Oxygen-17 Magnetic Resonance Study of the Oxygen Exchange of Methyl- and Butylarsonic Acids and Dimethylarsinic Acid with Solvent Water

The rates of the oxygen-exchange reactions of methyl- and butylarsonic acids and dimethylarsinic acid with solvent water were measured in the pH range below 6, and at temperatures between 50 and 90 °C, by using the 17O NMR line-broadening technique. For methylarsonic acid, the rate RMAA is expressed by the following rate law: RMAA = kh1[H+][H2A] + k1[H2A] + kh2[H+][H2A]2 + k2[H2A]2 + k3[H2A][HA−], where H2A and HA− designate CH3AsO3H2 and CH3AsO3H−, respectively. For butylarsonic acid, the rate law is expressed as: RBAA = kh1[H+][n–C4H9AsO3H2]. For dimethylarsinic acid, its conjugated acid is involved in the rate law as: RCAA = kh1′[(CH3)2AsO2H2+] + khh1′[H+][(CH3)2AsO2H2+]. The exchange rates of methyl- and butylarsonic acids and dimethylarsinic acid (50—90 °C) were analyzed in terms of the respective rate law to obtain the values of the rate constants involved in the rate laws. The activation parameters of these rate constants were obtained; for both the first- and second-order rate constants, the activ...