Hydrogenation, general mechanisms

Degradation of polyolefins such as polyethylene, polypropylene, polybutylene, and polybutadiene promoted by metals and other oxidants occurs via an oxidation and a photo-oxidative mechanism, the two being difficult to separate in environmental degradation. The general mechanism common to all these reactions is that shown in equation 9. The reactant radical may be produced by any suitable mechanism from the interaction of air or oxygen with polyolefins (42) to form peroxides, which are subsequentiy decomposed by ultraviolet radiation. These reaction intermediates abstract more hydrogen atoms from the polymer backbone, which is ultimately converted into a polymer with ketone functionahties and degraded by the Norrish mechanisms (eq. 

The hybridic nature of the Si—H bond is utili2ed to generate C—H bonds by ionic hydrogenation according to the foUowiag general mechanism, ia which a hydride is transferred to a carbocation. 

It is not difficult to incorporate this result into the general mechanism for hydrogen halide additions. These products are formed as the result of solvent competing with halide ion as the nucleophilic component in the addition. Solvent addition can occur via a concerted mechanism or by capture of a carbocation intermediate. Addition of a halide salt increases the likelihood of capture of a carbocation intermediate by halide ion. The effect of added halide salt can be detected kinetically. For example, the presence of tetramethylammonium... 

The general mechanism for electrophilic substitution suggests that groups other than hydrogen could be displaced, provided the electrophile attacked at the substituted carbon. Substitution at a site already having a substituent is called ipso substitution and has been observed in a number of circumstances. The ease of removal of a substituent depends on its ability to accommodate a positive charge. This fector determines whether the newly attached electrophile or the substituent is eliminated from the [Pg.588]

MMA onto cellulose was carried out by Hecker de Carvalho and Alfred using ammonium and potassium persulfates as radical initiators [30]. Radical initiators such as H2O2, BPO dicumylperoxide, TBHP, etc. have also been used successfully for grafting vinyl monomers onto hydrocarbon backbones, such as polypropylene and polyethylene. The general mechanism seems to be that when the polymer is exposed to vinyl monomers in the presence of peroxide under conditions that permit decomposition of the peroxide to free radicals, the monomer becomes attached to the backbone of the polymer and pendant chains of vinyl monomers are grown on the active sites. The basic mechanism involves abstraction of a hydrogen from the polymer to form a free radical to which monomer adds ... 

Despite the wide diversity of enzyme structures, most enzyme activity follows a general mechanism that has several reversible steps. In the first step, a reactant molecule known as a substrate (S) binds to a specific location on the enzyme (E), usually a groove or a pocket on the surface of the protein E + S ES The substrate binds to the active site through intermolecular interactions that usually include significant amounts of hydrogen bonding. 

Along with Cu ", MnO and Ag" , the two oxidation states of mercury are reduced by molecular hydrogen -Halpern " considers that hydrogen is oxidised by two general mechanisms corresponding to rate equations of the type... 

Typical non-enolising aldehydes are formaldehyde and benzaldehyde, which are oxidised by Co(III) Ce(IV) perchlorate and sulphate , and Mn(III) . The main kinetic features and the primary kinetic isotope effects are the same as for the analogous cyclohexanol oxidations (section 4.3.5) and it is highly probable that the same general mechanism operates. kif olko20 for Co(III) oxidation of formaldehyde is 1.81 (ref. 141), a value in agreement with the observed acid-retardation, i.e. not in accordance with abstraction of a hydroxylic hydrogen atom from H2C(OH)2-The V(V) perchlorate oxidations of formaldehyde and chloral hydrate display an unusual rate expression, viz. 

Catecholborane and pinacolborane are especially useful in hydroborations catalyzed by transition metals.163 Wilkinson s catalyst Rh(PPh3)3Cl is among those used frequently.164 The general mechanism for catalysis is believed to be similar to that for homogeneous hydrogenation and involves oxidative addition of the borane to the metal, generating a metal hydride.165... 

Van der Meerakker [28] gives a general mechanism for electroless processes involving various reductants. The mechanism assumes the formation of adsorbed hydrogen atoms, which can then be oxidized or desorbed as H2 gas. In the case of Co(II) reduction by hypophosphite, the relevant mechanism is as follows ... 

Fig. 5. General mechanism for 5 -deoxyadenosylcorrinoid catalyzed hydrogen transfer...

This overview is organized into several major sections. The first is a description of the cluster source, reactor, and the general mechanisms used to describe the reaction kinetics that will be studied. The next two sections describe the relatively simple reactions of hydrogen, nitrogen, methane, carbon monoxide, and oxygen reactions with a variety of metal clusters, followed by the more complicated dehydrogenation reactions of hydrocarbons with platinum clusters. The last section develops a model to rationalize the observed chemical behavior and describes several predictions that can be made from the model. 

In Scheme 1 is represented an idealized picture of the two possibilities for the hydrogenation of alkenes by metal complexes not containing an M—11 bond. One possibility involves initial coordination of the alkene followed by activation of H2 (alkene route). The other (more general) possibility is the hydride route, which involves initial reaction with H2 followed by coordination of the alkene. The second general mechanism, usually adopted by catalysts containing an M—H bond, is shown in Scheme 2. 

Another example is the influence of ultrasonic sound treatment. In chlorinated or bromi-nated solvents it leads to extreme rate accelerations and higher selectivities (Table 6)84. This observation was explained by the formation of hydrogen halide from the sonolysis of the solvent molecules, followed by protonation of the dienophiles and ordinary acid catalysis. Nevertheless, although there are quite a few aspects of the Diels-Alder reaction which are not totally understood, the general mechanisms leading to selectivities and catalysis are clear. 

Commonly, ortho elimination refers to a hydrogen transfer via a six-membered transition state at ortto-disubstituted aromatic compounds. In practice, the reacting entities are almost in position to form this six-membered transition state. The general mechanism of the ortho elimination is as follows ... 

In order to explain the formation of nortricyclene from 2-ea o-norbornanol, it is necessary to assume a back side attack at the hydrogen attached to carbon 6. The general mechanism here is similar to the trans elimination reaction as discussed under menthol, 1,4-cyclohexanediol, and bornanols. 

Odom JM, Peck HD Jr. 1981. Hydrogen cycling as a general mechanism for energy coupling in the sulfate-reducing bacteria, Desulfovibrio sp. FEMS Microbiol Lett 12 47-50. 

The general equation for the hydrogen separation mechanism may be formulated as follows (Wachter, 62) ... 

Taking into account all these considerations, the following general mechanism may be operating, when the hydrogenation of a Z geometric isomer is carried out in... 

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