Modulation of tautomeric equilibria by ionic clusters. Acetylacetone in solutions of lithium perchlorate-diethyl ether.

Abstract

Acetylacetone (2,4-pentanedione, 1) is a molecule whose tautomeric forms are in dynamic equilibrium. Concentrated salt solutions in nonaqueous solvents exert a remarkable influence on the keto-enol ratio of this beta-diketone. The keto content of 1 increases from 5% in pure diethyl ether to 84.5% in a 4.14 M lithium perchlorate-diethyl ether (LPDE) solution, a nearly 17-fold increase. The equilibrium expression, K = [keto]/[enol] = k(f)/k(r), exhibits a linear dependence on [LiClO(4)], with the formal order of participation of lithium ion in the equilibrium being 1.0. A kinetic analysis reveals that k(f) is independent of LPDE concentration, whereas k(r) displays an inverse dependence on salt concentration, indicating preferential coordination of the keto tautomer with Li(+). Although 1 exits as the enol in water only to the extent of 16%, the addition of lithium perchlorate further reduces this figure. In an aqueous 4.02 M LiClO(4) solution, acetylacetone enol accounts for only 4.6% of the total amount of 2,4-pentanedione present. It has also been found that acetylacetone itself is an excellent solvent for LiClO(4) as well as for NaClO(4) with solutions containing up to 7.5 M LiClO(4) attainable. The enol content of 1 decreases dramatically from 81% to 7.4% on going from the neat liquid to a solution of 6.39 M LiClO(4) in acetylacetone.