Singlet oxygen reactions, prevention

Another type of chemical change is initiated by light, which may trigger autolytic, that is, free radical (Type I) or singlet oxygen (Type II) reactions. These changes are routinely classified as oxidation. Rancidity in cosmetics, especially those containing unsaturated Hpids, is commonly prevented by use of antioxidants (qv). 

Of considerable practical importance is the concentration of substrate. Even fairly reactive compounds do not autoxidize readily at sub-millimolar concentrations [7a]. The rate of initiation is also crucial in promoting autoxidation. Although many autoxidation reactions are auto-initiated, the use of initiators such as d -tert-butyl peroxyoxalate (DBPO) [12a] or di-tcr -butyl hyponitrite (DTBN) [12b] can be beneficial. DBPO and DTBN are convenient sources of tcrt-butoxyl radicals in the temperature ranges of 20-60 and 20-90 °C, respectively (Scheme 2). They are especially effective when used in conjunction with an excess of tcrt-butyl hydroperoxide (TBHP), whose role is to trap product peroxyl radicals as hydroperoxides, thereby preventing side-reactions [13a,c. In contrast to auto-initiated autoxidation, the autoxidation of simple alkenes (e.g. 1-methylcyclohexene, Scheme 2) by the DBPO-TBHP procedure is of comparable preparative value to the widely used singlet oxygen route [13a,b]. 

P-carotene is an antioxidant that prevents singlet oxygen radicals from causing damage in living organisms, particularly in photosynthetic reaction centers and antenna systems. 

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