Kinetics of a slow step in the chelation of ethylenediaminetetra-acetic acid to chromium(III)
Abstract
The kinetics of a slow step in the reaction of [CrIIIYH3(Cl)2(H2O)] to give [CrIIIYH(H2O)] and [CrIIIY(H2O)]-(ethylenediaminetetra-acetate, EDTA = Y4–), and in the analogous reaction of [CrIIIYH3(Br)2(H2O)], have been studied in aqueous perchlorate media [µ 1·0M(LiClO4)] at temperatures in the range 1–22°. The reactions are believed to occur in two steps, the first of which is slow and rate determining. First-order kinetics are observed for this step (kobs) and the hydrogen-ion dependence is given by kobs(K+[H+])=k1[H+]+k2K, where k1 is the rate constant for the reaction of a protonated form of the complex (x is probably 2 in [CrIIIYHx(X)2(H2O)]x–3), k2 is the rate constant for the reaction of the complex with one less proton, and K is the acid dissociation constant which relates the two species. At 20° values of k1 and k2 are 0·15 × 10–2 and 1·43 × 10–2 sec.–1 respectively for the chloro-complex, and 1·72 × 10–2 and 12·2 × 10–2 sec.–1 for the bromo-complex. Enthalpies of activation for the reactions whose rate constants are k1 and k2 are within the range 16·6–17·5 kcal. mole–1, and the corresponding entropies of activation fall between –9·2 and –12·2 e.u.