KINETIC STUDY OF PROTON EXCHANGE BETWEEN AL(OH2)63+ ION AND WATER IN DILUTE ACID . PARTICIPATION OF WATER MOLECULES IN PROTON TRANSFER

Rates of proton exchange between A1(0H2)63+ and H2O were measured by nuclear magnetic resonance techniques in dilute solution between 0 and 50° and in the pH range 3.3-0.5. Proton exchange results largely from proton transfer rather than from the exchange of whole water molecules. Two kinetic processes were found: acid dissociation of Al(OH2)63+, and reaction between Al(OH2)63+, Al(OH2)5OH2+, and two or more water molecules. The rate constant for acid dissociation, 1.1 x 105sec-1 at 30°, is in close agreement with results obtained by the dissociation field method. Kinetic analysis and HOD-D2O solvent isotope effects show that acid dissociation is a bimolecular process in which the proton is transferred directly to an adjacent water molecule; e.g., (D20)5Al(ODH) + OD2 → (D2O)5AlOD + HOD2+. The specific rate of acid dissociation increases slightly with the chloride concentration. The second process is formulated conveniently as a symmetrical proton transfer involving hydrated Al(OH2)63+ and hydrated (H20)5AlOH2+. In H2O, this process appears to be diffusion controlled; the rate constant is high, 9 x 108 sec-1 M-1 at 30°, and the kinetic isotope effect in HOD-D20 solvent is fairly small. One of the steps in the kinetic scheme is the dissociation of the hydrogen-bonded hydration complex, (H2O)5Al(OH).H*OH. The rate constant for breaking the Al(OH).H*OH hydrogen bond in this process is found to be 4 x 107 sec-1 at 30°. Because of the difference in the participation by water molecules in the two processes, the kinetics for proton exchange is complex; rates are not simply the sum of additive contributions from two separate processes. © 1969, American Chemical Society. All rights reserved.