Equilibrium and kinetics of proton transfer from thiocarboxylic acids to ?-collidine

Abstract

NMR and IR spectroscopy have been used in studying the equilibrium in the reaction of proton transfer from thiocarboxylic acids RCOSH [R=CH3 (a), C6H5 (b) or CH2Cl (c)] to γ-collidine (d), and also the kinetics of CH/NH proton exchange between protonated γ-collidine and excess RCOSH. For compoundsa and b, partial protonation of the γ-collidine is observed; and for compound c, complete protonation. The heat of reaction of proton transfer with the participation of binary acidamine associates is 30 (a) and 45 (b) kJ/mole. The rate of proton exchange decreases and the activation energy E increases with increasing acidity of the RCOSH [E=44 (b) and 88 (c) kJ/mole] and with increasing basicity of the amine (Ed < ETEA), which, in accordance with the orders of reaction that were found for the exchanged components, is due to a mechanism in which the slow stage is proton transfer in the ion pair NH+...−SOCR. The thiocarboxylate ion of c is unstable; and after splitting out Cl−, it forms the compounds Cl(CH2COS)2− and (CH2COS)2.