| Popis: |
The decondensation of the Mo 7 O 24 6− ion was studied with a Durrum-Gibson stopped-flow spectrophotometer at 25 °C in NaCl as supporting electrolyte medium and ionic strength held constant at 3. The kinetic study of the fast reaction of paramolybdate ion and anionic monocharged base, B, Mo 7 O 24 6− + 8B + 4H 2 O → 7MoO 4 2− + 8HB + shows that the slow step of the decondensation is the simultaneous attack of Mo 7 O 24 6− by B and water. This step is followed by rapid reactions of neutralisation unobservable in our experimental conditions. The rate law can be written rate = { k w [H 2 O]+ k B [B]} [Mo 7 O 24 6− ]. For strong bases such as OH − , C 6 H 5 O − and NH 3 CH 2 CO 2 − the water hydrolysis is negligible in the rate limiting step and the reaction appears to be second order, first order in the polyanion and first order in the base and a S N 2 mechanism is conceivable. The rate constant, k B , increases with the strength of the base (or its nucleophilic reactivity). For weak bases which have p K a near 5 such as CH 3 CO 2 − C(CH 3 ) 3 CO 2 − and C 6 H 5 CO 2 − the decondensation is slower. The concentration and the nature of the base have no influence on the kinetic law. The reaction appears to be first order in Mo 7 O 24 6− . Thus, it is reasonable to assume that the attack on Mo 7 O 24 6− is due only to water. |
