Kinetics of the reaction of Ni(II) and acetylacetone

Abstract

The rate of reaction of Ni(II) ion and acetylacetone to form monoacetylacetonate nickel(II) ion was studied conductometrically by a stop-flow method. The solvents consisted of several mixtures of water in 2-propanol ranging from 9·85 to 14·45 per cent water by weight. The temperatures in this study were 8·96°C, 0·00°C and −10·25°C. The rate of formation of the monoacetylacetonate nickel(II) ion was found to be first order with respect to both nickel ion concentration and the concentration of the enol tautomer of acetylacetone, hence second order overall. Hydrogen ion initially present in concentration up to 5 × 10 −4 M had no apparent effect on the initial rate of reaction. The initial rate was also shown to be independent of the ionic strenght. In all solvents employed, the rate of first complex formation was complicated from the outset of reaction by the reversible nature of the process. Small changes in the dielectric constant of the solvent system were seen to have a large effect on the rate of formation of the complex. The log of the second order rate constant was found to be a linear function of 1 D in the range of dielectric constant of 33−22. Below this range, the plot shows a very rapidly increasing slope attributable to changes in the co-ordination sphere of Ni(II). The energy of activation in any solvent of fixed composition, ( E a) c , was mathematically separated into two components, energy of activation at constant dielectric, ( E a) D , and energy of activation at constant temperature, ( E a) T . The relationship: ( E a) C = ( E a) T + ( E a) D appears to hold at dielectric constant above 25.