Oxygen, Ozone, and Electrolysis

the cheapest oxidant, is used only rarely without irradiation and without catalysts. Examples of oxidations by air alone are the conversion of aldehydes into carboxylic acids (autoxidation) and the oxidation of acyl-oins to a-diketones. Usually, exposure to light, irradiation with ultraviolet light, or catalysts are needed. Under such circumstances, dehydrogenative coupling in benzylic positions takes place at very mild conditions [7]. In the presence of catalysts, terminal acetylenes are coupled to give diacetylenes [2], and anthracene is oxidized to anthraquinone [3]. Alcohols are converted into aldehydes or ketones with limited amounts of air [4, 5, 6, 7], Air oxidizes esters to keto esters [3], thiols to disulfides [9], and sulfoxides to sulfones [10. In the presence of mercuric bromide and under irradiation, methylene groups in allylic and benzylic positions are oxidized to carbonyls [11]. 

A reaction peculiar to singlet oxygen is the formation of hydroperoxides in the a positions with respect to double bonds [14, 15, 19, 21, 22, 23, 24, 25, 26] or aromatic rings [23, 27, 28, 29, 30] (equation 5). 

Instead of the fully accepted concerted mechanism via an ene reaction, a mechanism with perepoxides and dioxetanes as intermediates for hydroperoxides and carbonyl products, respectively, was proposed [21] (equation 6). 

Another type of oxidation typical of singlet oxygen is the formation of oxides or endooxides from conjugated dienes by 1,4-addition (Diels-Alder reaction) [13, 15, 17, 19, 24, 31, 32, 33, 34, 55] (equation 7). 

A less frequent reaction is the formation of dioxetanes from compounds containing double bonds [20, 36, 37, 38, 39, 40, 41, 42], Such compounds are not stable and disintegrate to dicarbonyl compounds (equation 8). 

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