ORGN 198 |
| Microwave-assisted synthesis represents a major breakthrough in synthetic chemistry methodology since virtually all types of thermally driven chemical reactions can be accelerated by microwaves. The control of temperature is of utmost importance not only for the outcome of reactions but also for the reproducibility of results. Therefore, a comparative study between two leading microwave unit suppliers has been conducted. In both single-mode units, the temperature is measured indirectly via an infrared sensor. Interestingly, the temperature profiles differ significantly under otherwise identical conditions (2 ml solvent, pre-selected temperature of 150°C, 200 W). These measured temperatures were then compared with temperatures determined using fibre optics and digital thermometers. Unpolar solvents or solvents with a small loss factor are notoriously difficult to heat in microwave units. Polar additives such as ionic liquids can be added to otherwise low-absorbing reaction mixtures in order to increase the absorbance level of the medium. Ionic liquids interact very efficiently with microwaves through the ionic conduction mechanism and are rapidly heated at rates easily exceeding 10°Cs-1 without any significant pressure build-up. As an alternative to the use of the rather expensive ionic liquids as solvents, several research groups have used ionic liquids as “doping agents” for microwave heating of otherwise nonpolar solvents. This technique is becoming increasingly popular, as demonstrated by the many recently published examples. This study suggests that not all ionic liquids are suitable as microwave engery/heat transfer reagents for unpolar solvents and that certain published results are based on erroneous temperature measurements. |
|
New Reactions and Methodology
8:00 AM-12:00 PM, Monday, 27 March 2006 Georgia World Congress Center -- C301, Oral
Division of Organic Chemistry |