A-hydrogen abstraction

Two other important commercial uses of initiators are in polymer cross-linking and polymer degradation. In a cross-linking reaction, atom abstraction, usually a hydrogen abstraction, occurs, followed by termination by coupling of two polymer radicals to form a covalent cross-link ... 

Osmium carbonyl (Os3(CO)i2) acts as a photoinitiator of vinyl polymerization [20], which can function without a halide additive. The mechanism of photoinitiation is by a hydrogen abstraction from monomer to pho-... 

The cancellation in GP effects in the state-to-state DCS are found [20-22, 26, 27, 29] at low impact parameters, when F(J) in Eq. (15) is chosen to include only contributions for which / < 9. It is well known [55,56] that most of the reactive scattering in this regime consists of head-on collisions, in which the reaction proceeds mainly by the H atom striking the H2 diatom at geometries that are close to linear. Most of the products are then formed by direct recoil in the backward (9 = 180°) region, this being typical behavior for a hydrogen-abstraction reaction. 

W.L. Gladfelter, University of Minnesota At this time, can you make any generalizations which would allow you to predict when you will see a-hydrogen abstraction versus B-hydro-gen abstraction ... 

Imido alkylidene complexes were first prepared by a reaction analogous to that shown In equation 6. Recently they have been prepared from imido alkyl complexes by well-behaved a-hydrogen abstraction reactions (16) Imido neopentylidene complexes seem to be more stable than oxo neopentylidene complexes, possibly because the oxo ligand is sterically more accessible to Lewis acids, including another tungsten center. 

In the presence of a proper second molecule bimolecular photochemical processes occur. Obviously such reactions can also occur in an intramolecular fashion in bifunctional molecules containing both reactive centres. These reactions are a) hydrogen abstraction by the excited molecule if the second molecule (or reactive centre) is a hydrogen donor RH (1.14) b) photodimerisation (1.15) c) photoaddition or photocycloaddition (1.16) d) electron transfer (1.17), if no bonding takes place between the reactants (or reactive centres). ... 

It has been shown that abstraction of an a or p hydrogen from the ozonide also can occur. In fact, a-hydrogen abstraction can occur up to 2.5 times faster than Criegee fragmentation [36], As an example, the proposed mechanism for the reaction of ds-2-butene with ozone is shown in Figure 4. 

Mo and W alkylidene complexes usually are prepared from M(VI) dialkyl complexes by some variant of the a hydrogen abstraction reaction (Eq. 7 other ligands omitted) [5,41]. 

To explore the mechanism of allylic hydroxylation, three probe substrates, 3,3,6,6-tetradeuterocyclohexene, methylene cyclohexane, and /l-pinenc, were studied (113). Each substrate yielded a mixture of two allylic alcohols formed as a consequence of either retention or rearrangement of the double bond. The observation of a significant deuterium isotope effect (4-5) in the oxidation of 3,3,6,6-tetradeuterocyclohexene together with the formation of a mixture of un-rearranged and rearranged allylic alcohols from all three substrates is most consistent with a hydrogen abstraction-oxygen rebound mechanism (Fig. 4.48). 

Moro-Oka et al. (1976) have reported that the oxidation of 9,10-dihydroanthracene by K02 solubilized in DMSO by 18-crown-6 gives mainly the dehydrogenated product, anthracene. Under the same conditions, 1,4-hexadiene is dehydrogenated to benzene. The authors proposed a mechanism in which the superoxide ion acts as a hydrogen-abstracting agent only. The oxidations of anthrone (to anthraquinone), fluorene (to fluorenone), xanthene (to xanthone) and diphenylmethane (to benzophenone) are also initiated by hydrogen abstraction. 

Allylic intermediate, 27 185-187 ii-Allylic nickel intermediates, 33 15-18, 22 Allylic oxidation, see Oxidation, allylic Allylic species, 30 21 formation, isomerization, 30 18-19 free allyl radicals, 30 149 a-hydrogen abstraction, 30 147 Allyl methacrylate oxidation, 41 305 Allyls... 

The a-hydrogen abstraction leads to an a ketoperoxide, which has been tentatively identified by Pitts in the case of cM-2-butene. Further reactions of the ketoperoxide include the formation of the normal products, i.e., the carbonyl products that can also be explained by the Criegee mechanism. The /3-hydrogen abstraction accounts for the ob-... 

C—H bond is sufficiently weak, the hydrogen can be abstracted by O2, forming a product ion, (M-H) as shown in Figure 6 and Eq. (2) [23]. Thus, the transition between an adduct ion to a hydrogen abstracted ion is controlled by the acidity of the hydrogen and can be observed for other anions such as Cl ... 

The order of activity per unit surface area was equal to that in the case of selfcondensation of acetone and in agreement with the order of basicity of the solids, namely, SrO > CaO > MgO. However, the authors found that the rate-determining step for aldol condensation of n-butyraldehyde is the a-hydrogen abstraction by the active sites, which are the surface ions. The differences in rate-determining step and active sites in the condensation of butyraldehyde and aldol condensation of the acetone were attributed to differences in acidity of the a-hydrogen in the two molecules. CaO was slightly more active than MgO at 273 K after a reaction time of 1 h, maximum conversions of 41% were observed with selectivities to 2-ethyl-3-hydroxy-hexanal and to the corresponding Tishchenko reaction product (2-ethyl-3-hydroxy- -hexyl butyrate) of 39.8 and 56.9%, respectively. 

In this reaction system traces of ethane formed by the termination and probably traces of formaldehyde (formed by a hydrogen abstraction by formyl) are also formed, but their concentrations are always much less than those of CH4 and CO. 

Hofmann-Lofjler-Freytag Reaction It is a photochemically initiated decomposition of N-haloamines in acidic solution. First d-halo-amines are formed, these are then converted into pyrrolidines by intramolecular nucleophilic substitution, which involves a hydrogen abstraction. 

The temperature dependence is that expected for a hydrogen abstraction i.e., the rate constant increases with temperature (left side of Fig. 6.11). 

The primary reaction of Type 2 photoinitiators is a hydrogen abstraction from the tertiary amine by a triplet excited ketone. The amino radical thus formed is sufficiently active to initiate the polymerization of vinyl monomers Scheme 2. 

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