Kinetics of dehalogenation of chloroform by a metal-chelate system

Abstract

The kinetics of dehalogenation of chloroform by mixtures of bis( N-butylsalicylideneiminato)nickel(II) and N,N,N′,N′-tetramethylethylenediamine in 50% (v/v) methanol-chloroform at 60–75°, measured as rates of chloride release were followed potentiometrically. Dehalogenation occurs at a negligible rate in the absence of nickel(II) compound. At a constant amine concentration (4·29 × 10 −3 M), the observed pseudo-first order rate constants varied directly with nickel concentration (0·125 − 0·750 × 10 −3 M). Two mechanisms are proposed for dehalogenation routes in the absence and presence of nickel(II) compound. The former involves proton abstraction as a rate-determining step; the second mechanism involves chelate exchange leading to a transition state in which the enhanced attraction of nickel(II) ion for an incipient chloride ion causes a weakening of the carbon-chlorine bond of the coordinated chloroform molecule. The latter mechanism is consistent with the scope of the reaction and with the observed entropy of activation. The reaction followed the rate expression rate = k ψ [diamine] t = ( k 1 + k 2[Ni] T )[diamine] T . A least-squares treatment of the data obtained at 63° gave values of k 1 = 0·098±0·047 sec −1 and k 2 = 0·582±0·015 sec −1 M −1. Activation parameters were calculated. The Arrhenius energy of activation was 17·±0·6 kcal/mole over the range 60–75° ([Ni] = 2·5 × 10 −3 M, [Diamine] t = 4·29 × 10 −3 M). The enthalpy and entropy of activation were calculated to be 16·5±0·6 kcal/mole and −27 e.u., respectively.