The transition state conformational effect on the activation energy of ethyl acetate neutral hydrolysis

Abstract

We report a first-principles study on ethyl acetate neutral hydrolysis in which we focus on the activation energy variation resulting from the conformational effect in the transition state. We use the conformers of ethyl formate, ethyl acetate, ethyl fluoroacetate, and ethyl chloroacetate as the ester models and one water molecule with a one-step reaction mechanism. We also consider the long-range interaction and the surrounding water in the form of PCM. Our results show that the various conformers yield a significant range of activation energy. Moreover, the gauche conformer has lower activation energy than the trans conformer. The activation energy in its own right is lowered by the halogen atoms. Finally, we remark that the long-range correction and PCM stabilize the transition state geometry but raise the activation energy.

A Density-Functional Study of the Conformational Preference of Acetylcholine in the Neutral Hydrolysis

“…All conformers have an average bonding orbital of 0.5464 C(sp 2.90 ) + 0.8375 O(sp 2.19 ). This bonding is relatively weaker than the C1-O2 bond of ethyl acetate, which is 0.5898 C(sp 1.91 ) + 0.8076 O(sp 1.42 ) [28]. It suggests that the neutral hydrolysis of ACh + is easier than that of ethyl acetate.…”

“…We used B3LYP functionals and the 6-311++G(d,p) basis set integrated in Gaussian 09 software [32]. The use of B3LYP functionals follows its success in our previous similar study on chemical reactions [28,33,34] and other similar cases [35,36]. The optimization-routine calculations are to obtain the stable structures and the total electronic energy of ACh + Br -, water, and AA and, more importantly, to find the stable conformers of ACh + and Ch + .…”

“…The optimization-routine calculations are to obtain the stable structures and the total electronic energy of ACh + Br -, water, and AA and, more importantly, to find the stable conformers of ACh + and Ch + . For the TS, we followed the same procedure used in our previous study [28], where we applied the TS optimization and the intrinsic reaction coordinate routines of calculation. Besides the energy and the structure, we also calculated the charge population using the Natural Bond Orbital (NBO) program [37].…”

“…This site is typical for the ester family. According to our study on ethyl acetate neutral hydrolysis [28], the activated complex (ACh + -water) forms between C1 and O3, the nucleophilic site of water. The electrophilic site of the ACh + conformers gives a hint to the cleaving location during the hydrolysis.…”

“…We consider two important things: a one-step mechanism for the reaction model and the conformation of ACh + backbone dihedral angles. Despite its simplicity, the former worked well in revealing the conformational effects in the ethyl acetate neutral hydrolysis [28]. Therefore, we can focus on the conformation in one particular transition state (TS).…”