Are Strong Bronsted Acids Necessarily Strong Lewis Acids?

摘要

The Broensted and Lowry acid base theory is based on the capacity of protondonation or acceptance (in the presence or absence of a solvent) whereas theLewis acid base theory is based on the propensity of electron pair acceptanceor donation. We explore through DFT calculation the obvious question whetherthese two theories are in conformity with each other. We use pKa as thedescriptor for the Broensted and Lowry acidity. The DFT descriptors likeionization potential, electron affinity, electronegativity, hardness and globalelectrophilicity are computed for 58 organic and inorganic acids. Thefractional electron transfer, del(N) and the associated energy change, del(E)for the reaction of these acids with trimethyl amine (a strong base) are usedas the possible descriptors for the Lewis acidity. A near exponential decreasein del(N) and (-del(E)) values is observed in general with an increase in pKavalues. The findings reveal that a stronger Broensted acid in most casesbehaves as a stronger Lewis acid as well. However it is not necessarily truefor all acids.