Hydrogen transfer ability mechanism

Returning now to the question of the initiation step of the cationic hydrocarbon reactions, there are essentially two schools of thought on the subject. One is that the catalyst, itself, has the intrinsic ability to extract a hydride ion from the hydrocarbon or to distort a carbon-hydrogen bond to such an extent that the net result is the same (Milliken et al., 39). The other mechanism is the formation of a cationic complex by the simple addition of a proton to olefinic impurities, either initially present or formed in small amounts by thermal cracking or oxidation. This complex, however small its concentration on the surface of the catalyst, can then start a chain reaction through the hydrogen transfer mechanism, i.e., it can extract a hydride ion from a saturated hydrocarbon, which then becomes the propagating complex (Hansford, 43, 47 Thomas, 18). 

The reaction between the photoexcited carbonyl compound and an amine occurs with substantially greater facility than that with most other hydrogen donors. The rate constants for triplet quenching by amines show little dependence on the amine a-C-H bond strength. However, the ability of the amine to release an electron is important.- - This is in keeping with a mechanism of radical generation which involves initial electron (or charge) transfer from the amine to the photoexcited carbonyl compound. Loss of a proton from the resultant complex (exciplex) results in an a-aminoalkyl radical which initiates polymerization. The... 

The only antimalarial drugs whose mechanisms of action are reasonably well understood are the drugs that inhibit the parasite s ability to synthesize folic acid. Parasites cannot use preformed folic acid and therefore must synthesize this compound from the following precursors obtained from their host p-aminobenzoic acid (PABA), pteridine, and glutamic acid. The dihydrofolic acid formed from these precursors must then be hydrogenated to form tetrahydrofoUc acid. The latter compound is the coenzyme that acts as an acceptor of a variety of one-carbon units. The transfer of one-carbon units is important in the synthesis of the pyrimidines and purines, which are essential in nucleic acid synthesis. 

In vitro tests, used in evaluation of antioxidant properties make use of the ability of antioxidants to quench free radicals. Based on this mechanism, the methods are divided into two groups SET - single electron transfer, and HAT - hydrogen atom transfer. Reactions with antioxidants in assays with the DPPH radical, ABTS and the Folin-Ciocalteu reagent both operate according to the SET and HAT mechanism. Due to the kinetics of the reaction, they are included in the... 

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