Toward a quantitative evaluation of Lewis acid strength. A 13CNMR study of the interaction of boron trifluoride with diethyl ether1Dedicated to the memory of Russell S. Drago.1

The rate of exchange between boron trifluoride diethyl etherate and free diethyl ether can be conveniently measured by 13C dynamic NMR spectroscopy (DNMR) between 25°C and −50°C in dichloromethane solution. The same rates and activation parameters are found by the line shape analysis of either the methyl or the methylene group. The rates do not vary with the ratio of free ether to complex between 0.61 and 2.4, indicating a rate-determining unimolecular decomposition of the latter. The literature claim of a bimolecular mechanism for the ether exchange is thus found to be incorrect. An enthalpy of activation (ΔH‡) of 9.7 kcal/mol was calculated for this decomposition. This value is a good measure of the strength of interaction of BF3 with diethyl ether, because the recombination to form the complex (2) should have a very low energy barrier, and it is close to the ΔH° value of 10.9 kcal/mol determined by Brown and Adams for the decomposition of 2 to gaseous diethyl ether and BF3. The 13CDNMR method can be applied in principle to the interaction of ether with other Lewis acids, thus providing a method for comparison of Lewis acid strengths.