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.