P936: Seeking Goldilocks: Expanding catalyzed ester hydrolysis lab for physical and/or organic chemistry laboratories

Author: Rachel Mason , University of Texas at Tyler, USA

Co-Author: Sean C. Butler, Caleb M. Bunton and S. Scott Maldonado, University of Texas at Tyler , USA; Abraham Pineda, Tyler Junior College, USA

Date: 8/6/14

Time: 5:15 PM6:30 PM

Room: LIB

Related Symposium: S33

A versatile experiment for undergraduate physical and/or organic chemistry students, which can be used to illustrate the impacts of steric hindrance and/or electron induction on reaction rate and explore catalyst effectiveness is presented. The classic imidazole catalyzed ester hydrolysis of p-nitrophenyl acetate (NPA) laboratory exercise exposes students to the use of pseudo-first order conditions to extract rate constants for catalyzed and uncatalyzed reactions by monitoring the hydrolysis product’s UV-Vis absorption.1,2 NPA analogs with bulky groups restricting access to the cleavage site, such as 4-nitrophenyl trimethylacetate, are compared to illustrate steric effects. Likewise, NPA analogs with strong electron withdrawing groups, such as 4-nitrophenyl trifluoroacetate confirm inductive effects in a concrete manner. Effectiveness of catalysts of varying nucleophilicity are compared. Change in entropy, enthalpy and Gibbs free energy of the various transition states are obtained through Eyring plot analysis of the observed rate constant’s temperature dependence.3 The flexible framework of the experiment allows the lab to be unique from a student perspective each term while keeping the instructor’s preparation essentially identical by addressing a wide range of concepts from the same data sets. Lombardo, A. J.Chem.Ed 1982, 59, 887. Halpern, A.M. and McBane, G.C. Experimental Physical Chemistry: A Laboratory Textbook. 3rd ed. W.H. Freeman &Company: New York, 2006 pp 23.1-23.12 Bender, M.L. and Turnquest, B.W. JACS 1956, 79, 1652