Chlorination Chemistry 4. Ab Initio Study of the Addition, Metathesis, and Isomerization Channels Governing the Reaction of Chlorine Atom with Propargyl Chloride

Highly correlated ab initio molecular orbital calculations have been used to map out the potential energy surface corresponding to the reaction of Cl + propargyl chloride (C₃H₃Cl) in the gas phase. Ten transition state structures governing the mechanism of the title reaction were computed at seven different levels of theory up to QCISD(T)/6-311+G(d,p)//QCISD(T)/6-31+G(d,p). Chlorine atom additions at the center and end unsaturated carbons are barrierless processes forming incipient, chemically activated 2,3-dichloro-1-propen-1-yl and 1,3-dichloro-1-propen-2-yl radicals, respectively. The incipient radicals cannot isomerize by transferring a hydrogen atom because the governing transition state structures reside at ≥ 95 kJ·mol^-1 above the initial reactants. The presence of a transition state for chlorine atom transfer below the energy of the initial reactants enables isomerization between the incipient radicals. A second accessible chlorine transfer transition state enables the 1,3-dichloro-1-propen-2-yl radical to isomerize into the more stable 1,2-dichloroallyl radical. The chemically activated 1,3-dichloro-1-propen-2-yl radical also undergoes bond scission of the chlorine atom in the chloromethyl group forming chloroallene and Cl atom. Chlorine atom attack on the chloromethyl group encounters metathesis transition states for HCl and Cl₂ formation at 5 kJ·mol^-1 and 124 kJ·mol^-1 above the initial reactants, respectively. The accord demonstrated between master equation calculations, based on the ab initio results, and experimental data validate the proposed reaction mechanism and predict that the dominant products of chlorine atom addition to propagryl chloride are chloroallene and the 1,2-dichloroallyl radical at 298 K and HCl, chloropropargyl radical, and chloroallene at 1000 K.