Keto-enol tautomerism. Relative stabilization of enol isomers

Abstract

Ab initio molecular orbital theory is used to examine the effect of simple π-electron-accepting substituents (Li, BeH, BH2) on the keto-enol equilibrium in the acetaldehyde-vinyl alcohol system. The enol-keto energy difference is increased slightly by α-Li substitution and decreased to a near-zero value by a BH2 substituent. The results are rationalized in terms of geminal interactions. The conformational preferences of substituted acetaldehyde systems (XCH2CHO; X = Li, BeH, BH2, CH3, F) are discussed. Three classes of rotational potential functions are distinguished on the basis of the σ-electron-donating or -accepting nature of the substituent.