Theoretical study of neighboring carbonyl group participation in the elimination kinetics of chloroketones in the gas phase

The gas-phase elimination of kinetics 4-chlorobutan-2-one, 5-chloropentan-2-one, and 4-chloro-1-phenylbutan-1-one has been studied using electronic structure methods: B3LYP/6-31G(d,p), B3LYP/6-31++G(d,p), MPW91PW91/6-31G(d,p), MPW91PW91/6-31++G(d,p), PBEPBE/6-31G(d,p), PBEPBE /6-31++G(d,p), and MP2/6-31++G(d,p). The above-mentioned substrates produce hydrogen chloride and the corresponding unsaturated ketone. Calculation results of 4-chlorobutan-2-one suggest a non-synchronous four-membered cyclic transition state (TS) type of mechanism. However, in the case of 5-chloropentan-2-one and 4-chloro-1-phenylbutan-1-one, the carbonyl group assists anchimerically through a polar five-membered cyclic TS mechanism. The polarization of the C - Cl bond, in the sense of C^δ+.Cl^δ-, is a rate-determining step in these elimination reactions. The significant increase in rates in the elimination of 5-chloropentan-2-one and 4-chloro-1-phenylbutan- 1-one is attributed to neighboring group participation due to the oxygen of the carbonyl group assisting the C - Cl bond polarization in the TS.