Trichloroisocyanuric acid (TCCA) and carboxamide interactions in TCCA/NaNO2 triggered nitration of pyrrole and indole in aqueous aprotic media: A kinetic correlation of solvent properties with reactivity

Abstract

AbstractThis study deals with the trichloroisocyanuric acid (TCCA) interactions with carboxamides like formamide (FMA), N,N′‐dimethyl formamide (DMF), and N,N′‐dimethyl acetamide (DMA) interactions during the nitration of heterocyclic compounds (HC) like pyrrole and indole in the presence of excess of [NaNO2] over the concentrations of all other reactants. All the reactions were performed in aqueous acetonitrile media containing carboxamide under acid‐free conditions. Kinetics of the reactions revealed first order in [nitrating agent] and [HC] under otherwise similar conditions. To gain an insight into the reactive species and role of added carboxamide (FAA, DMF, DMA, etc.), the observed rates of the nitration reaction (log k) were analyzed as a function of (1/D), ([D − 1]/[2D + 1]), mole fraction (nx), and volume (%) of carboxamide, 1/viscosity, density refractive index function), and Hildebrand solubility parameter plots. Linear regression analysis gave very good correlation coefficients (R2 values), which indicate the importance of several solvent properties in addition to the role of dielectric constant (D) of the reaction media. Multiple linear solvent energy relationships suggested by Abraham, Koppel, Palm, and Taft also afforded very good correlation coefficient (R2 values), showing the importance of cumulative effect of solvent properties. Besides these features, the negative entropies of activation (∆S#) suggest greater solvation in the transition state. Isokinetic temperature (β) values for different protocols were very close to the experimental temperature range (303‐323 K), indicating the importance of both enthalpy and entropy factors in controlling the reaction.