Photooxidation of 2-Propanol

Anatase is much more active than rutile in the liquid-phase photooxidation of 2-propanol, and the observed reactivity compared well with that of gaseous alcohol on platinized titania [106a]. Because most early studies employed the alcohol as a sacrificial reagent for the photochemical production of hydrogen, the organic product was often not analyzed and little effort was devoted to the selective activation of alcohols in the presence of other functional groups. In the oxidation of car-... 

Several studies have been done to establish the chain-carrying species in the oxidation of alcohol (29). The absence of peroxide in the photooxidation of 2-propanol suggests either that the chain carrier is a hydroperoxy radical and not the 2-hydroxy-2-propylperoxy radical or that 2-hydroperoxy-2-propanol is too unstable to accumulate, if formed under these conditions. Only the hydroperoxy radical could be trapped by the spin-trapping reagent phenyl-N-f-butylnitrone in the oxidation of 2-propanol at 25°C (28). The reported high rate constant (30) for the decomposition of the 2-hydroxy-2-propylperoxy radical to acetone and hydroperoxy radical also implies that the hydroperoxy radical is the predominant chain carrier. On the other hand, Schenck and his co-workers (31) isolated 2-hydroperoxy-2-propanol in the photooxidation of 2-propanol with 313-nm light. 

Certain features of alcohol photo-oxidation are connected with the presence of sensitizers. As found by Backstrom [60], the rate of photooxidation of 2-propanol (with benzophenone as sensitizer) is inversely proportional the oxygen pressure. Dependence of the 2-propanol oxidation rate on alcohol concentration and on light intensity, /, (with anthraquinone as sensitizer) is expressed as... 

When the reaction was carried out under N2 atmosphere, the biphenyl yield was increased up to >92%. Compound 705 is therefore likely to be formed via the photooxidation of the intermediate 702 with an 02 molecule dissolved in 2-propanol. 

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