Ethylene dissociative mechanism

Crabtree looked at vinylic C-H activation using the unsubstituted complex trispyrazolylborate-bis-ethyleneiridium(I). Irradiation leads to the formation of the vinyl hydride complex by a mechanism that involves initial ethylene dissociation. Evidence for this pathway stems from the observed inhibition by added ethylene (Eq. 37) [127]. 

From the results we concluded that the dissociative mechanism originally suggested for the ethylene-deuterium exchange is valid and is responsible for the exchange involving other unsaturated, saturated and aromatic hydrocarbons and oxygenated compounds. 

Mechanism 6.11 shows the steps in the free-radical polymerization of ethylene. Dissociation of a peroxide initiates the process in step 1. The resulting peroxy radical adds to the carbon-carbon double bond in step 2, giving a new radical, which then adds to a second molecule of ethylene in step 3. The carbon-carbon bond-forming process in step 3 can be repeated thousands of times to give long carbon chains. 

A dissociative mechanism is proposed for the aquation of // a -[Cr(en)2Cl2] ion from studies in aqueous-organic solvent mixtures (0—50% MeOH, ethylene glycol, glycerol, acetone, and dioxan). Plots of In A with e ( = dielectric constant of solvent) or the Grunwald-Winstein T-values are linear, and comparison with the analogous Co systems indicates far less charge separation in the transition state. 

The hydrogenation of ethylene is catalyzed by many metals orders of the reaction in ethylene and hydrogen are given in Table 2.2. We assume the following reaction mechanism, based on molecular adsorption of ethylene, dissociative adsorption of hydrogen, and the stepwise addition of hydrogen atoms to ethylene on the surface of the catalyst... 

Although collision-stabilized reaction complexes take part in chain propagation, the complex spectra of ions observed for ethylene and acetylene suggest that this mechanism undoubtedly must compete with consecutive reactions of species produced by unimolecular dissociation of the complexes and by collisional dissociation of other ions. ... 

From this discussion one would expect a clear distinction between ionic and nonionic types of precursors. Two complications exist. The first is caused by the possibility that the same product or intermediate may be produced by a mechanism involving ionic species and by one involving nonionic ones. For example, there can be little doubt that hydrogen may be produced from ethylene by dissociation of both an excited state and an excited ion,... 

Another possibility is that carbene species are generated via the dissociative adsorption of ethylene onto two adjacent chromium sites [71]. A second ethylene molecule then forms an alkyl chain bridge between the two chromium sites this can subsequently propagate via either the Cossee or the Green-Rooney mechanism. 

Block copolymerization was carried out in the bulk polymerization of St using 18 as the polymeric iniferter. The block copolymer was isolated with 63-72 % yield by solvent extraction. In contrast with the polymerization of MMA with 6, the St polymerization with 18 as the polymeric iniferter does not proceed via the livingradical polymerization mechanism,because the co-chain end of the block copolymer 19 in Eq. (22) has the penta-substituted ethane structure, of which the C-C bond will dissociate less frequently than the C-C bond of hexa-substituted ethanes, e.g., the co-chain end of 18. This result agrees with the fact that the polymerization of St with 6 does not proceed through a living radical polymerization mechanism. Therefore, 18 is suitably used for the block copolymerization of 1,1-diubstituted ethylenes such as methacrylonitrile and alkyl methacrylates [83]. 

Fahey (16) suggests that intermediate 3 dissociates formaldehyde he finds supportive evidence in the rhodium-based system by observation of minor yields of 1,3-dioxolane, the ethylene glycol trapped acetal of formaldehyde. For reasons to be discussed later, we believe the formation of free formaldehyde is not on the principal reaction pathway. (c) We have also rejected two aspects of the reaction mechanism proposed by Keim, Berger, and Schlupp (15a) (i) the production of formates via alcoholysis of a formyl-cobalt bond, and (ii) the production of ethylene glycol via the cooperation of two cobalt centers. Neither of these proposals accords with the observed kinetic orders and the time invariant ratios of primary products. 

Correlations between surface species and emitted secondary ions are based on characterization of the surface adlayer by adsorption and thermal desorption measurements. It is shown that the secondary ion ratios RuC+/Ru+ and R CTVRuJ can be quantitatively related to the amount of nondesorbable surface carbon formed by the dissociative adsorption of ethylene. In addition, emitted hydrocarbon-containing secondary ions can be directly related to hydrocarbon species on the surface, thus allowing a relatively detailed analysis of the hydrocarbon species present. The latter results are consistent with ejection mechanisms involving intact emission and simple fragmentation of parent hydrocarbon species. 

In any case, such chain transfer processes lead to a continuous increase of the molecular weight of the alkylaluminum compounds during the polymerization. It is, however, possible that alkylaluminum molecules having a low molecular weight are regenerated by a mechanism similar to those reported in the study of the kinetic behavior of ethylene polymerization, in the presence of trialkylaluminum 36) or through a dissociation to a hydride ... 

He concludes that the first (associative) mechanism gives values nearest the observed heat of adsorption determined by Beeck (30), and is therefore accepted as nearest the truth (34) (Qo (calculated) = 42 kcal./ mole Qo (observed) = 58 kcal./mole). Experiments on tungsten and nickel films (Beeck (35), Trapnell (36), and more recent work in Rideal s laboratory) have shown that when ethylene is added to a clean metal surface ethane appears in the gas phase. A self hydrogenation mechanism must be operative and at least in these cases dissociation of ethylene must occur on the catalyst. It is suggested that the calculations might be complicated by the energy of bond strain in the adsorption of an ethylene molecule to the fixed lattice distances of the metal. 

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