Reactions in Homogeneous Media Showing MW Rate Enhancement

Reactions in Homogeneous Media Showing MW Rate Enhancement 

As mentioned earlier, a number of reactions initially observed to show MW rate enhancements compared with conventionally heated reactions at the same temperature, have since, with more careful comparison, been shown to occur at the same rate under the two heating modes. Other reactions, such as Knoevenagel reactions in ethanol solution (vide supra, Schs. 15 and 16), have been shown to have modest rate enhancements, occurring typically 2 or 3 times faster under MW heating than under conventional heating at the same temperature. These rate increases are not surprising considering that solvents superheat by 10 °C or more on MW reflux, particularly as the reaction mixtures were not stirred. 

On the other hand, there have been a few reports of MW assisted reactions at atmospheric pressure which have been reported to show more substantial rate enhancements, and these will now be discussed. 

Scola et al. [67] studied the kinetics of the MW cure of a phenylethyl-terminated imide polymer model compound and an oligomer using a variable frequency MW source and found that the activation energy of the MW cures were 68% and 51% of the thermal cure respectively. It should be noted that the reactions were performed in the liquid phase in the absence of solvent. 

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