The Complex Formation of Boron Fluoride with Lower Olefins at Low Temperatures. II. Isotope Effects and Absorption Spectra of Boron Fluoride-Butene Systems

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The equilibrium constants of isotopic exchange between boron fluoride gas and boron fluoride in 1-butene and cis-2-butene solutions have been found to be 1.017±0.002 and 1.020±0.002 at －112°C, and 1.012±0.002 and 1.015±0.002 at －95°C. In boron fiuoride-trans-2-butene system, the equilibrium constant is 1.011±0.002 at －112°C and 1.006±0.002 at －95°C. When 1-butene and ethylene are dissolved in liquid boron fluoride, the intensity or the wave number of the V ← N absorption bands in mono-olefins is changed and the fine structure of these bands disappear. Thus, the following conclusion, more decisive than that of the preceding work, has been obtained: Boron fluoride can form liquid σ-complexes with polar mono-olefins with electron-releasing substituents. On the other hand, with nonpolar mono-olefins it cannot form σ-complexes but only unstable π-complexes. Although σ-complexes are not the intermediate products in reactions by boron fluoride catalysts, it is possible that these reactions proceed through π-complexs. However, the rate-determining step does not involve the initial formation of π-complexes. It has also been suggeted that the equilibrium constant may be proportional to the ratio of dative to no-bond structure in the complex, and it has been found that the larger the equilibrium constant, the higher become the stability and the catalytic activity of the complexes.
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