Role of Chelation and Resonance on the Intrinsic Acidity and Basicity of Tropolone.

The gas-phase basicity and acidity of tropolone have been determined by Fourier transform ion cyclotron resonance mass spectrometry (FT ICR) techniques. Ab initio calculations at the MP2/-6311+G(d,p) level were carried out to describe the effects that protonation and deprotonation have on the aromaticity and hence on the stability of the system. Experimental and calculated energetics of protonation and deprotonation are in excellent agreement. Our analysis shows that both the protonated and the deprotonated species are stabilized by resonance. As a consequence the acidity of tropolone (341.3 kcal/mol) is enhanced, and this compound is found to be surprisingly as acidic as benzoic acid. This is in good agreement with the results reported in the literature and obtained in DMSO solutions. The intramolecular hydrogen bond becomes significantly weaker upon protonation, and this effect tends to counterbalance the resonance stabilization of the cation. As a consequence, tropolone is found to be slightly less basic than tropone.