Background: The influence of the microwave effect in organic reactions is still not a completely
clear issue. Higher temperature region (hot spots) in organic reactions have been observed and
attributed to the selective heating mechanism of microwave irradiation. The accumulated heat in the
polar reactants results in the acceleration of certain reactions. However, whether lower temperature
region (cool spots) exists is not clear.
Methods: On the basis of the microwave selective heating mechanism, less polar reactant allyl 4-
methylphenyl ether was selected as a molecular probe and conducted Claisen rearrangement in more
polar solvent N-methylpyrolinone in different concentrations. The temperatures of reactions under different
conditions were deduced from Arrhenius equation. The dielectric constants of starting material
allyl 4-methylphenyl ether, product 2-allyl-4-methylphenol, and solvent N-methylpyrolinone were determined
under 2.54 GHz microwave irradiation conditions by use of the capillary method.
Results: Lower temperature regions were observed in the Claisen rearrangement of allyl 4-
methylphenyl ether in N-methylpyrolinone only in low concentration. However, they were not in high
concentrations. The results of determination indicate that product 2-allyl-4-methylphenol shows higher
polarity than reactant allyl 4-methylphenyl ether. This is a possible reason why lower temperature
regions were not observed in reactions performed in higher concentrations because much more polar
product was generated. Stirring speed does not affect temperature difference possibly due to the reason
that microwave frequency is huge as compared with the stirring speed.
Conclusion: Lower temperature regions were observed in the Claisen rearrangement of allyl 4-
methylphenyl ether in N-methylpyrolidinone, an intramolecular reaction with a less polar reactant in a
more polar solvent, followed by the first order kinetic. The results reveal the reason why some organic
reactions cannot be accelerated because their reactants are located in lower reaction temperature regions