Chain-breaking hydrogen donor

The chemical mechanisms involved in the action of antioxidants have been discussed in standard texts [9,63-66] and the reader is directed to these and the references they contain for more detailed information. Two complementary antioxidant mechanisms are frequently used synergistically in polyolefins. The first is the kinetic chain-breaking hydrogen donor, (CB-D) mechanism in which chain-propagating peroxyl radicals (POO ) are preferentially reduced to hydroperoxide by the antioxidant (AH). 

Another approach to safer stabilization is to use a biological antioxidant such as vitamin E (a-tocopherol is the active form of vitamin E, AO-9, Table la). It is essentially a hindered phenol which acts as an effective chain breaking donor antioxidant, donating a hydrogen to ROO to yield a very stable tocopheroxyl radical, a-Tocopherol is a very effective melt stabilizer in polyolefins that offers high protection to the polymer at very low concentration [41], (Table 2). 

Primary antioxidants, also termed as chain-breaking antioxidants, interfere with the chain reaction in Scheme 2.1, by trapping radicals or labile hydrogen atom donors. These are exemplified by hindered phenols and alkylarylamines. Scheme 2.8 schematically demonstrates the scavenging activity of a typical hindered phenol. 

In summary, we have found that the oxidation of these partially hindered amines occurs to give two stable nitroxyl radicals which can function as chain breaking acceptors. The primary radical formed is easily oxidized and thus could also act as a hydrogen atom donor in route to the second stable nitroxyl racical. 

Chain-breaking antioxidants (a) free-radical traps, (b) electron donors, (c) hydrogen donors. 

Hydrogen donors are referred to as chain breaking donors. In particular, hydrogen donors decompose peroxides into inert products. Hydrogen donors are classified as primary antioxidants, because they access secondary products in the chain of the autoxidation reaction. The mechanism that interferes the autoxidation reaction is shown in Eq. 19.5. 

Radical chain processes break down whenever the velocity of a termination reaction is comparable to the velocity of the rate-controlling step in a chain reaction. This situation would occur, for example, if one attempted to use EtsSiH as the hydrogen atom donor in the alkyl halide reduction sequence in Figure 4.6 and employed typical tin-hydride reaction conditions because the rate constant for reaction of the silane with an alkyl radical is 4 orders of magnitude smaller than that for reaction of Bu3SnH. Such a slow reaction would not lead to a synthetically useful nonchain sequence, however, because no radical is persistent in this case. In fact, a silane-based radical chain reduction of an alkyl halide could be accomplished successfully if the velocity of the initiation reaction was reduced enough such that it (and, hence, also the velocity of alkyl radical termination... 

Two important features emerge from our examination of these three examples of membrane protein structure. First, the parts of the protein that interact with the hydrophobic parts of the membrane are coated with nonpolar amino acid side chains, whereas those parts that interact with the aqueous environment are much more hydrophilic. Second, the structures positioned within the membrane are quite regular and, in particular, all backbone hydrogen-bond donors and acceptors participate in hydrogen bonds. Breaking a hydrogen bond within a membrane is quite unfavorable, because little or no water is present to compete for the polar groups. 

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