Microwave Chemistry

The microwave-assisted synthesis of aspirin has been conducted in the study, using different catalysts. Four acid catalysts and four basic catalysts were used in the experiment in which percentage yield was calculated; one student was working without a catalyst. As the table demonstrates, the non-catalytic method produced the best yield, both using the recrystallization procedure and the solvent-free approach. Using this knowledge, I will be conducting my own experiment, investigating the effects of irradiation intensity, using the non-catalytic and solvent-free approach, to both maximize the yield and to follow the greenest, most environmentally-friendly pathway to aspirin synthesis.

Microwave Chemistry: Text

The diagram below is the reaction mechanism for the non-catalytic synthesis of aspirin ("A Greener Approach To Aspirin Synthesis Using Microwave Irradiation")

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Reactions using microwaves are not only faster and cleaner than those which use conventional heating methods, they also produce highest yields. Microwave Chemistry is used to improve and simplify synthesis of many organic reactions; it is more efficient and more environmentally sound. Microwaves have a wavelength of between 0.01 and 1 m, and a frequency range between 0.3 and 30 GHz. “Microwave energy is a natural phenomenon which can be induced when electric current flows through a conductor, for example, an antenna, a transmitter chip, or a magnetron.”

Microwave Chemistry: Text

In order for the reaction to occur and for the transformation to take place, the reactants “must collide in the correct geometrical orientation to become activated to a higher level transition state (ETS).” Microwave irradiation, however, does not affect the Activation Energy (Ea), but instead it “provides the momentum needed to overcome this barrier and complete the reaction more quickly than conventional heating methods.” The reason as to why microwave heating is so much faster than conventional heating is because it “occurs on a molecular level as opposed to relying on convection currents and thermal conductivity when using conventional heating methods.” (Bassyouni, F.A., Abu-Bakr, S.M. & Rehim, M.A. Evolution of microwave irradiation and its application in green chemistry and biosciences.

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Non-polar substances are heated poorly under microwave conditions, due to the nature of the radiation. Additionally, some reactions may require temperatures unattainable through the absorption of the reagents. Both these problems “can be overcome by the use of a susceptor, an inert compound that efficiently absorbs microwave radiation and transfers the thermal energy to other compounds that are poor radiation absorbers or to the reaction medium.”

A susceptor commonly used is graphite, since most forms of carbon interact strongly with microwaves. “Powdered amorphous carbon and graphite rapidly (within 1 min) reach very high temperatures (>1000 °C) upon irradiation and, for this reason, graphite has been widely employed as a microwave susceptor. The amount of graphite can be varied. In some cases, a catalytic amount of graphite (10% or less than 10% by weight) is sufficient to induce rapid and strong heating of the reaction medium. However, in most cases the amount of graphite is at least equal to or greater than the amount of reagents, thus resulting in a graphite-supported microwave process.” (T. Besson and C. O. Kappe , Microwave Susceptors, Microwaves in Organic Synthesis: Third Edition , A. de la Hoz and A. Loupy, Wiley-VCH, Weinheim, 2012, pp. 297–346)

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