Substituent Effects on Neutral and Ionized C=C and C=O Double Bonds and their Implications for the Stability Order of Keto/Enol Tautomers

By using ab initio molecular orbital (MO) calculations substituent effects on monosubstituted neutral as well as cationic ethylene and formaldehyde are studied. The relative stabilizations caused by substitution have been evaluated by means of isodesmic reactions employing a complete “first row sweep”. The so-obtained results have been used to investigate the relative stabilizations of simple, substituted neutral and cationic keto/enol pairs, π-donating and -accepting as well as (σ-accepting substituents were found to stabilize the neutral C=C bond thermochemically. The C==O double bond is highly stabilized by 7r-donating and σ-accepting substituents, and the stabilization effects are much larger compared to those found for the corresponding C=C double bond systems. Substantial stabilization is provided by a strong polarization of the double bond. Substituents which are able to reinforce an already existing bond polarization are stabilizing most efficiently. In the cationic species, the ability of the substituent to donate negative charge, thereby compensating the electron deficiency caused by ionization, is found to be of prime importance. Both a- and π-donors provide substantial stabilization. The relative stabilizations for C==O double bond containing cationic species are smaller for 7r-donating groups and larger for σ-donors compared to the corresponding cationic ethylene derivatives. These findings also apply for neutral and ionized acetaldehyde and vinyl alcohol derivatives. The effect on the respective keto/enol energy differences is discussed.