Kinetics and mechanism of the aminolysis of S-ethoxycarbonyl O-ethyl dithiocarbonate and related pyrothiocarbonates

Abstract

The reactions of S-ethoxycarbonyl O-ethyl dithiocarbonate (1), bis(ethoxythiocarbonyl) sulphide (4), and bis(ethoxycarbonyl) sulphide (5) with pyrrolidine, piperidine, and morpholine in 95% aqueous ethanol have been subjected to kinetic study. The rate equation found for all the reactions is k_obs=k_o+k_N[N], where k_obs is the pseudo-first-order rate constant (amines were in excess over the substrates) and N represents the free amine. The rate law accounts for reaction schemes with zwitterionic tetrahedral intermediates, EtOC(S^–)(NH⁺)SCXOEt and EtOC(O^–)(NH⁺)SCXOEt, where X is O or S, formation of which from reactants is the rate-determining step. The intermediates break down to products faster than their deprotonation by the solvent or another amine molecule. This is confirmed by estimation of the rate constants involved in the schemes and evaluation of the pK_a value of the intermediates. From the k_N values obtained it is deduced that amines attack at the CS group of the substrate faster than at the CO group, although the nature of both the substrate and the amine could change this order of reactivity. Activation parameters are reported for all the reactions, and it was found that ΔS^‡ was lower for attack for CO compared with that at CS. This is explained in terms of higher solvation of the transition state for CO attack relative to that for CS attack.