Bronsted nitro groups

The relative sizes of the Hammett p and Bronsted a constants will determine the relative rate of 5-nitrosalicylamide. If intramolecular base catalysis applies, then 5-nitrosalicylamide should hydrolyse more rapidly, since the nitro group will increase the susceptibility of the amide bond to attack by hydroxide ion and increase the efficiency of the phenolic hydroxyl as a general acid catalyst. The value of Jtobs at the plateau region was found to be 18 times smaller for the 5-nitrosalicylamide than for salicylamide a mechanism of intramolecular general base catalysis is, therefore, the preferred mechanism. 

This carbanion further adds to the carbonyl compound to form an intermediate, which removes a proton from water to give the final nitro aldol product (Figure 3.9). Solid bases without Bronsted hydroxy groups such as decarbonated hydrotalcites are not active in this reaction. The Mg-Al hydrotalcite catalyst is non-toxic and can be recycled with little loss in activity.42... 

General base catalysis of the cyclization of ethyl 2-hydroxymethyl 4 nitrobenzoate [35], where the acyl group is further activated by electron withdrawal by the nitro-substituent, is characterized by a Bronsted coefficient of 0 97, i.e. unity within the limits of error, suggesting that proton transfer is a diffusion-controlled process (Fife... 

Data for the reactions of several cyclic tertiary amines with phenyl, 4-nitro-phenyl and 2,4-dinitrophenyl acetates, at 25°C and ionic strength 1.0, appear in Table 40, and as a Bronsted plot in Fig. 20. The usual irregularities of such plots for nucleophilic attack are evident. Linear relationships between log k and pKa are generally found for groups of compounds of closely similar structure, as for the substituted pyridines in Fig. 20. The data for the two tricyclic amines fall on separate curves, and the points for imidazole clearly fall on neither of the first two sets of lines. The separate lines for the reactions of particular classes of nucleophile are approximately parallel, as is usually found. 

Nucleophilic and Radicaloid Substitution Reactions of Monocyclic Thio-phens. - The rate constants for the reaction of 5-substituted 2-bromo-3-nitrothiophens, in methanol, with various substituted anilines to give 5-substituted iV-(3-nitro-2-thienyl)anilines have been measured at various temperatures, It was found that the sensitivity parameters that were obtained from Hammett and Bronsted correlations were practically independent of the starting system. The kinetics of piperidino-substitution of methyl 2-methoxy-3-nitrothiophen-5-carboxylate and 5-acetyl-2-methoxy-3-nitrothio-phen, in which the methoxy-group is the leaving group, have been investigated and the mechanism has been discussed. 

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