Kinetics and mechanisms of oxidation by metal ions, part XII [1] oxidation of formaldehyde by alkaline osmium(VIII)

✍ Scribed by Minakshi Vishnoi; Kamla Sharma; Raj N. Mehrotra

• Publisher: John Wiley and Sons
• Year: 1991
• Tongue: English
• Weight: 622 KB
• Volume: 23
• Category: Article
• ISSN: 0538-8066
• DOI: 10.1002/kin.550230504

✦ Synopsis

The stopped-flow measurements on the disappearance of alkaline osmium(VII1) at 402 nm indicated a first-order dependence each in [Os(VIII)] and [HCHO]. The pseudo first-order rate constant hobs ([HCHO] & [Os(VIII)]) decreased with increasing [OH-]. The ionic strength, however, had no effect on kobs. The rapid scan spectra of the reaction mixture indicated the formation of an inert complex which absorbs at 319 nm. Therefore the rate determining step is considered to involve the bimolecular collision between Os04(OH)2'and hydrated formaldehyde. The values of the rate limiting constant k and the equilibrium constant Khn for the formation of the alkoxide ion from the reaction of hydrated formaldehyde with OHare evaluated. The equilibrium constant Kho, within the experimental limits, is independent of temperature. The pK, value, calculated from Kho, is 13.62 f 0.05 which is in good agreement with the pK, value 13.27 for formaldehyde. The activation parameters, AH' = 40 f 2 k J mol-' and AS' = -51 2 6 JK-' mol-', for the rate limiting constant k are determined.