Enolization enthalpies for aliphatic carbonyl and thiocarbonyl compounds

The enolization enthalpies at 0 and 298 K for nine aliphatic carbonyl and nine aliphatic thiocarbonyl compounds have been determined from ab initio CBS-4 (complete basis set fourth-order) model calculations. The enolization enthalpies for the aliphatic carbonyl compounds were found to be 9-24 kcal/mol, but they were about 8-11 kcal/mol higher than those for the corresponding thiocarbonyl compounds. The lower enolization enthalpies for the thiocarbonyl compounds are attributed to the > C=S being much weaker than the > C=O double bond strength. The enolization enthalpies for the cycloketones and thiocycloketones were found to be significantly dependent upon the size of the cyclic ring. The solvent effects on the enolization enthalpy have also been discussed.