Other Reactions in Polar Solvents

We have investigated a number of reactions in polar solvents, most of which had been previously reported to occur more rapidly under microwave heating than classical heating in open vessels, to see if there are any significant MW rate enhancements, which could suggest to the involvement of a specific MW effect [19, 20]. 

It should be pointed out that many of these reports compared reaction times of MW heated reactions with times previously reported in the literature for the same reactions under conventional heating. Unfortunately the conventionally heated reactions are often complete in times which are much shorter than those quoted and it is important to perform direct and careful comparisons between MW and thermal reactions, using the same quantities of reagents and solvents and the same reaction temperature. 

We have found it convenient to compare MW and conventional reactions using reflux conditions, since the temperatures are constant at the boiling point of the solvent. To eliminate the problem of the time required to reach the reflux temperature, reaction mixtures without one of the reactants or catalyst are heated to reflux and then the other reactant or catalyst quickly added. The reflux times required to give similar yields for a reaction, taken only partially to completion by MW and classical heating, are then compared. Small rate enhancements might still be expected merely because of superheating by up to 40 °C by the MW [39, 40, 46], and localized heating 

Comparison between reaction rates using MW and conventional heating should preferably be performed using kinetic studies [19, 33, 48], where rate constants can be evaluated or plots of yield of product versus reaction time for the conventional and MW reactions can be compared. 

In their initial studies on this reaction, Westaway and Gedye [30] observed slightly increased rates both in the presence of ethanol and in the absence of a solvent, when MW heating was used in an open vessel. Since it was difficult to measure the temperature accurately in the MW experiment, the reaction in ethanol was later performed under reflux [19]. The yield of the product was found to be 73% in 8 min using MW reflux compared with a yield of 71% in 20 min under conventional reflux, representing a rate enhancement of approximately 2.5 times. 

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