H atom abstraction

In dibenzothiophene-S,S-dioxide the S atom is in a ring, and hence more constrained. The yield of SOz in the radiolysis is linear with the dose to about 13 Mrad after which it levels off as in p,p -ditolyI sulfone. However, the yield of S02 in this case is much lower (a factor of 25) than in the case of p,p -ditolyl sulfone (G = 0.002 compared to G = 0.05). This stability of the dibenzothiophene sulfone could be partially due to back reaction to reform the parent sulfone and partially due to more efficient energy delocalization. The expected biphenylene product was not detected due to limitations of the analytical method. Bowmer and O Donnell70 studied the volatile products in y-radiolysis of dialkyl, alkyl aryl and diaryl sulfones. Table 2 gives the radiolytic yields of S02 and of the hydrocarbon products of the alkyl or aryl radicals. The hydrocarbon products are those obtained either by H atom abstraction or by radical combination. The authors69 suggested the mechanism... 

Hydrogen, which is a major product in the radiolysis of most hydrocarbon polymers, is only a minor product in the radiolysis of poly(olefin sulfone)s, although it is of the largest yield between the minor products. Hydrogen is formed by H atoms combination or by an H atom abstracting hydrogen... 

The formation of adduct is followed by fragmentation and subsequent H-atom abstraction reaction from the sulfinic acid produced. Strong acid solutions of aromatic sulfoxides like thianthrene 5-oxide (7) or phenothiazine 5-oxide (8) gives rise to ESR signals, which... 

Ionic Reactions in TD/D2 Methane Mixtures. Previous investigation of the radiolysis of D2 containing small quantities of CH4 demonstrated that at low conversions all products anticipated from the H atom abstraction sequence except CH3D are absent from 125° to —196°C. and that the temperature coefficient of the rate of CH3D formation between 25° and 125 °C. is much too small for a purely atomic and free-radical reaction sequence (8). These observations are confirmed by new data presented in Table I. The new data also demonstrate the initial value of G(CH3D) is independent of temperature at 25°C. and below. 

Figure 1 Rate constants for H-atom abstraction from a variety of reducing systems by primary alkyl radicals at 80 C.

Since the recognition of the role of chlorine in catalytic ozone destruction, increasing effort has been devoted to finding replacements. In most cases reported so far, the replacements are partially halogenated molecules that retain one or more hydrogen atoms (HCFCs and HFC s). The presence of H-atoms gives HO a handle (via H-atom abstractions such as R4) for their tropospheric... 

The chemical reactivity of the organoruthenium and -osmium porphyrin complexes varies considerably, with some complexes (M(Por)R2, M(Por)R and Os(OEP)(NO)R) at least moderately air stable, while most are light sensitive and Stability is improved by handling them in the dark. Chemical transformations directly involving the methyl group have been observed for Ru(TTP) NO)Me, which inserts SO2 to form Ru(TTP)(N0) 0S(0)Me and Ru(OEP)Me which undergoes H- atom abstraction reactions with the radical trap TEMPO in benzene solution to yield Ru(OEP)(CO)(TEMPO). Isotope labeling studies indicate that the carbonyl carbon atom is derived from the methyl carbon atom. "" Reaction of... 

The measured [ OH]/[ OH] branching ratio versus inverse temperature is plotted in Fig. 4. If the two species are produced by two parallel pathways, the total reaction rate is a simple sum of the two pathway-resolved rates. In this case, the data points in an Arrhenius plot should fall on a straight line with a slope proportional to the difference in activation energies for the two competing pathways. A fit to the data in Fig. 4 yields the result that the barrier to O atom abstraction is 1.0 0.4kcal mol larger than for H atom abstraction. Although... 

CH3S(0)CH2)2 is observed indicating that H atoms abstract hydrogen from DMSO to form CH3S(0)CH2 which dimerizes to (CH3S(0)CH2)2- The radical CH3S(0)CH2 is not found in the reaction of OH" with DMSO. ... 

Further oxidation of an alkoxy radical (RO ), via H-atom abstraction at the carbon adjacent to the oxygen s radical center, leads to the formation of an aldehyde. 

Hydroxyl radical reacts with VOCs via either an H-atom abstraction or radical-addition mechanism (the latter is possible if the VOC is an alkene). The nascent... 

The initiator-derived radical products generate a-tocopheroxyl radicals (2) from a-tocopherol (1). The radicals 2 are further oxidized to ort/io-quinone methide 3 in a formal H-atom abstraction, thereby converting benzoyloxy radicals to benzoic acid and phenyl radicals to benzene. The generated o-QM 3 adds benzoic acid in a [ 1,4] -addition process, whereas it cannot add benzene in such a fashion. This pathway accounts for the observed occurrence of benzoate 11 and simultaneous absence of a 5 a-phenyl derivative and readily explains the observed products without having to involve the hypothetical C-centered radical 10. 

An interesting body of work merits the reader s attention concerning the Eley-Rideal reaction of H-atom abstraction of chlorine atoms adsorbed on gold38-41 ... 

Peeters, J., Vandenberk, S., Piessens, E, Pultan, V. (1999) H-atom abstraction in reactions of cyclic polyalkenes with OH. Chemosphere 38, 1189-1193. 

The IPM parameters for hydrogen transfer atom in alkoxyl radicals are presented in Table 6.12. Isomerization proceeds via the formation of a six-membered activated complex, and the activation energy for the thermally neutral isomerization of alkoxyl radicals is equal to 53.4 kJ mol-1. These parameters were used for the calculation of the activation energies for isomerization of several alkoxyl radicals via Eqns. (6.7, 6.8, 6.12) (see Table 6.14). The activation energies for the bimolecular reaction of hydrogen atom (H-atom) abstraction by the alkoxyl radical and intramolecular isomerization are virtually the same. 

Enthalpies, Activation Energies, and Rate Constants of Intramolecular H-Atom Abstraction in Alkoxyl Radicals Calculated by the I PM Method [36]... 

The experimental data on free radical isomerization reactions with H-atom abstraction were analyzed by the IPM method [36]. The kinetic parameters of isomerization were compared with those of the intermolecular H-atom abstraction (see Chapter 2). The results... 

The intramolecular hydrogen atom transfer occurs with lower activation energies in comparison with the intermolecular transfer (see the values of Ee for both types of reactions in Table 6.11). The values of the activation energies of intramolecular radical H-atom abstraction calculated by the IPM method are given in Table 6.15. 

H" atom can both abstract hydrogen atoms and add to the double bonds. However it was found that the predominant reaction is the addition to the double bond. From the absorption of the cyclohexadienyl radical (formed by H abstraction) in acidic solution containing t-butanol (to scavenge the OH radicals) it was concluded14 that only 22% and 7% of the H atoms abstract hydrogen from 1,4- and 1,3-cyclohexadiene, respectively. 

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