Aromatic hydrocarbons reactions with singlet oxygen

The examples of reaction rates of O2 in Figure 12.7 show that O2 reacts only with deprotonated species (e.g., phenolate anions) that is, the apparent rate constants decrease in the pH region below the pKa of the chemical. Singlet oxygen is selective it is an electrophile that reacts only with particular functional chemical structures such as are present in 1,3 dienes (see chemical structure of fiirfuiyl alcohol) or polycondensed aromatic hydrocarbons (with delocalized 7T electron bonds) or in sulfides or mercaptans (Hoigne, 1990). 

Three typical water-sohible substrates including the polycyclic aromatic hydrocarbon 3, the cyclohexadienic derivative 5 and the olefin 7 were oxidized in order to assess the efficiency of the oxidizing system H202/Ca(0H)2. These substrates represent standard types of singlet oxygen reactions, namely [4 + 2] cycloaddhion and the ene reactioa The dis pearance of the substrates was monitored by HPLC and the products were identified 1 conq>arison with genuine sanq>les, photochemically obtained. E q)erimental conditions aixl results are summarized in Table 1. 

Oxygen is known to form metastable complexes with olefins and some aromatic hydrocarbons which are capable of absorbing sunlight UV wavelengths, unlike the parent hydrocarbons (Khalil and Kasha, 1978). In some cases, the existence of these complexes may lead to reactions of the substrate hydrocarbons or formation of singlet oxygen (see Section 4.A.3). 

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