MOLECULAR MODELLING OF THE KINETICS AND THERMODYNAMICS OF GAS-PHASE THERMAL DECOMPOSITION OF XANTHATES

Abstract

Pyrolysis of xanthates (organo sulphur compounds) provides valuable synthetic routes to higher yield of olefins which has become an attraction in polymer chemistry. Thermal decomposition of unsubstituted xanthates has high activation barriers but alkyl substituents provide positive inductive effects which enhances kinetics and thermodynamics of the reaction. However, there is scanty information on the gas phase thermal decomposition of substituted xanthates. Therefore, this research was designed to investigate the effect of progressive methylation on kinetics and thermodynamics of the gas-phase thermal decomposition of some α- and β- substituted alkyl xanthates. Quantum mechanical approach (density functional theory [B3LYP/6-311++G**]) was employed to model the progressive methylation of O-alkyl S-methyl xanthates at the α- and β- carbon positions (O-ethyl, O-npropyl, O-ipropyl, O-nbutyl, O-ibutyl, O-tbutyl) at 629K. Molecular mechanics force field was used to obtain conformers and the most stable conformer of the compounds was further subjected to geometric calculations. Reaction path calculations were carried out on the most stable conformer of each compound and the progress of the reactions was followed by the Wiberg bond indices [average bond indices (δBav), percentage bond evolution (%Ev) and synchronicities (Sy)]. The geometric parameters [bond length, bond angle, dihedral angle and atomic charge distribution] at ground state, transition state and products were calculated using standard method. The data obtained were used to calculate the kinetics [rate constant (k), pre– exponential factors (A), Activation energy (Ea)] and thermodynamic parameters [Change in enthalpy (∆H*), change in entropy (∆S*) and Gibbs free energy (∆G*)] of the substituted alkyl xanthates. The modelled alkyl xanthates revealed the formation of acetylenes, carbonylsulphide and thiol. The energy of formation of stable conformers of the different derivatives ranged from -50.93 to +16.00 kJ/mol. Reaction path showed that the reaction involved a concerted six–membered transition state with bond lengths: C-O (2.08Å, bond breaking); C-H (1.24Å, bond breaking) and S-H (1.83Å, bond making). The %Ev ranged from 65 to 77 for C-O breaking, 30 for C-C formation and 39 to 43 for S-H formation. These showed that breaking of C-O was the most advanced process hence C-O bond breaking was the rate determining step. The least advanced process was the formation of iii C-C and S-H bonds. The δBav ranged from 0.478 to 0.485 indicating that the transition states have an early character, while Sy ranged from 0.899 to 0.932 for O-alkyl S-methyl xanthate indicating that the mechanism corresponds to highly asynchronous process. The Ea, A and k ranged from 166.20 to 149.18 kJ/mol, (4.90 to 7.18) x1011 and 1.04x10-3 to 4.30 s-1 , respectively while, the thermodynamic parameters ranged from 161.34 to 128.04 kJ/mol (∆H*); -24.00 to -31.16 J/mol(∆S*); and 142.90 to 178.44 kJ/mol (∆G*). These parameters decreased with progressive methylation, and with corresponding increase in rate constant of thermal decomposition. Progressive methylation in gas phase at the α- and/or β- position of O-alkyl S methyl xanthates lowered thermodynamic parameters and activation energy with corresponding increase in entropy change and rate of reaction.