Kinetics and mechanism of the oxidation of phenols by the oxochromium(IV) ion

Journal Article · Tue Dec 07 23:00:00 EST 1993 · Inorganic Chemistry; (United States)

DOI:https://doi.org/10.1021/ic00077a023· OSTI ID:5088061

Al-Ajlouni, A; Bakac, A; Espenson, J H ^[1]

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1. Iowa State Univ., Ames, IA (United States)

The oxidation of phenols by the pentaaquaoxochromium(IV) ion, (H[sub 2]O)[sub 5]CrO[sup 2+], in acidic aqueous solutions yields p-benzoquinone as a major product. Small amounts of the unstable 4,4'-biphenoquinone were also produced, as evidenced by an increase (fast stage) and then a decrease (slow stage) in absorbance at 400 nm where there is an intense absorption band. The fast stage is first-order in CrO[sup 2+] and first-order in phenol. The slow stage follows a first-order exponential decay. There is a large kinetic isotope effect for the first stage, k[sub H]/k[sub D] = 14.7, on deuteration of the hydroxylic hydrogen. There is, however, no kinetic isotope effect of deuteration of the C-H hydrogen, K[sub H]/k[sub D] [approximately] 1.0. The rate constants for different meta-substituted phenols follow the Hammett relationship with [rho] = 1.7. The activation parameters are [Delta]H = 15.2 [+-] 1.5 kJ mol[sup [minus]1] and [Delta]S = 144 [+-] 15 J mol[sup [minus]1] K[sup [minus]1]. The authors propose a mechanism according to which the phenols are first oxidized by one electron to the corresponding phenoxyl radicals; this occurs by hydrogen atom abstraction. The superoxochromium(III) ion, (H[sub 2]O)[sub 5]CrO[sub 2][sup 2+], which is also present in solution, then oxidizes the phenoxyl radical to benzoquinone. The rate constants for both stages increase with increasing ionic strength of the medium. The solution acidity was found to decrease the rate of the first stage and enhance the rate of the second, but both effects are relatively small. The activation parameters for the second stage are [Delta]H = 46 [+-] 4 kJ mol[sup [minus]1] and [Delta]S = -129 [+-] 11 J mol[sup [minus]1] K[sup [minus]1].

• OSTI ID: 5088061
• Journal Information: Inorganic Chemistry; (United States), Journal Name: Inorganic Chemistry; (United States) Vol. 32:25; ISSN 0020-1669; ISSN INOCAJ
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
400201* -- Chemical & Physicochemical Properties
400202 -- Isotope Effects
Isotope Exchange
& Isotope Separation
ABSORPTION SPECTROSCOPY
AROMATICS
CATIONS
CHALCOGENIDES
CHARGED PARTICLES
CHEMICAL REACTION KINETICS
CHEMICAL REACTIONS
CHROMIUM COMPOUNDS
CHROMIUM OXIDES
HYDRATES
HYDROXY COMPOUNDS
IONS
ISOTOPE EFFECTS
KINETICS
ORGANIC COMPOUNDS
OXIDATION
OXIDES
OXYGEN COMPOUNDS
PHENOLS
PHYSICAL PROPERTIES
RADICALS
REACTION KINETICS
SPECTROSCOPY
THERMODYNAMIC PROPERTIES
TRANSITION ELEMENT COMPOUNDS