Effect of solvent on the kinetics of the reaction between dialkylbenzimidamides and 4-nitrophenylnitromethane

Abstract

Rate constants have been determined for the proton-transfer reaction of 4-nitrophenylnitromethane, NO₂C₆H₄CH₂NO₂, with two NN-dialkylbenzimidamides, HNC(C₆H₅)NR₂, where R = 1-butyl, or 1-propyl, in the temperature range 273–303 K in some aprotic solvents of low polarity. In the concentration ranges used the reaction to form an ion-pair product is first-order in acid and first-order in benzimidamide. The reactions were monitored spectrophotometrically employing the stopped-flow method. An examination of the relationships of the forward proton-transfer rate constants at 298 K, in six solvents, for the dibutyl compound to a solvent permittivity function and to the solvent polarity parameter (E_T) shows that a polar activated complex develops; this is compatible with reaction in which ions are produced. The corresponding kinetic parameters for the dipropyl analogue in three solvents are consistent with this pattern. There is no significant effect upon the kinetic parameters of variation of the alkyl group in the benzimidamide. The energies of activation for forward proton transfer are small, in the range 10–23 kJ mol^–1, while in the reverse direction the energies of activation range is 65–114 kJ mol^–1. A Brønsted-type plot using the forward rate constants and the derived equilibrium constants at 298 K yields a coefficient which is indicative of a significant degree of proton transfer in the transition state. However, the highly exothermic nature of the reaction and the low energies of activation in the forward direction suggest a reactant-like activated complex. Provisional results (forward rate constants and energies of activation) for deuteron transfer from the deuterated acid to both benzimidamides in di-1-butyl ether and in chlorobenzene are reported. The hydrogen-ion transfer results are compared with those reported for reaction of the title acid with 1,1,3,3-tetramethylguanidine.