Kinetics of Diels-Alder reactions in critical binary solutions

The kinetics of the Diels-Alder (D-A) reactions of N-ethylmaleimide with 2,3-dimethyl-1,3-butadiene and of 9,10-dimethyl-anthracene with diethyl maleate in two critical binary solutions n-tetradecane + dimethyl carbonate and n-nonane + diethyl maleate have been studied by the three-wavelength UV-spectrophotometry. It was found that neither speeding up nor slowing down effect for the first reaction was observed, which obeyed the Griffiths-Wheeler rule; while a very weak slowing down effect was detected for the second reaction, but significantly weaker as compared with the prediction by the Griffiths-Wheeler rule. It was attributed to that only less than 0.002% of diethyl maleate changed to the products during the reaction, hence the concentration of diethyl maleate may be considered as a fixed thermodynamic density in the critical reaction system, resulting in no thermodynamic slowing down being detected.