Characterization of Keto-Enol Tautomerism of Acetyl Derivatives from the Analysis of Energy, Chemical Potential, and Hardness

We present a theoretical study of the substituent effects on keto ⇌ enol equilibria in 10 acetyl derivatives (CH 3 COX, X = H, OH, CH 3,OCH 3, NH 2, N(CH 3 ) 2, OCHO, F, Cl and Br). The analysis performed in terms of the potential energy, electronic chemical potential, and molecular hardness leads to the following results: (a) in the whole series, the keto isomers are more stable than the enol ones; (b) it is shown that the HSAB principle may explain the relative stability of the keto and the enol species; (c) accurate activation barriers are predicted from a modified Marcus-like equation that contains the force constants associated to reactants and products and to the imaginary frequency of the transition states; (d) the energy and hardness profiles are opposite to each other, verifying the principle of the maximum hardness.