1,5-H-abstraction

With sulfoxides, OH reacts mainly by addition to the S-0 double bond [DMSO reaction (14) 92% k = 7 x 109 dm3 mol-1 s-1]. The resulting adduct has not been detected, because it decomposes very rapidly by 3-fragmentation [reaction (15) Dixon et al. 1964 Norman and Gilbert 1967 Veltwisch et al. 1980], 

The HO-H bond dissociation energy (BDE) is 499 kj mol-1, while the C-H bonds in saturated hydrocarbons are much weaker (BDE = 376-410 kj mol-1 Berkowitz et al. 1994 for a compilation, see Chap. 6). Thus, there is a considerable driving force for H-abstraction reactions by -OH. On the other hand, vinylic hydrogens are relatively tightly bound, and an addition to the C-C double bond is always favored over an H-abstraction of vinylic or aromatic hydrogens. Hence, in the case of ethene, no vinylic radicals are formed (Soylemez and von Sonntag 1980), and with benzene and its derivatives the formation of phenyl-type radicals has never been conclusively established. 

Despite the considerable driving force for the H-abstraction reaction, there is some remarkable selectivity. Primary hydrogens (-CH3) are less likely abstracted than secondary (-CH2 ) and tertiary (-CH-) ones (Asmus et al. 1973). In addition, neighboring substituents that can stabilize the resulting radical by elec- 

Dihydrouracil, an isomer of glycine anhydride, has two kinds of carbon-bound hydrogen atoms. Those activated by the neighboring NH-group react much more readily (90%) than those next to the carbonyl function (ca. 5% Schuch-mann et al. 1984 for details, see Chap. 10). Thus, a high regioselectivity is again observed. 

In the case of amines, protonation that withdraws electron density from the center of reaction lowers the rate of reaction by a factor of 30 (Das and von Sonntag 1986). Besides H-abstraction from carbon [reactions (18) and (21)], the formation of N-centered radical cations is observed [reactions (19)/(22) and (20) for amino acids see, e.g Bonifacic et al. 1998 Hobel and von Sonntag 1998]. Reaction (20) is also an H-abstraction reaction. The ET reaction (19)/(22) may proceed via a (bona-fide, very short-lived) adduct (Chap. 7). 

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Rate of secondary H abstraction 72 6 39 Rate of primary H abstraction 28 4 1... 

The active site on the surface of selective propylene ammoxidation catalyst contains three critical functionalities associated with the specific metal components of the catalyst (37—39) an a-H abstraction component such as Sb ", or Te" " an olefin chemisorption and oxygen or nitrogen insertion component such as Mo " or and a redox couple such as Fe " /Fe " or Ce " /Ce" " to enhance transfer of lattice oxygen between the bulk and surface... 

In H abstraction, a hydrogen radical reacts with a molecule (primarily a paraffin) and produces a hydrogen molecule and a radical. In the same way, a methyl radical reacts to produce a radical and methane. Similar reactions with other radicals (ethyl and propyl) can also occur. In addition, some radicals like H, CH, etc, are added to olefins to form heavier radicals. 

Saturated large rings may form nitrogen radicals by H abstraction from N, or abstraction may occur in the a- or /3-positions in nonnitrogen systems. Oxepane gives the radical in the 2-position, with subsequent cleavage and reclosure of the intermediate carbenoid to cyclohexanol (Section 5.17.2.1.5). In unsaturated large systems a variety of reactions, unexceptional in their nature, are found. Some azepines can be brominated by A -bromosuc-cinimide others decompose under similar conditions (Section 5.16.3.7). 

Grafting reactions onto a polymer backbone with a polymeric initiator have recently been reported by Hazer [56-60]. Active polystyrene [56], active polymethyl methacrylate [57], or macroazoinitiator [58,59] was mixed with a biopolyester polyhydroxynonanaate [60] (PHN) or polybutadiene to be carried out by thermal grafting reactions. The grafting reactions of PHN with polymer radicals may proceed by H-abstraction from the tertier carbon atom in the same manner as free radical modification reactions of polypropylene or polyhy-droxybutyratevalerate [61,62]. 

Hurley and Testa (Ref 17) exposed nitrobenzene in isopropyl alcohol, degassed and in air, to a mercury lamp at 3660A Products in the absence of air were acetone and phenyl-hydroxylamine (PHA). In air PHA was oxidized to nitro sob enzene which couples with PHA to form azoxybenzene. They hypothesized that the triplet molecule abstracted H-atoms from the solvent no effect was noted with ben zene as solvent. They also worked with nitrobenzene in isopropyl alcohol-water mixts containing HC1 with a mercury lamp at 3660A (Ref 18), and found that the quantum yields depended on pH and isopropyl alcohol content, but were independent of oxygen with acid present. Their conclusion was that the quantum yield consisted of two parts, H abstraction by the triplet, and protonation of the triplet... 

The formation of dimethyl sulfide, dimethyl sulfone, and methane (by H-abstraction) observed in these photolyses is thus accounted for. Hydrogen abstraction by the methylsulfinyl radical affords methanesulfenic acid, CH3SOH, a very reactive molecule, which rapidly undergoes a series of secondary reactions to produce the methanesulfonic acid, methyl methanethiolsulfonate (CH3S02SCH3), and dimethyl disulfide which were also observed during these photolyses. 

Thus we think of the chemical ionization of paraffins as involving a randomly located electrophilic attack of the reactant ion on the paraffin molecule, which is then followed by an essentially localized reaction. The reactions can involve either the C-H electrons or the C-C electrons. In the former case an H- ion is abstracted (Reactions 6 and 7, for example), and in the latter a kind of alkyl ion displacement (Reactions 8 and 9) occurs. However, the H abstraction reaction produces an ion oi m/e = MW — 1 regardless of the carbon atom from which the abstraction occurs, but the alkyl ion displacement reaction will give fragment alkyl ions of different m /e values. Thus the much larger intensity of the MW — 1 alkyl ion is explained. From the relative intensities of the MW — 1 ion (about 32%) and the sum of the intensities of the smaller fragment ions (about 68%), we must conclude that the attacking ion effects C-C bond fission about twice as often as C-H fission. 

Thus, the process of hydride ion abstraction from a primary position is approximately thermoneutral, and hence we must conclude that it is an energetically allowed process, although possibly with a relatively small reaction rate. A process competing with primary H abstraction (Reaction 13) is methide ion abstraction (Reaction 11, loss of CH4 from the... 

MW + 1 ion), and this reaction is 35-40 kcal./mole exothermic. We might expect that the rate for this reaction would be appreciably greater than the rate for primary H abstraction, and we consequently postulate that primary H abstraction to yield MW — 1 ions will not occur to any significant extent. The other process we must consider is a simple extension of Reaction 12—namely, beta fission at a branch point. However, we now wish to consider the case where the branch point is a quaternary carbon. As a typical example ... 

Aya Ayabe, Y., Matsuda, H. Abstracts of the 16th Meeting of the Polarographic Society of Japan, 1970, p. 51. 

Reaction (31) shows an example of hydrosilylation of ketones, i.e., reduction of 4-ferf-butyl-cyclohexanone affordir mainly the tmns isomer, indicating that the axial H-abstraction is favored7... 

The reaction has been extended to ketones, carboxylic acids and esters (all of which couple a to the C=0 group), and amides (which couple a to the nitrogen) by running it in the presence of H2. ° Under these eonditions it is likely that the excited Hg abstracts H from H2, and that the remaining H- abstracts H from the substrate. 

The argument of the directing effect of lone pairs on the substiment [92] easily extends to the alkyl cases. The orbital interaction (Scheme 20) [103] in the pere-poxide quasi-intermediate suggests the stabilization occurs by the simultaneous interaction of O with two allylic hydrogens on the same side of the alkene. Photooxygenation of trisubstituted olefins revealed a strong preference for H-abstraction from disubstituted side of the double bond [104, 105],... 

The partial oxidation of propylene occurs via a similar mechanism, although the surface structure of the bismuth-molybdenum oxide is much more complicated than in Fig. 9.17. As Fig. 9.18 shows, crystallographically different oxygen atoms play different roles. Bridging O atoms between Bi and Mo are believed to be responsible for C-H activation and H abstraction from the methyl group, after which the propylene adsorbs in the form of an allyl group (H2C=CH-CH2). This is most likely the rate-determining step of the mechanism. Terminal O atoms bound to Mo are considered to be those that insert in the hydrocarbon. Sites located on bismuth activate and dissociate the O2 which fills the vacancies left in the coordination of molybdenum after acrolein desorption. 

Selective oxidation and ammoxldatlon of propylene over bismuth molybdate catalysts occur by a redox mechanism whereby lattice oxygen (or Isoelectronlc NH) Is Inserted Into an allyllc Intermediate, formed via or-H abstraction from the olefin. The resulting anion vacancies are eventually filled by lattice oxygen which originates from gaseous oxygen dlssoclatlvely chemisorbed at surface sites which are spatially and structurally distinct from the sites of olefin oxidation. Mechanistic details about the... 

Lyons and coworkers studied the ESR spectra of bakelite polysulfone [—CgH4— O—CgH4—SO2—CgH4—O—CgH4—C(CH3)2—] y-irradiated at 77 K and found features characteristic of at least four radicals, the cyclohexadienyl radical, formed from addition to the aromatic ring, methylene groups (— CH2) formed from H abstraction from the methyl group, phenoxy radicals and peroxy radicals. 

Dissociation of the gases SiH4 and H2 by electron impact will create reactive species (radicals) and/or neutrals (Si2H6 and even higher-order silanes [195-198]). Atomic hydrogen is an important particle because it is formed in nearly all electron impact collisions, and the H-abstraction reaction [199, 200] of (di)silane is an important process, as is seen from sensitivity study. Dissociation of SiHa can create different SiH (with x = 0, 1,2, 3) radicals. Only silylene (SiH2) and... 

Figure 7.26. Photo-induced hydrogen abstraction from the y-carbon leads to biradical 72, which can (a) revert to the starting ketone, (b) cyclize, or (c) cleave the 2,3-CC bond. The structure for y-H abstraction for the starting ketone is also shown and the ideal parameters defined and listed.

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