Kinetics and mechanism of permanganate oxidation of phenylphosphinic acid in acid perchlorate solution

Abstract

The stoichiometry of the title reaction is expressed by the equation, MnO₄^- + 2C₆H₅HP(O)OH + 4H⁺ →Mn(III) +2C₆H₅PO(OH)₂ + 2H₂O. The kinetics and the rapid scan of the reaction mixture suggest the formation of the complexes by

C₆H₅HP(O)OH (<b>C1</b>, <i>K</i>₁<i>) </i>and C₆H₅HP(O)O^- (<b>C2</b>, <i>K</i>₂<i>)</i>with MnO₄^- ion. The equilibrium constants of the complexes are H⁺ dependent. It is suggested that the complexes are formed by hydrogen bonds. P-H .... O-Mn, and the redox involve the dissociation of H^- from the P-H bond causing transfer of two electrons from substrate to MnO₄^- through -O- bridge. The relative stability of the complex <b>C2</b> > <b>C1 </b>because K₁ < <i>K</i>₂<i> </i>and, <i>k</i>₁ > <i>k</i>₂<i> </i>where <i>k</i>₁ and <i>k</i>₂<i> </i>are the rate constants for the decomposition of the complexes <b>C1</b> and <b>C2</b><i>. </i>The kinetics in perchlorate solution (μ = 1mol dm^-3 LiClO₄) under psedo-first order conditions ([PPA] ˃˃ [MnO₄ ^-]) indicated that the H⁺ catalysed rate correlates with the expression <i>k</i>_obs(<i>K</i>_a + [H⁺] = <i>a </i>+ <i>b</i>[H⁺] + c[H⁺]²where <i>k</i>_obs is the pseudo-first order rate constant and <i>K</i>_a is the dissociation constant of C₆H₅HP(O)OH. The [H⁺]²term is likely due to the acid catalysis of <b>C1</b>. The activation parameters corresponding to <i>k</i>₁ and <i>k</i>₂<i> </i>are reported.