How Fast Can This Thing Go?

A Fall semester experiment that we have been doing for a number of years is a kinetic study of the hydrolysis of 2-chloro-2-methylpropane (tert-butyl chloride). This goes well with the quintessential SN2/SN1 chapter covered Fall semester. It is also one of the very few kinetic experiments I have seen that is appropriate for Organic Chemistry. The hydrolysis reaction is followed by monitoring the pH of the reaction with an indicator (bromophenol blue) which changes from blue at pH > 4.6 to yellow at pH < 3.0. A 0.15 mL volume of 0.1 M sodium hydroxide is added at the beginning of the reaction (final volume of 5 mL). An initial concentration of 0.003 M NaOH calculates to a pH of 11.5 according to the simplistic formula of “pH = 14 + log [NaOH].” Following to the same equation the NaOH would have to be almost completely consumed before the pH would reach the acidic pH 4 (10^-10 M). Since the initial concentration of the substrate is 10X the initial concentration of NaOH this represents the 10% completion of the reaction. It seems somewhat counterintuitive that the reaction is monitored by the concentration of nucleophile which theoretically has no effect on the SN1 reaction rate. I reality, the reaction is very sensitive to the amount of NaOH added. So sensitive that it tends to be difficult to get consistent readings both within a particular pair of students and between pairs of students in the same lab session. I ask students to submit their data before they leave the lab. Students consult the class data to contrast and compare the possible correctness of their data. The experiment also involves the energy of activation (Ea) calculation using the measured rates at different temperatures. A previous post on this reaction.

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