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Children learn through play, and they learn more than adults might expect. Science experiments are a great way to spark their curiosity, get their minds active, and encourage them to do something that doesn’t involve a screen. 14187-32-7, C20H24O6. A document type is Article, introducing its new discovery., Application In Synthesis of Dibenzo-18-crown-6
The mechanisms and the activation parameters of decomplexation have been determined by 23Na NMR for dibenzo-24-crown-8 (DB24C8)-NaPF6 in nitromethane, dibenzo-18-crown-6 (DB18C6)-NaX in acetonitrile (X = BF4-, BPh4-), and DB18C6-NaY in nitromethane (Y = PF6-, BPh4-).For DB24C8-NaPF6 in nitromethane, the decomplexation follows a bimolecular exchange mechanism for T > 2E-3 M, characterized by DeltaH(excit.) = 30 +/- 2 kJ mol-1 and DeltaS(excit.) = -37 +/- 10 J mol-1 K-1.At lower sodium concentrations, the mechanism is predominantly unimolecular with DeltaG(excit.)300 ca. 63 kJ mol-1.For Na+-DB18C6 in acetonitrile, the mechanism is purely unimolecular with DeltaH(excit.) = 40 +/- 2kJ mol-1 and DeltaS(excit.) = -44 +/- 8 J mol-1 K-1.In nitromethane, the bimolecular exchange mechanism is in competition with the unimolecular one.The conributions of the two mechanisms have been separated from the observed rate constants: at 300 K, DeltaG(excit.)bi = 48 +/- 4 kJ mol-1 and DeltaG(excit.)uni = 60 +/- 3 kJ mol-1.The activation parameters have been determined for the unimolecular decomplexation mechanism: DeltaH(excit.) = 37 +/- 3 kJ mol-1 and DeltaS(excit.) = -78 +/- 8 J mol-1 K-1.The comparison with literature data showed that the unimolecular decomplexation mechanism is favored in high-donicity solvents, despite a higher activation enthalpy, which is compensated by a higher activation entropy.It is suggested that the unimolecular decomplexation of Na+-DB18C6 involves a desolvation step accompanying conformational changes.
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Reference：
Chiral Catalysts,
Chiral catalysts – SlideShare