The Kinetic Study of the Oxygen-Exchange Reaction of Some Alkylarsonates and Dialkylarsinates with Water

Article abstract of DOI:10.1246/bcsj.65.1397

The rates (Rs) of the oxygen exchange reaction of alkylarsonates (methyl-, butyl-, hexyl-, and benzyl-) and dialkylarsinates (dimethyl- and dibutyl-) with solvent water were measured in the pH range greater than 6, and at temperatures between 30 and 0 deg C.For arsonates, the rate (R) is expressed by the following rate law: R = k₁2-> + k₂-> + k₃ + k₄->² +k₅-> + k₆², where A^2-, HA^-, and H2A designate R'AsO3^2-, R'AsO3H^-, and R'AsO3H2 (R' = alkyl), respectively.For arsinates, R = k₁-> + k₂ (pH > 9).The exchange rates of arsonates at 0 and 30 deg C were analyzed in terms of the rate law to obtain the values of the rate constants (k₁ - k₆).The activation parameters were obtained for the k₁ paths of both arsonates and arsinates, which are the reaction between R'AsO3^2- or R'2AsO2^- and water.It is noticeable that the values of the activation entropies of the arsonates were found to be negative, while those of the arsinate were found to be positive.It was suggested that the oxygen exchange of arsonate ions proceeds via an S_N2 mechanism, while that of arsinates preceeds via S_N1.