Introduction to Microwave Chemistry

Microwave synthesis gives organic chemists more time to expand their scientific creativity, test new theories and develop new processes.

Microwave synthesis represents a major breakthrough in synthetic chemistry methodology, a dramatic change in the way chemical synthesis is performed and in the way it is perceived in the scientific community. Conventional heating, long known to be inefficient and time-consuming, has been recognized to be creatively limiting as well. Microwave synthesis gives organic chemists more time to expand their scientific creativity, test new theories and develop new processes. Instead of spending hours or even days synthesizing a single compound, chemists can now perform that same reaction in minutes. In concert with a rapidly expanding applications base, microwave synthesis can be effectively applied to any reaction scheme, creating faster reactions, improving yields, and producing cleaner chemistries.

In addition, microwave synthesis creates completely new possibilities in performing chemical transformations. Because microwaves can transfer energy directly to the reactive species, so-called “molecular heating”, they can promote transformations that are currently not possible using conventional heat. This is creating a new realm in synthetic organic chemistry.

Microwaves also provide chemists with the option to perform “cool reactions”. Energy is applied directly to the reactants, however. The bulk heating is minimized by use of simultaneous cooling. This allows for enhanced reactions of larger, more heat sensitive molecules (e.g. proteins), as the temperatures are low enough to eliminate thermal degradation. This will provide some exciting new opportunities and an important new tool for proteomics and genomics research.

Recent microwave hardware advancements now provide a range of affordable, flexible tools for the synthetic chemist. This new technology, coupled with the rapidly expanding knowledge and applications base, will cause a major shift towards microwave synthesis in the next few years. As Victor Hugo, the famous French novelist and poet wrote, “An invasion of armies can be resisted, but not an idea whose time has come.” Microwave synthesis is an idea whose time has come and whose impact will be truly monumental on the world of chemistry.

 

History

The development of microwave technology was stimulated by World War II, when the magnetron was designed to generate fixed frequency microwaves for RADAR devices.1,2 Percy LeBaron Spencer of the Raytheon Company accidentally discovered that microwave energy could cook food when a candy bar in his pocket melted while he was experimenting with radar waves. Further investigation showed that microwaves could increase the internal temperature of foods much quicker than a conventional oven. This ultimately led to the introduction of the first commercial microwave oven for home use in 1954.

Investigation into the industrial applications for microwave energy also began in the 1950s and has continued to the present. Microwave energy has found many uses including irradiating coal to remove sulfur and other pollutants, rubber vulcanization, product drying, moisture and fat analysis of food products, and solvent extraction applications. Wet ashing or digestion procedures for biological and geological samples have also become very important analytical tools. As improvements and simplifications were made in magnetron design, the prices of domestic ovens fell significantly. Consequently, research done in the latter half of the 20th century was performed in modified domestic microwave ovens. The effects of microwave irradiation in organic synthesis were not explored until the mid 1980s. The first two papers on microwave-enhanced organic chemistry were published in 1986 and many organic chemists have since discovered the benefits of using microwave energy to drive synthetic reactions.3,4 Until recently, most of this research has been executed in multi-mode domestic microwave ovens, which have proven to be problematic. These ovens are not designed for the rigors of laboratory usage: acids and solvents corrode the interiors quickly; there are no safety controls, temperature or pressure monitoring; and the cavities are not designed to withstand the resulting explosive force from a vessel failure in runaway reactions.

In the 1980s, companies began to address these issues by manufacturing industrial microwave ovens specifically designed for use in laboratories. These multi-mode systems featured corrosion-resistant stainless steel cavities with reinforced doors, temperature and pressure monitoring, and automatic safety controls. They have worked well for doing large-scale laboratory applications, but they have some fundamental limitations in performing small-scale synthetic chemistry. Recently, single-mode technology, which provides more uniform and concentrated microwave power, has become available. These newer systems represent a breakthrough in providing new capabilities for doing microwave synthesis and are a key factor in the rapid expansion of this field of science.