30 2 x 3xo it h it 57 58 ii chemistry of dioxygen 257

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~ 30 2 + 'X ~ 3XO it H it (5.7) (5.8)
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II. CHEMISTRY OF DIOXYGEN 257 But such a reaction pathway would give a high activation barrier, because the excited triplet states of even unsaturated molecules are typically 40-70 kcallmol less stable than the ground state, and those of saturated hydrocarbons are much higherY Likewise, a pathway in which O 2 is excited to a singlet state that then reacts with the substrate would be spin-allowed (Reactions 5.9 and 5.10). The high reactivity of singlet dioxygen, generated by photochemical or chemical means, is well-documented. 18 ,19 However, such a pathway for a reaction of dioxygen, which is initially in its ground triplet state, would also require a high activation energy, since the lowest-energy singlet excited state of dioxygen is 22.5 kcall mol higher in energy than ground-state triplet dioxygen. 15 16 30 2 + 22.5 kcal/mol -----;> '0 2 tt tt (5.9) 'X ---3> 'xo H H (5.10) Moreover, the products of typical reactions of singlet-state dioxygen with or- ganic substrates (Reactions 5.1I and 5.12, for example) are quite different in character from the reactions of dioxygen with organic substrates catalyzed by oxygenase enzymes (see Section V): (5.1I) (5.12) One pathway for a direct reaction of triplet ground-state dioxygen with a singlet ground-state organic substrate that can occur readily without a catalyst begins with the one-electron oxidation of the substrate by dioxygen. The prod- ucts of such a reaction would be two doublets, i.e., superoxide and the one- electron oxidized substrate, each having one unpaired electron (Reaction 5.13). These free radicals can diffuse apart and then recombine with their spins paired (Reaction 5. 14). 30 2 + 'X ---3> 20 2 - + 2X + tt H t t (5.13) (5.14) Such a mechanism has been shown to occur for the reaction of dioxygen with reduced flavins shown in Reaction (5.15).20
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258 (5.15) However, this pathway requires that the substrate be able to reduce dioxygen to superoxide, a reaction that requires an unusually strong reducing agent (such as a reduced flavin), since dioxygen is not a particularly strong one-electron oxi- dizing agent (see Table 5.1 and discussion above). Typical organic substrates in enzymatic and nonenzymatic oxygenation reactions usually are not sufficiently strong reducing agents to reduce dioxygen to superoxide; so this pathway is not commonly observed. The result of these kinetic barriers to dioxygen reactions with most organic molecules is that uncatalyzed reactions of this type are usually quite slow. An exception to this rule is an oxidation pathway known as free-radical autoxida- tion. c. Free-Radical Autoxidation The term free-radical autoxidation describes a reaction pathway in which diox- ygen reacts with an organic substrate to give an oxygenated product in a free- radical chain process that requires an initiator in order to get the chain reaction started.
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