Toward a quantitative evaluation of Lewis acid strength. A C-13 NMR Study of the interaction of boron trifluoride with diethyl ether

The rate of exchange between boron trifluoride diethyl etherate and free di ethyl ether can be conveniently measured by C-13 dynamic NMR spectroscopy ( DNMR) between 25 degrees C and -50 degrees C in dichloromethane solution. T he same rates and activation parameters are found by the line shape analysi s of either the methyl or the methylene group. The rates do not vary with t he ratio of free ether to complex between 0.61 and 2.4, indicating a rate-d etermining unimolecular decomposition of the latter. The literature: claim of a bimolecular mechanism for the ether exchange is thus found to be incor rect. An enthalpy of activation (Delta H++) of 9.7 kcal/mol was calculated for this decomposition. This value is a good measure of the strength of int eraction of BF3 with diethyl ether, because the recombination to form the c omplex (2) should have a very low energy barrier, and it is close to the De lta H degrees value of 10.9 kcal/mol determined by Brown and Adams for the decomposition of 2 to gaseous diethyl ether and BF3. The C-13 DNMR method c an be applied in principle to the interaction of ether with other Lewis aci ds, thus providing a method for comparison of Lewis acid strengths. (C) 199 9 Elsevier Science B.V. All rights reserved.