Abstract: There are many competitive reactions in chemical reactions, especially in organic chemical reactions. However, how to determine the main products and their yields in the competitive reactions whose kinetically-controlled product (KP) and thermodynamically-controlled product (TP) are inconsistent is of significance to analyze the compositions and proportions of products and to obtain high-yield target product and is also a problem puzzled the researchers. In this study, a critical time tc was defined as the time when the yield of KP is equal to that of TP and critical time ratio ζ was defined as the ratio of reaction time t to tc. Based on numerical solution and further analysis, formulas of tc and ratio of yields ry=c(KP)/c(TP) were obtained as follows: tc = pK(KP)/k(TP) and ry = {rk, ζ<0.3/rk; 1/(2ζ?1), 5/rk<ζ<log2(rK/5+1); 1/rK, ζ>log2(5rK)}. Based upon above formulas, the main products of Diels?Alder reaction between furan and maleimide were investigated. The results indicated that the main product is endo-product and exo-product when temperature is below 50 ℃ and above 65 ℃, respectively, which is in good agreement with experimental results that endo-product and exo-product were obtained at 25 ℃ and 90 ℃, respectively. This verified the accuracy of the obtained formula in determining the main product in the competitive reaction. Moreover, studies on chlorination of nucleosides showed that the APT charge and electrostatic potential parameters are only applicable for kinetically-controlled reaction to predict main products, but not for thermodynamically-controlled reaction.

Key words: Competitive reaction, Main product, Yield; Kinetically-controlled product, Thermodynamically-controlled product, Quantum chemical computation