Pedagogical Comparison of Five Reactions Performed under Microwave Heating in Multi-Mode versus Mono-Mode Ovens: Diels–Alder Cycloaddition, Wittig Salt Formation, E2 Dehydrohalogenation To Form an Alkyne, Williamson Ether Synthesis, and Fischer Esterification

Abstract

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.

Some Like It Hot: Experimentally Determining ΔΔH⧧, ΔΔS⧧, and ΔΔG⧧ between Kinetic and Thermodynamic Diels–Alder Pathways Using Microwave-Assisted Synthesis

“…Incorporation of microwave-assisted synthetic techniques is becoming increasingly popular across multiple disciplines of research and chemistry education. − Numerous undergraduate experiments using commercial reactors have been developed, including Friedel–Crafts acylations, aldol condensations and conjugate additions, , Wolff–Kishner reductions, carbonyl-based additions and substitutions, ,,,, and synthesis of fluorescent inorganic materials . Domestic microwave ovens have been successfully used as a cost-effective synthetic tool in the undergraduate lab − but suffer from poor efficiency and reproducibility.…”

“…Cycloadditions are one class of reactions in which focused microwave irradiation can be used to achieve quantitative yields in minutes. − More specifically, the Diels–Alder cycloaddition is a concerted, thermally allowed [4 + 2] process yielding a product that contains a new or additional 6-membered ring. Numerous microwave-assisted Diels–Alder procedures are present in literature, − including adaptations designed for use in the undergraduate laboratory. ,,,,,− Conventional Diels–Alder reactions occur through interaction of the highest occupied molecular orbital (HOMO) of a diene and the lowest unoccupied molecular orbital (LUMO) of a dienophile. The rates of Diels–Alder reactions are system dependent, as completion times range from a few seconds to many days under ambient conditions.…”

Synthesis of substituted N-phenylmaleimides and use in a Diels–Alder reaction: a green multi-step synthesis for an undergraduate organic chemistry laboratory

“…One of these experiments utilizes a green Diels-Alder by performing a solvent-free reaction at room temperature (73). A couple of undergraduate experiments have been published utilizing maleimides as the dienophile in the Diels-Alder reaction (78,79).…”

Microwave Extraction of Eugenol from Cloves: A Greener Undergraduate Experiment for the Organic Chemistry Lab

“…Recent articles describe the use of microwave ovens in the undergraduate laboratory [10,11] and highlight the convenience and time savings associated with microwave methods. This microwave technique not only extracts the essential oils at a low enough temperature to prevent decomposition, but the microwave's lower energy demand makes it a greener alternative to existing methods.…”