Theoretical study of DABCO-based ionic liquid: synthesis and reaction mechanism

Quaternary ammonium salt obtained from the Menshutkin reaction between DABCO and benzyl chloride has been used in the synthesis of a novel Bronsted acidic ionic liquid (IL), namely 1-benzyl-4-(sulfobutyl)-diaza-bicyclo-octane hydrogen sulfate. The reaction of DABCO with benzyl chloride is a crucial step in the synthesis of this IL. Density functional theory calculations at B3LYP/6-31G(d,p) level have been employed to investigate the mechanism of Menshutkin reaction by calculating the energy barriers through possible transition states i.e., five-membered ring transition state and S(N)2 transition state in gas phase and in diethyl ether as a solvent. It was found that while DABCO reacts with benzyl chloride through the well-known S(N)2 transition state mechanism, the corresponding reaction with chlorodiphenylmethane can proceed through both S(N)2 and five-membered ring transition state mechanism. However, S(N)2 transition state mechanism is still the strongly preferred one out of the two possible mechanisms. The electronic structure analysis shows that solvent effects and enhanced resonance stabilization may play a decisive role in guiding the reaction pathway.