KINETICS OF PROTON-TRANSFER OF 3,5-HEPTANEDIONE, 2,6-DIMETHYL-3,5-HEPTANEDIONE, AND DIBENZOYLMETHANE WITH AMINES IN 50-PERCENT ME2SO-50-PERCENT WATER - EFFECT OF STERIC CROWDING AND PI-OVERLAP ON INTRINSIC RATE CONSTANTS

Rates of reversible deprotonation of 3,5-heptanedione (6b), 2,6-dimethyl-3,5-heptanedione (6c), and dibenzoylmethane (6d) by several primary aliphatic amines, by piperidine and morpholine, and by hydroxide ion (6b and 6d only) have been measured in 50% Me2SO-50% water (v/v) at 20-degrees-C. Apparent pK(a)'s as well as the pK(a) values of the keto and the enol forms, and the enolization equilibrium constants (K(T)) were also determined. The pK(a) and K(T) values show the same trends observed previously in water. The intrinsic rate constants for the reactions of 6b and 6c with a given family of amines (primary aliphatic or secondary alicyclic) are the same and also equal to those for the reaction of acetylacetone (6a) with the same amines determined previously. These results indicate that steric effects play an insignificant role in the reactions of 6a, 6b, and 6c. The intrinsic rate constants for the deprotonation of 6d are approximately three fold lower than for 6a-c. This reduction is shown not to be caused by a steric effect but by pi-overlap with the phenyl groups in the enolate ion.