POLYOXOMETHALATE (POM) OXIDATION OF PHENOLIC COMPOUNDS: EFFECT OF AROMATIC SUBSTITUENT GROUPS ON REACTION MECHANISM
The reaction kinetics of polyoxometalate (POM), K5SiVW11O40 ·12H2O, oxidation of a series of substituted phenols in sodium acetate buffer (I=0.2 and pH 5.0) were investigated. Second-order reaction rates were observed and found to increase with a decrease in the H+. Electron-transfer from neutral phenols (k1) was slower than the corresponding phenoxide anions (k2). The reaction rate was highly dependent on the nature of substituent groups. The observed rate constants were found to increase in the order p-methoxyphenol (1) > p-methylphenol (2) > w-methoxyphenol (3) > m-methylphenol (4) > Phenol (5) > p-chlorophenol (6) > p-bromophenol (7) > m-chlorophenol (8) > m-nitrophenol (P). A Hammett plot of the data reveals a better correlation with σ than σ. These results indicate a reaction involving the formation of an electron-deficient phenoxy radical intermediate with the rate-determining step being an electron-transfer from the neutral substrate by the POM. The activation energy was decreased in order p-methylphenol (2), p-propylphenol (10), p-pentylphenol (11), and p-octylphenol (72) as the size of substrate was increased although free energy of activation of 2, 10, 11 and 72 are almost the same values. This indicates that there was a steric factor among those model compounds in the reaction with POM.
reaction kinetics polyozometalate ozidation
Yong Sik Kim Hou-min Chang John F. Kadla
Biomaterials Chemistry, Faculty of Forestry, University of British Columbia, Vancouver, BC, Canada College of Natural Resources, North Carolina State University, Raleigh, NC 27695-8005, USA
国际会议
广州
英文
46-50
2006-11-08（万方平台首次上网日期，不代表论文的发表时间）