title: Enhancing the rate of the Diels-Alder reaction using CO2 + ethanol and CO2 + n-hexane mixed solvents of different phase regions.
abstract: The reaction rate of the Diels-Alder reaction between N-ethylmaleimide and 9-hydroxymethylanthrance in CO2 + ethanol and CO2 + hexane mixed solvents of different compositions were determined by in situ UV/vis spectroscopy at 318.15 K and different pressures. The density of the mixed solvents at different pressures was also determined and the isothermal compressibility was calculated using the density data. The activation volume of the reaction was calculated based on the dependence of rate constant (kc) on pressure. It was demonstrated that the kc was very sensitive to the pressure in the mixed solvents near the critical region and the kc increased dramatically as pressure approached dew points, critical point, and bubble points of the mixed solvents. However, the kc in the mixed solvents outside the critical region or in pure CO2 was not sensitive to pressure. At suitable conditions, kc could be 40 times larger than that in acetonitrile. The activation volume of the reaction was nearly independent of pressure as the pressure was much higher than the phase separation pressure of the mixed solvents, while it increased considerably as pressure approached the bubble points, critical point, and dew points from high pressure. The clustering of the solvent molecules with the reactants and the activated complex in the reaction systems near the phase boundary in the critical region may be the main reason for the interesting phenomena observed. This work also shows that, using pure CO2 as the solvent, the reaction cannot be carried out in the critical region of the solvent due to the limitations of the reactants, while it can be conducted in the critical region of mixed solvents of suitable compositions, where the solvents are highly compressible and the reaction rate can be tuned effectively by pressure.
authors.affiliation: Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China.
authors.forename: Bo
authors.initials: B
authors.lastname: Wang
authors.forename: Buxing
authors.initials: B
authors.lastname: Han
authors.forename: Tao
authors.initials: T
authors.lastname: Jiang
authors.forename: Zhaofu
authors.initials: Z
authors.lastname: Zhang
authors.forename: Ye
authors.initials: Y
authors.lastname: Xie
authors.forename: Wenjing
authors.initials: W
authors.lastname: Li
authors.forename: Weize
authors.initials: W
authors.lastname: Wu
mesh_terms: D000873:Anthracenes / Q000138:chemical synthesis* / Q000737:chemistry; D001952:Bridged-Ring Compounds / Q000138:chemical synthesis* / Q000737:chemistry; D002245:Carbon Dioxide / Q000737:chemistry*; D000431:Ethanol / Q000737:chemistry*; D005033:Ethylmaleimide / Q000138:chemical synthesis* / Q000737:chemistry; D006586:Hexanes / Q000737:chemistry*; D007700:Kinetics; D015394:Molecular Structure; D044367:Phase Transition; D011312:Pressure; D012680:Sensitivity and Specificity; D012997:Solvents / Q000737:chemistry; D013056:Spectrophotometry, Ultraviolet / Q000379:methods; D013499:Surface Properties
chemical_list: C512556:9-hydroxymethylanthrance; D000873:Anthracenes; D001952:Bridged-Ring Compounds; D006586:Hexanes; D012997:Solvents; D002245:Carbon Dioxide; C026385:n-hexane; D000431:Ethanol; D005033:Ethylmaleimide