Thermodynamic and kinetic models for acid chloride formation: A computational and theoretical mechanistic study

The structural evolution pathways leading to the conversion of some α-chlorinated carboxylic acids (Cl_nH_3-nCCOOH, n = 0, 1, 2, 3) to their respective acid chlorides by thionyl chloride are investigated via density functional theoretical (DFT) modelling. For all compounds where n = 0-3, acid chloride formation occurs via two competing pathways, consisting of three activation barriers in both cases, all of which are enthalpy-controlled and moderate (ΔG^ǂ < 190 kJ mol^-1). Though both pathways are not limited by the same step, they are both composed of only cyclic activated complexes. Rapid intra-molecular small molecule transfer (HCl) allows one pathway to be slight more productive than the other. Whereas all acids evolve via both competing pathways, the evolution of formic acid occurs exclusively via that which involves intramolecular HCl transfer where all the constituent transition states are formed quasi-synchronously. Results for both pathways are summarized in a detail kinetic model which, of-course, is based on the thermodynamic profiles.