Spin trapping, hydrogen abstraction

Unfortunately, due to the above shortcomings of hydroxylamine derivatives as spin traps, the uncertainties of the mechanism of their reactions with superoxide are added. Although it is supposed that nitroxide radicals are formed by oxidation with superoxide (Reaction (6)), this reaction cannot be an elemental stage because superoxide cannot abstract a hydrogen atom. 

This simple expedient appears to have been employed first by Lunazzi et al. (1972), who compared the reactivities of ethylbenzene and a methylthiophene towards hydrogen abstraction by t-butoxyl radicals using MNP as trap (Scheme 3). Features in the spectra of the spin adducts of thienylmethyl and... 

Atom abstraction from the cr-carbon atom in spin adducts, which leads to nitrones, and thence in some instances to spin adducts of these nitrones (e.g. Scheme 9), is undoubtedly a general phenomenon. For this sequence to be observed it is necessary that some, or all, of the following conditions are fulfilled. (0 The nitrone is more reactive towards radical addition than is the original spin trap and/or the latter is substantially consumed in the reaction. (h) The initial spin adduct is particularly susceptible to hydrogen-atom loss (and probably reacts this way by disproportionation). ( 7) The secondary spin adduct is particularly persistent. 

A number of reports on the thermal decomposition of peroxides have been published. The thermal decompositions of f-butyl peroxyacetate and f-butyl peroxypivalate, of HCOH and a kinetic study of the acid-induced decomposition of di-f-butyl peroxide in n-heptane at high temperatures and pressures have been reported. Thermolysis of substituted f-butyl (2-phenylprop-2-yl) peroxides gave acetophenone as the major product, formed via fragmentation of intermediate alkoxy radicals RCH2C(Ph)(Me)0. A study of the thermolysis mechanism of di-f-butyl and di-f-amyl peroxide by ESR and spin-trapping techniques has been reported. The di-f-amyloxy radical has been trapped for the first time. jS-Scission reaction is much faster in di-f-amyloxyl radicals than in r-butoxyl radicals. The radicals derived from di-f-butyl peroxide are more reactive towards hydrogen abstraction from toluene than those derived from di-f-amyl peroxide. 

Adam W, Arnold MA, Nau WM, Pischel U, Saha-Moller CR (2001b) Structure-dependent reactivity of oxyfunctionalized acetophenones in the photooxidation of DNA base oxidation and strand breaks through photolytical radical formation (spin trapping, EPR spectroscopy, transient kinetics) versus photosensitization (electron transfer, hydrogen-atom-abstraction). Nucleic Acids... 

The relative homolytic reactivities of 2- and 3-methylthiophenes toward hydrogen abstraction from the side chain by tert-butoxyl radicals to form 98 and 99 have been investigated by the spin-trapping method. The... 

The results of spin-trapping experiments (18) also support this view no radicals were trapped on photolysis of benzil in benzene solutions of compounds I and II (see Section II. A. 1) but in methanol solutions of II, CH2OH radicals were scavenged, indicating that hydrogen abstraction by photoexeited benzil had occurred (18). Thus the behaviour of benzil in these experiments contrasted markedly with that of benzoin and benzoin methyl ether, described previously. 

Reaction 6 is the hydrogen atom abstraction from PVA, an important propagation reaction. Since it was shown by the spin-trapping technique using 2-methyl-2-nitrosopropane that only the tertiary hydrogen of polypropylene was abstracted by the alkoxy radical (28), it was assumed that the first attack was also exclusively on the tertiary hydrogen of PVA. The a-hydroxyperoxy radical formed by the addition of oxygen (Reaction... 

As spin trap in these experiments, pentamethylnitrosobenzene (PMNB) was used. The first problem was to find if the benzoyl radical as an initiator could add to the double bonds of the model compoimds. Benzoyl radicals produced from benzil by a laser beam output of 308 nm added to 2-heptene (formula 6) but not to the other two model compounds. In addition allyl radicals are formed by hydrogen abstraction (formula 7). 

Spin trapping with PMNB was applied to the radicals derived from initiator decomposition (formula 3) and their subsequent reactions with the model compounds (formula 5). Both initiator radicals could be trapped and identified. When model compounds were present during UV-irradiation, new radicals were identified from the ESR spectra. For dihydrocyclopentadiene (DHCPD) only one trapped radical was found and for ethylidene norbornane (ENB) two radicals. By comparison with computer simulated ESR spectra, it is concluded that the radicals of these model compounds are all allyl radicals (formula 8 and 9) formed by hydrogen abstraction from the models. Radical (8 a) has two stereoisomers but they have closely the same ESR spectra when trapped and cannot be separated. Radical (8 b) has two resonance structures (shift of double bond in the ethylidene group) but only one radical (8 b) is trapped, probably due to steric hinderance for trapping the methin radical. The DHCPD radical (formula 9) has two steric forms because the two allylic hydrogens are not identical. Once they are formed, the spin trap can only approach from one side and only one of the steric forms is trapped as shown in the ESR spectrum. 

---