Calculated pKEnol Values for Enols of Carboxylic Acid Derivatives HCC(OH)X (X = OH, NH2, NMe2, OMe, OCHO, F, Cl, Br)

The energies of acetic acid and its CH 3 C( O)X derivatives and of the corresponding enols H 2 C C(OH)X, for X = OH, NH 2, NMe 2, OMe, OCHO, F, Cl, and Br, were calculated by the MO ab initio method MP2(full)/6-31G**, by single-point CCSD(T)(full)/6-311G**/MP2(full)/6-31G**), and by the hybrid density functional method B3LYP/6-31G**. The calculated p K Enol = −log K Enol values for the “keto”/enol equilibria are all high (17.5−24.0) and follow the order for X:H < alkyl < OCHO < Br∼Cl < F < NH 2 < NMe 2 < OH, OMe. By using the appropriate isodesmic reactions, it was shown that all substituents stabilize more the acid species than its enol (where Br and Cl are slightly destabilizing). All the computational methods display a similar trend. The structures and energies of the various conformations of both species are given. Comparison with the scarce experimental p K Enol data shows a reasonable agreement with the calculations.