ENOLIZATION OF ALDEHYDES AND KETONES - STRUCTURAL EFFECTS ON CONCERTED ACID-BASE CATALYSIS

The third-order term (k(AB)) for the concerted acid-base catalyzed eno lization of a selection of simple aldehydes and ketones has been measu red in a series of substituted acetic acids at 25 degrees C at constan t ionic strength 2.0 (NaNO3). While there is no direct correlation of the magnitude of the third-order term with either the rate constants f or 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 implie s that the concerted pathway is important only when both the general a cid 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 ot her ketones (0.59), as measured in acetic acid buffers. The resultant Bronsted beta(AB) value of 0.20 found for propanal (2) is consistent w ith the overall observation that concerted catalysis is largely indepe ndent of the buffering species, and that process is overall base catal yzed. The solvent isotope effect on the concerted acid-base catalyzed enolization rate term, k(AB)(H2O)/k(AB)(D2O) = 1.33, indicates that th e transition state for proton transfer to the carbonyl is more advance d than in the case of ketones. In general we have found that carbonyl compounds with large measured (or estimated) enol contents show signif icant third-order terms.