO coefficient in the carbene position is low, judging from the electron densities in nitro- and cyanobenzene radical anions 89 ). Furthermore, these groups deactivate all ring positions towards electrophilic substitution. An example from the nitrene field, which shows that a p-cyano group may decelerate ring expansion, will be discussed in Section IV. 1. 

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