Kinetic profiles are easily measured for catalyzed ester hydrolysis of nitrophenyl acetate. Since the product, 4-nitrophenol, is UV-active, the reaction rates can be directly collected using UV-vis spectroscopy at ambient temperatures and pressures.<sup>1,2</sup> These features of the reaction make it an attractive pedagogical option for undergraduate physical chemistry labs, since it demonstrates how to find rate constants for catalyzed and uncatalyzed pathways using pseudo-first order assumptions. This study aims to redesign the lab to also investigate the kinetic effects of modified substrates, which would extend the scope of the lab to include organic chemistry as well. By comparative analysis of multiple substrate analogs, the rates of hydrolysis can infer the steric and inductive influence of the modified acetyl groups. We use 4-nitrophenyl acetate, 4-nitrophenyl trifluoroacetate, 4-nitrophenyl trimethylacetate, and 4-nitrophenyl chloroformate. Effectiveness of two catalysts are also compared – imidazole and 4-Dimethylaminopyridine. The substrate compounds afford a comparison of the control group, 4-nitrophenyl acetate, with bulky groups restricting access to the cleavage site (4-nitrophenyl trimethylacetate) and also with strong electron withdrawing groups (4-nitrophenyl trifluoroacetate and 4-nitrophenyl chloroformate). Thus, steric and inductive effects are quantified in conjunction with rate analysis. Eyring plot analysis of the observed rate constant’s temperature dependence gives the change in entropy, enthalpy, and Gibb’s free energy of the various transition states.<sup>3</sup> The flexible framework allows the lab to be unique from a student’s perspective each term while keeping the instructor’s preparation essentially identical.

Diversifying classic ester hydrolysis to illustrate catalytic, inductive and steric effects on rate in physical and organic chemistry laboratories

ACS Spring 2022

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