Thermoanalytical investigations of some chloro-organic pesticides and related compounds

Abstract

The thermal features of several chloro-organic pesticides, namely lindane, HCB, PCNB, p, p'-DDT, dicofol and p, p'-methoxychlor, as well as some related compounds were studied using thermoanalytical methods (DTA, TG, DTG) and by the application of standard procedures to identify some of the reaction products. Heating of the compounds at a constant rate leads to their total volatilization. Most of the compounds studied undergo simple evaporation or sublimation with rising temperature. The volatilization of p, p'-DDT, o, p'-DDT, dicofol and p, p'-methoxychlor is, however, accompanied by chemical changes in the system. It has been revealed that the primary step in the decomposition of p, p'-DDT is the elimination of HCl and formation of p, p'-DDE. The decomposition of o, p'-DDT and p, p'-methoxychlor presumably begins with an analogous process. In the case of the compounds exhibiting simple volatilization patterns, the thermodynamics and kinetics of the process were examined on the basis of the TG curves. The enthalpy of volatilization was evaluated from the Clausius-Clapeyron equation. The kinetics were analysed using a standard approach assuming that the rate is described by an equation comprising the term characterizing a kinetic model for the process and the Arrhenius term accounting for the influence of temperature on the process. This procedure revealed that the rate determining step is well represented by the Polanyi-Wigner model without any activation barrier over that resulting from the thermochemical requirements. The enthalpies of formation of some of the compounds were evaluated on the basis of the Benson's group additivity scheme. These characteristics together with the information on the thermal behaviour of these derivatives enabled the possible pathways for the thermal decomposition of DDT isomers, α-chloro-DDT, dicofol and p, p'-methoxychlor to be revealed.