Thermochemical parameters of chlorinated compounds of pyridine

Abstract

Thermochemical and geometrical parameters of all chlorinated compounds of pyridine were calculated with the CBS-QB3 composite method. Standard entropies, standard Gibbs free energies of formation, standard enthalpies of formation, and heat capacities were computed and compared with their corresponding available experimental data. Our calculated enthalpy values agree well with a rather limited corresponding experimental data. Adjacent chlorinated sites in pyridine was found to incur a thermodynamic penalty of 5.0 kcal/mol. While chlorination of pyridine is carried out at elevated temperatures in the gas and solvent media, acquiring the trend underpinning chlorination sequence at room temperature provides an insightful mechanistic insight. For this reason, we calculated Fukui indices for electrophilic substitution and attempted to link obtained values with thermodynamic stability orderings computed at 25 °C. Overall, the pattern and degree of chlorination induces very minor geometrical differences in reference to the unsubstituted pyridine. Calculated Fukui indices predicts the chlorination sequence as follows; 2-chloro → 2,5-dichloro → 2,3,6-trichloro → 2,3,5,6-tetrachloro → 2,3,4,5,6-pentachloropyridine. However, a significant positive charge accumulated in the N atom of the ortho-Wheland-type adduct renders its thermodynamically unstable by 8 kcal/mol in reference to the meta-Wheland intermediate. Overall, the sequence of chlorination is most likely to be sensitive to kinetics factors rather than thermodynamic attributes; i.e., energies required to form the Wheland-type intermediates.