Kinetic Investigation of Thermal Decomposition Reactions of Podophyllotoxin and Its Derivatives

The thermal behavior and thermal decomposition kinetic parameters of podophyllotoxin (1) and 4 derivatives, picropodophyllin (2), deoxypodophyllotoxin (3),beta-apopicropodophyllin (4), podophyllotoxone (5) in a temperatare-programmed mode have been investigated by means of DSC and TG-DTG. The kinetic model functions in differential and integral forms of the thermal decomposition reactions mentioned above for first stage were established. The kinetic parameters of the apparent activation energy Ea and per-exponential factor A were obtained from analysis of the TG-DTG curves by integral and differential methods. The most probable kinetic model function of the decomposition reaction in differential form was (1-a)(2) for compounds 1-3, (3)\(2) circle a(-1/2) for compound 4 and (2)\(1) (1-a)circle[-1n (1-a)](-1) for compound 5. The values of E-a indicated that the reactivity of compounds 1-5 was increased in the order: 5 < 4 < 2 < 1 < 3. The val ues of the entropy of activation Delta S-#, enthalpy of activation Delta H-# and free energy of activation Delta G(#) of the reactions were estimated. The values of Delta G(#) indicated that the thermal stability of compounds 1-3 with the same f(a) was increased in the order: 2 < 3 < 1.