Benzene nitration, reaction profiles

There are certain limitations to the usefulness of nitration in aqueous sulphuric acid. Because of the behaviour of the rate profile for benzene, comparisons should strictly be made below 68% sulphuric acid ( 2.5 fig. 2.5) rates relative to benzene vary in the range 68-80% sulphuric acid, and at the higher end of this range are not entirely measures of relative reactivity. For deactivated compounds this limitation is not very important, but for activated compounds it is linked with a fundamental limit to the significance of the concept of aromatic reactivity as already discussed ( 2.5), nitration in sulphuric acid cannot differentiate amongst compounds not less than about 38 times more reactive than benzene. At this point differentiation disappears because reactions occur at the encounter rate. 

In the section dealing with electrophilic attack at carbon some results on indazole homocyclic reactivity were presented nitration at position 5 (Section 4.04.2.1.4(ii)), sulfon-ation at position 7 (Section 4.04.2.1.4(iii)) and bromination at positions 5 and 7 (Section 4.04.2.1.4(v)). The orientation depends on the nature (cationic, neutral or anionic) of the indazole. Protonation, for instance, deactivates the heterocycle and directs the attack towards the fused benzene ring. A careful study of the nitration of indazoles at positions 2, 3, 5 or 7 has been published by Habraken (7UOC3084) who described the synthesis of several dinitroindazoles (5,7 5,6 3,5 3,6 3,4 3,7). The kinetics of the nitration of indazole to form the 5-nitro derivative have been determined (72JCS(P2)632). The rate profile at acidities below 90% sulfuric acid shows that the reaction involves the conjugate acid of indazole. 

Here, BDC is 1,4-benzene-dicarboxylate (terephthalic acid) and Zn40(BDC)3 represents the MOF-5 imit composition. The reaction equilibrium can be shifted toward formation of the MOF product by tuning the concentration profiles of the solvent, the water released, or the nitrate ions produced. Since esterification reactions can be driven in both directions without difficulty, it appears evident that the stability of MOF materials in applications could depend on polar protic environments and pH values (128). 

Once a likely Br0nsted acid site was determined from the above protocol, a number of acetyl nitrate orientations around the acid site were generated and optimized. The lowest energy orientation was chosen to study the reaction of acetyl nitrate with toluene. The toluene was placed close to the acetyl nitrate NO2 moiety for two scenarios electrophilic para attack and electrophilic ortho attack. The H-beta zeolite, acetyl nitrate, and toluene models were then optimized to give the reactant minima for subsequent transition state searches. The mechanism for the reactant, intermediate, and product minima and transition states for para and ortho toluene nitration were partially inspired by the mechanistic work of Olah and coworkers " and the woik of Silva and Nascimento" for the nitration of benzene in the gas phase and via formyl nitrate and a 5-T (pentameric) cluster carved from beta zeolite, respectively. From these minima, the appropriate transition states were found using the transition state algorithm described earlier to build potential energy profiles for the nitration of toluene by acetyl nitrate in the presence of the beta acid site catalyst. 

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