Enolization of Aldehydes and Ketones:  Structural Effects on Concerted Acid−Base Catalysis

The third-order term ( k AB ) for the concerted acid−base catalyzed enolization of a selection of simple aldehydes and ketones has been measured in a series of substituted acetic acids at 25 °C at constant ionic strength 2.0 (NaNO 3 ). While there is no direct correlation of the magnitude of the third-order term with either the rate constants for acid ( k A ) or base ( k B ) catalysis, a simple log−log relationship exists between the product of the consecutive rate constants ( k A · k B ) and the concerted (third order) rate constants ( k AB ). This implies that the concerted pathway is important only when both the general acid and the general base terms are significant; this will be useful in designing other systems which might show such concerted catalysis. In the case of aldehydes, a slope of 0.97 was found for this plot, which compares to the result for 4-substituted cyclohexanones (0.51) and other ketones (0.59), as measured in acetic acid buffers. The resultant Brønsted β AB value of 0.20 found for propanal (2) is consistent with the overall observation that concerted catalysis is largely independent of the buffering species, and that process is overall base catalyzed. The solvent isotope effect on the concerted acid−base catalyzed enolization rate term, k AB (H 2 O)/ k AB (D 2 O) = 1.33, indicates that the transition state for proton transfer to the carbonyl is more advanced than in the case of ketones. In general we have found that carbonyl compounds with large measured (or estimated) enol contents show significant third-order terms.