Carbon-centered radicals, autoxidation

Cupric salts have not proven to be particularly useful in hydrocarbon oxidation. They do, however, exhibit interesting characteristics. Cupric ion has a singular ability to compete with oxygen for carbon-centered radicals in autoxidation (compare eq. (32), where X can be carboxylate, halide, etc., with eq. (3)) ... 

In most cases, radical A will react very rapidly (to begin the propagation phase of autoxidation) with molecular oxygen, to form a new radical AOO, commonly referred to as a peroxy radical. In the case of carbon-centered radicals, this step is extremely fast (usually approximating diffusion control). 

One possible way of overcoming this problem is to introduce into the reaction mixture a compound that decomposes at a constant rate to free radicals (X ) capable of extracting a hydrogen atom from the PUFAs (RH) and consequently initiating the autoxidation process. The compounds most frequently used for this are the so-called azo-initiators (X-N=N=X), which thermally decompose to highly reactive carbon-centered radicals. ... 

According to this particular mechanism, P-carotene is capable of scavenging peroxyl radicals (Reaction 12.26). The resulting carbon centered radical (ROO-P-CAR ) reacts rapidly and reversibly with oxygen to form a new, chain-carrying peroxyl radical (ROO-P-CAR-OO ). The carbon centered radical is resonance stabilized to such an extent that when the oxygen pressure is lowered the equilibrium of the Reaction 12.27 shifts sufficiently to the left, to effectively lower the concentration of the peroxyl radicals and hence reduces the amount of autoxidation in the system."" Furthermore, the P-carotene radical addnct can also undergo termination by reaction with another peroxyl radical (Reactions 12.27 and 12.28). 

While the sterically hindered phenols react preferentially with oxygen-centered radicals such as peroxy and alkoxy, rather than with carbon-centered radicals, benzofuranones can scavenge both types of radicals. Accordingly, a ben-zofuranone can be repeatedly positioned at key locations around the autoxidation... 

High heat treatment of milk exposes free thiol groups that autoxidize to form diiyl radicals, Oj and HjOj. Thiols may exert an indirect prooxidmit effect through this mechanism (4). The thiols are mainly derived from modification of the milk flit globule membrane and the serum proteins, particularly P-lactoglobulin. In freeze-dried milk proteins, radicals were detected by ESR spectroscopy, and spectral characteristics were consistent with carbon-centered radicals (7). 

Autoxidation is achemical reaction that usually takes place at ambient temperatures between atmospheric oxygen and a lipid substrate. In the presence of an initiator such as light, heat, or metal ions, unsaturated lipids (LH) form carbon-centered alkyl radicals (L ) ... 

ROS produced by sugars and glycated protein autoxidation participate in oxidizing of already glycated proteins and affect other proteins (H24). These reactions are catalyzed by metal ions (W17). The presence of metal ions may, moreover, initiate the Fenton reaction and produce hydroxyl radicals. A carbon-centered 1-hydroxyalkyl radical was found during autoxidation of glyceraldehyde (T12). 

The initiation reaction is the hemolytic abstraction of hydrogen to form a carbon-centered alkyl radical in the presence of an initiator. Under normal oxygen pressure, the alkyl radical reacts rapidly with oxygen to form the peroxy radical, which in turn reacts with more unsaturated lipids to form hydroperoxides. The lipid-free radical thus formed can further react with oxygen to form a peroxy radical. Hence, the autoxidation is a free radical chain reaction. Because the rate of reaction between the alkyl radical and oxygen is fast, most of the free radicals are in the form of the peroxy radical. Consequently, the major termination takes place via the interaction between two peroxy radicals. 

In the case of sp carbon center oxyfunctionalization with hydrogen peroxide in the presence of POMs, the possibility of autoxidation seems to be ruled out, since similar results were obtained under an argon atmosphere, in comparison with those achieved in the presence of air. Moreover, radical processes can be ruled out, as the presence of a radical scavenger in the reaction media did not inhibit the formation of any reaction products [52],... 

On the other hand, participation of free radicals should be considered in most cases of oxygenations. Both of carbon and oxygen centered radicals can take part in the oxygenation processes. Apart from the apparent autoxidation process, it is not easy to differentiate explicitly metal-based mechanisms from free radical mechanisms. Different types of the radical-clock reagents have been developed for detection of radicals of different lifetime, especially in the discussions on the radical-rebound mechanisms. 

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