A Kinetic Study on Aminolysis of S-4-Nitrophenyl Thiobenzoate in H2O Containing 20 mol % DMSO and 44 wt % EtOH: Effect of Medium on Reactivity and Mechanism

Abstract

Second-order rate constants (<TEX>$k_N$</TEX>) have been measured for nucleophilic substitution reactions of S-4-nitrophenyl thiobenzoate with a series of alicyclic secondary amines in <TEX>$H_2O$</TEX> containing 20 mol % DMSO at 25.0 <TEX>${\pm}$</TEX> 0.1 <TEX>${^{\circ}C}$</TEX>. The Br<TEX>$\phi$</TEX>nsted-type plot exhibits a downward curvature, i.e., <TEX>$\beta_{nuc}$</TEX> decreases from 0.94 to 0.34 as the amine basicity increases. The reactions in the aqueous DMSO have also been suggested to proceed through a zwitterionic tetrahedral intermediate (T<TEX>${\pm}$</TEX>) with change in the RDS on the basis of the curved Br<TEX>$\phi$</TEX>nsted-type plot. The reactions in the aqueous DMSO exhibit larger <TEX>$k_N$</TEX> values than those in the aqueous EtOH. The macroscopic rate constants (<TEX>$k_N$</TEX>) for the reactions in the two solvent systems have been dissected into the microscopic rate constants (<TEX>$k_1\;and\;k_2/k_{-1}$</TEX> ratio) to investigate effect of medium on reactivity in the microscopic level. It has been found that the <TEX>$k_2/k_{-1}$</TEX> ratios are similar for the reactions in the two solvent systems, while <TEX>$k_1$</TEX> values are larger for the reactions in 20 mol % DMSO than for those in 44 wt % EtOH, indicating that the larger <TEX>$k_1$</TEX> is mainly responsible for the larger <TEX>$k_N$</TEX>. It has been suggested that the transition state is more stabilized in 20 mol % DMSO through mutual polarizability interaction than in 44 wt % EtOH through H-bonding interaction.