Photo-oxygenation of saturated hydrocarbons using uranyl ions
Bergfeldt, Trevor Marlin
The photo-oxygenation of 2-methylpropane and cyclohexane using visible light in aqueous acidic uranyl ion solutions at ambient temperature and pressure has been undertaken. For 2-methylpropane in the absence of oxygen, the main product (≈90%) is 2-methyl-2-propanol with a quantum yield of 0.021 ± 0.001. In the presence of molecular oxygen, both 2-methyl-2-propanol and 2-propanone (acetone) are found. Based on this, and results of gamma radiolysis of aqueous 2-methylpropane to give 'tert'-butyl radical by electronically excited uranyl ion is proposed. In the absence of oxygen, the quantum yield of 2-methyl-2-propanol shows a sigmoidal dependence on the concentration of perchloric acid. A two-species kinetic model involving an acid-base dissociation of the uranyl ion in the excited state accounts for the observed features. The addition of sodium perchlorate to the 2-methylpropane system has an inhibitory effect on the quantum yield. Excited-state ion pairing between the uranyl ion and perchlorate anion is proposed. Consequently, the two-species acid-base model is expanded upon to yield a three-species acid-base-perchlorate model that seems to account for the results from 0.01-0.4 M perchloric acid concentration. Potassium peroxydisulfate is proven effective to increase the quantum yield of 2-methyl-2-propanol from 0.021 ± 0.001 to greater than unity (1.5 ± 0.1) indicating the existence and importance of thermal chain reactions involving sulfate radical anion. The quantum yield of 2-methyl-2-propanol is found to be dependent on the concentrations of 2-methylpropane, perchloric acid and potassium peroxydisulfate, and inversely dependent on the light intensity. The net consumption of uranyl ion is zero in the presence of potassium peroxydisulfate meaning that the uranyl ion is regenerated, making this a catalytic cycle in terms of uranyl ion. The oxygenation of cyclohexane using photo-excited aqueous uranyl ion gives cyclohexanol and cyclohexanone as the two main isolated products (54%). The overall mechanism is expected to be similar to that for the 2-methylpropane system. Refluxing of this substrate with a reducing agent (CaH2 or LiAlH4) is required prior to photolysis to achieve consistent quantum yields of both products due to thermal autoxidation reactions. The presence of molecular oxygen is found to be important in determining the ratio of alcohol to ketone in the product distribution. Potassium peroxydisulfate significantly enhances the quantum yield of cyclohexanone, leaving the quantum yield of cyclohexanol relatively unchanged (as compared to without added potassium peroxydisulfate), while uranyl ion is not consumed. Possible reactions involving cyclohexyl and cyclohexyl peroxyl radicals are given to account for the experimental results.
DegreeDoctor of Philosophy (Ph.D.)
Copyright DateSeptember 2001
saturated hydrocarbon oxidation