Possible reaction mechanisms

We have already noted (p. 16) that the interaction of an olefin with hydrogen or deuterium may lead to the occurrence of any of a number of processes. There is much evidence to suggest that each of these processes may be accounted for by considering a number of elementary steps in which a hydrogen atom, from a meantime unspecified source, is added to or removed from an adsorbed hydrocarbon species. 

It has been observed [23,91,92] that when an unsaturated hydrocarbon is reacted with (a) equilibrated and (b) non-equilibrated hydrogen—deuterium mixtures, the deuteroalkane distributions are identical. Such observations indicate that the direct addition of a hydrogen molecule across the olefinic bond does not occur, and provides strong evidence for the formation of a half-hydrogenated state , that is, an adsorbed alkyl radical, first suggested by Horiuti and Polanyi [81], as a relatively stable reaction intermediate. The process of hydrogenation may thus be represented as 

Under the conditions generally used for olefin hydrogenation, the alkane, once formed, is unreactive [93] and its readsorption on the surface can be neglected. 

The processes of olefin exchange, double bond migration and cis—trans isomerisation, observed to occur concomitant with hydrogenation, may be accounted for by considering that the formation of the half-hydrogenated state is reversible. For olefin exchange we can write 

Similar mechanisms may be written for double bond migration 

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VinylmetlQfleneeyelopropane rearranges in the gas phase to 3-metlQflenecyclopen-tene. Two possible reaction mechanisms are ... 

Given the above possible reaction mechanism, it is then intriguing to speculate that another approach to the same stereoselective reduction of a vinyl sulphone could be achieved by the use of a suitably sterically hindered organosilane, as outlined in equation (64). Such a reaction would provide an interesting test for the stereoelectronics of a conjugate addition reaction by a second-row heteroatom to a vinyl sulphone. 

C15-0125. At least three possible reaction mechanisms exist for the reaction of hydrogen with halogens, H2 + 2 2 H JT. Determine the rate law predicted by each of them ... 

Figure 3 Possible reaction mechanism of carboxylation of phenol via...

A possible reaction mechanism involves the initial generation of the carbene 86 via the deprotonation of SIEt HCl 85 by the Grignard reagent. The carbene 86 binds with CoCl to generate the complex 87 which is also reduced by the Grignard... 

A possible reaction mechanism shown in Scheme 7-10 includes (a) oxidative addition of the S-H bond to Pd(0), (b) insertion of the allene into the Pd-H bond to form the tt-allyl palladium 38, (c) reductive elimination of allyl sulfide, (d) oxidative addition of the I-aryl bond into the Pd(0), (e) insertion of CO into the Pd-C bond, (f) insertion of the tethered C=C into the Pd-C(O) bond, and (g) P-elimination to form 37 followed by the formation of [baseHjI and Pd(0). 

Two possible reaction mechanisms, A and B, are envisioned as shown in Scheme 7-15 and Scheme 7-16, respectively. 

SCHEME 2.21 Two possible reaction mechanisms for the initiation step of the lignin polymerization, investigated at PCM-B3LYP/6-311G(2df,p)//B3LYP/6-31G(d,p) level of theory. 

The earlier studies of aqueous solution of Cd2+ containing mercaptan were found to produce colloidal CdS by sonication [89] under nitrogen atmosphere. The H atoms produced during sonication attacked on molecules possessing a thiol moiety and lead to either H atoms extraction or H2S formation. The proportion of H2 to H2S was highly dependent on the type of mercaptan. Possible reaction mechanism was reported as follows [90],... 

Figure 5. Possible reaction mechanisms leading to the formation of protonated ammonia clusters from neutral clusters. Taken with permission from ref. 65.

In order to explain the large specific incorporation of protium into the 1 position of the spent naphthalene-d8, the possible reaction mechanisms of exchange need to be examined. Naphthalene may accept a H atom from coal to initiate the reaction as shown... 

Fig. 7.7 A possible reaction mechanism for the formation of acetic acid from CO and CH3SH at...

Glutaraldehyde is the most popular b/s-aldchydc homobifunctional crosslinker in use today. Flowever, a glance at glutaraldehyde s structure is not indicative of the complexity of its possible reaction mechanisms. Reactions with proteins and other amine-containing molecules would be expected to proceed through the formation of Schiff bases. Subsequent reduction with sodium cyanoborohydride or another suitable reductant would yield stable secondary amine... 

We might take a purist s approach and attempt to use kinetic theory to describe the dissolution and precipitation of each mineral that might appear in the calculation. Such an approach, although appealing and conceptually correct, is seldom practical. The database required to support the calculation would have to include rate laws for every possible reaction mechanism for each of perhaps hundreds of minerals. Even unstable minerals that can be neglected in equilibrium models would have to be included in the database, since they might well form in a kinetic model (see Section 26.4, Ostwald s Step Rule). If we are to allow new minerals to form, furthermore, it will be necessary to describe how quickly each mineral can nucleate on each possible substrate. 

Two molecules of carbon monoxide were successively incorporated into an epoxide in the presence of a cobalt catalyst and a phase transfer agent [29]. When styrene oxide was treated with carbon monoxide (0.1 MPa), excess methyl iodide, NaOH (0.50 M), and catalytic amounts of Co2(CO)8 and hexadecyltrimethylammonium bromide in benzene, 3-hydroxy-4-phenyl-2(5H)-furanone was produced in 65% yield (Scheme 7). A possible reaction mechanism was proposed as shown in Scheme 8 Addition of an in situ... 

Ar—N=N—Ar). A number of possible reaction mechanisms have been suggested at different times and, at this time, the position is not settled. No heavy-atom KIE work has been reported for the acid catalysed reaction, but such experiments have been carried out for the photochemical reaction37 which gives the 2-hydroxy product and which is known to be intramolecular. There is an absence of a KIE when [15N,15N ] material is used, which at least reveals that if the proposed intermediate 36 is involved, then the rate-limiting step must be its formation and not its subsequent reaction since N—O bond fission cannot be part of the slow step. 

A possible reaction mechanism based on these results is shown in Scheme 6, where Pyc plays a dual catalytic role both in the ORR (dark reaction) and the SOR (light reaction). It is noteworthy that, since the Pyc is opaque, only [Ru(bpy)3] can be used to absorb the light in the membrane. The active Pyc site is reported to be an efficient catalyst for the ORR and hence, the purging O2 is essential for the formation of H2O2 during the reaction. The control experiment in pure H2O2 gave only -47% conversion with poor selectivity (Table 4). However, the assistance of Pyc and [Ru(bpy)3] in the SOR is supported by the indirect electrochemical studies. 

Concerning the possible reaction mechanism, the activated lithium can both reduce the nickel(II) salt to give nickel(O) and react with the crystallization water present in the metallic salt to produce molecular hydrogen. However, no gas evolution was observed in the reaction, so the hydrogen initially formed was probably adsorbed on the surface of the active nickel to perform like hydrogenation without hydrogen in the sense that no external source of molecular hydrogen was employed. 

Under these circumstances any discussion of the possible reaction mechanism will be somewhat speculative. For future work, however, it may be helpful to compare the various possible pathways for the interconversion of sulfur rings ... 

Possible reaction mechanisms which explain the trends observed in product qualities are illustrated in reactions (1) - (4) below. 

For the photolysis of ferf-butyl nitrite a possible reaction mechanism (Scheme 6) consists of the production of ferf-butoxy radicals (equation 3), followed by their decomposition to give acetone and methyl radicals (equation 4). The latter are trapped by the nitric oxide liberated in the first step (equation 5). However, the absence of ethane production in the actual experiments suggested that an intramolecular formation of nitrosomethane is unlikely ". ... 

SCHEME 6. Possible reaction mechanism for the formation of nitrosomethane in a free radical reaction... 

Scheme 36 Possible reaction mechanism for the bismuth-catalyzed carbo-oxycarbonylation of alkynyl esters...

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