Five reactions were rate-accelerated relative to the standard reflux workup in both multi-mode and mono-mode microwave ovens, and the results were compared to determine whether the sequential processing of a mono-mode unit could provide for better lab logistics and pedagogy. Conditions were optimized so that yields matched in both types of microwave ovens for a Diels−Alder cycloaddition, Wittig salt formation, Fischer esterifications, an E2 alkyne formation, and Williamson ether synthesis. Typically, a 10-fold rate acceleration was observed under mono-mode heating versus multi-mode heating, reducing the total run-time between 1.5 and 3.0 min per sample, which rivals the batch run-time of a multi-mode unit in ∼16 student lab sections. Thus, the mono-mode microwave oven required a similar quantity of total reaction time in the lab, allowing students to run their experiments individually with less wait-time, competition for chemicals, equipment, and instrumentation and to complete the experiments in the lab period.
Microwave heating enhanced the rate of three reactions
typically performed in our undergraduate organic chemistry laboratory:
a Diels−Alder cycloaddition, a Wittig salt formation, and a
Williamson ether synthesis. Ninety-minute refluxes were shortened
to 10 min using a laboratory-grade microwave oven. In addition, yields
improved for the Wittig salt and ether preparations. The Diels-Alder
cycloaddition of N-phenylmaleimide and 1,3-cyclohexadiene
in absolute ethanol in a closed vessel and 130 °C gave 84−90%
yields. Formation of the Wittig salt, benzyltriphenylphosphonium chloride,
from triphenylphosphine and benzyl chloride gave yields greater than
90% when heated in acetonitrile at 200 °C Yields doubled in the
Williamson ether synthesis of 2-ethoxynaphthalene, from 2-naphthol,
methanolic potassium hydroxide, and iodoethane with microwave heating
at 130 °C.
An inquiry-based experiment for the organic chemistry laboratory was developed to provide students with a cognitively rich research experience. Student teams were charged with optimizing the reaction conditions for the Williamson ether synthesis of 2fluorophenetole from 2-fluorophenol, ethyl bromide, and potassium carbonate in either absolute ethanol or acetonitrile. Microwave acceleration of this S N 2 reaction followed by rapid GC−MS analysis allowed students to review results, revise experimental conditions, and repeat the modified reaction and its analysis within a 3 h laboratory period. Variables that could be manipulated were reagent equivalencies, solvents and their volume, microwave reaction time, temperature, and wattage. Post-laboratory, all team data were available to the entire class for additional evaluation and each student was required to suggest new conditions for improvement in his/her laboratory report.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.