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Linear and nonlinear free energy relationships in nucleophilic substitution reaction of 2-bromo-5-nitrothiophene with morpholine



The reaction of 2-bromo-5-nitrothiophene with morpholine was studied as an aromatic nucleophilic substitution reaction in various compositions of methanol with ethyl acetate and aqueous solution of methanol, ethanol, and propane-2-ol at 25°C. The second-order rate coefficients of the reaction were spectrophotometerically determined. It was shown that a mounting trend with the mole fraction of water in aqueous solution of alcohols and a mild decreasing with the mole fraction of ethyl acetate in methanol–ethyl acetate mixtures. Solvent effect investigations based on linear free energy relationship (LFER) confirm that polarity has a major effect, whereas the hydrogen-bond donor and acceptor abilities of the media have a minor effect on the reaction rate. A nonlinear free energy relationship based on preferential solvation hypothesis showed differences between the microsphere solvation of the solute and the bulk composition of the solvents, and nonideal behavior was observed in the trend of rate coefficients, which was consistent with LFER results. © 2012 Wiley Periodicals, Inc. Int J Chem Kinet 45: 59–67, 2013