Origin of the Acidity of Enols and Carboxylic Acids

The origin of the acidity of carboxylic acids and enols has been examined via ab initio MO calculations at the MP2/6-31+G** and Becke3LYP/6-31+G** levels as well as with the CBS-4 and CBS-Q model chemistries. The changes in electron populations that occur on ionization were examined in some detail. In the conversion of vinyl alcohol to its alkoxide ion, the σ electrons of the proton are donated to the oxygen. The repulsion between the increased σ density at oxygen and its π electrons leads to transfer of π electron density to the C C double bond. At the same time, the negative charge at oxygen repels the σ and π electrons of the attached carbon. A part of the negative charge appears at the hydrogens that are in the nodal plane of the π system. With longer chains, σ/π polarization leads to compensating charge shifts so as to minimize the differences between adjacent carbons. When a carbonyl group is attached, as in carboxylic acids and enols, it serves to stabilize the system by accepting some π electron density (∼0.1 e) and by a stabilizing electrostatic interaction between the positively charged carbonyl carbon and the adjacent negatively charged atom. The stabilization is on the order of 40 kcal/mol for carboxylic acids and 28 kcal/mol for enols. The effect of polar solvents on relative acidities also was examined using a new reaction field model, and experimental observations were reproduced.