Ethylene formation mechanism

Organometallic radicals are important intermediates in biological and catalytic reactions. The structure and formation mechanism of radicals trapped in y-irradiated molecular sieves exposed to methanol and ethylene have been studied by EPR spectroscopy. It was found that Ag CH2OH+ radical with one-electron bond between Ag and C is formed by the attack of -CH2OH hydroxymethyl radical on Ag+ cation. 

The predominant formation of ethylene in the early stages of the reduction of Cp2Fe2(C0K with LAH raises the possibility that ethylene is formed by an alternate mechanism. An especially attractive mechanism for ethylene formation in this system is shown in Scheme 4. 

A new mechanism, called the methane-formaldehyde mechanism, has been put forward for the transformation of the equilibrium mixture of methanol and dimethyl ether, that is, for the formation of the first C-C bond.643 This, actually, is a modification of the carbocation mechanism that suggested the formation of ethanol by methanol attaching to the incipient carbocation CH3+ from surface methoxy.460,462 This mechanism (Scheme 3.3) is consistent with experimental observations and indicates that methane is not a byproduct and ethanol is the initial product in the first C-C bond formation. Trimethyloxonium ion, proposed to be an intermediate in the formation of ethyl methyl ether,447 was proposed to be excluded as an intermediate for the C-C bond formation.641 The suggested role of impurities in methanol as the reason for ethylene formation is highly speculative and unsubstantiated. 

Ethylene is rather inert, but it is metabolized slowly, some of it to ethylene glycol.326 Plants store N-malonyl-ACC as a metabolically inert pool. Excess ACC can be deaminated in a PLP-dependent reaction to 2-oxobutyrate (step k, Fig. 24-16), a process that also occurs in bacteria able to subsist on ACC.327/327a There may also be other mechanisms for ethylene formation, e.g., peroxidation of lipids during scenescence of leaves.328 See also Chapter 31, Section G. 

Ethylene formation. Propose a mechanism for the conversion of -adenosylmethionine into 1-aminocyclopropane-l-carboxylate (ACC) by ACC synthase, a PLP enzyme. What is the other product ... 

Ethylene formation is fastidiously controlled in plants (as are other hormone productions) and the oxidative conversion of sp -hybridized C(3) and C(4) of methionine to sp -hybridized ethylene (139) via the (now you see it, now you don t) three-membered ring mechanism is a pretty act of metabolic functional group conjuring. 

Steps (1), (3), (4), (5), (6), (7), and (9) written down for Mechanism I are relevant. The rate of hydrogen adsorption will be proportional to the hydrogen pressure, as will the rate of formation of pairs of vinyls. In the absence of hydrogen desorption, hydrogen atoms liberated by vinyl reversal [step (5)] must either combine with acetylene (which, as an overall reaction, is facile), or else combine with vinyl, which is the envisaged mechanism of step (7). Thus, if vinyl disproportion is the important route for ethylene formation then, equating again the rates of formation and removal of vinyl. 

The reaction of ethanol with ammonia on zeolite catalysts leads to ethylamine. If, however, the reaction is carried out in the presence of oxygen, then pyridine is formed [53]. MFI type catalysts H-ZSM-5 and B-MFI are particularly suitable for this purpose. Thus, a mixture of ethanol, NH3, H2O and O2 (molar ratio 3 1 6 9) reacts on B-MFI at 330 °C and WHSV 0.17 h 1 to yield pyridine with 48 % selectivity at 24 % conversion. At 360 °C the conversion is 81% but there is increased ethylene formation at the expense of pyridine. Further by-products include diethyl ether, acetaldehyde, ethylamine, picolines, acetonitrile and CO2. When applying H-mordenite, HY or silica-alumina under similar conditions pyridine yields are very low and ethylene is the main product. The one-dimensional zeolite H-Nu-10 (TON) turned out to be another pyridine-forming catalyst 54]. A mechanism starting with partial oxidation of ethanol to acetaldehyde followed by aldolization, reaction with ammonia, cyclization and aromatization can be envisaged. An intriguing question is why pyridine is the main product and not methylpyridines (picolines). It has been suggested in this connection that zeolite radical sites induced Ci-species formation. 

These results show that, together with the typical carbocation cracking, a radical cracking mechanism is also taking place, especially on highly steam dealuminated zeolites. This radical mechanism, which will become proportionally more important when the framework aluminium content of the zeolite will decrease, could explain the strong increase in ethylene formation and decrease in branched/unbranched C4 ratio when decreasing the unit cell size of the zeolite. 

Elstner EE, Saran M, Bors W and LengfelderE (1978) Oxygen activation in isolated chloroplasts. Mechanism offerredoxin-dependent ethylene formation from methionine. Eur J Biochem 89 61-66... 

Discussion at the First European Workshop on Catalytic Methane Conversion also addressed the effect of chloride addition to catalysts. It was stated that the addition of halides significantly enhanced Cj selectivities and that the enhancement could be observed by cofeeding chlorine or methyl chloride. The mechanism of ethylene formation over chloride-containing catalysts has been described in terms of a purely gas-phase dehydrogenation of ethane. ... 

Some microorganisms in culture show methionine-dependent ethylene formation. In studies with Escherichia coli, 2-oxo-4-methylthiobutyrate (KMB) produced from methionine by transamination was suggested as the precursor of ethylene [19], and subsequently a cell-free system which produced ethylene from KMB in the presence of NAD(P)H, EDTA-Fe and oxygen was established [20]. An enzyme which catalysed a similar ethylene-forming activity was purified from Cryptococcus albidus [15]. The purified enzyme of molecular mass 62 kDa turned out to be NADH EDTA-Fe oxidoreductase. The proposed mechanism involves reduction of EDTA-Fe to EDTA-Fe by the enzyme, reduction of oxygen to superoxide by EDTA-Fe, of hydrogen peroxide to hydroxyl radical, and oxidation of KMB by hydroxyl radical to ethylene. However, an extensive physiological evaluation of this enzyme must be done before it can... 

Domain Structure and Domain Formation Mechanism of ABA and AB Block Copolymers of Ethylene Oxide and Isoprene... 

The domain structure and crystalline texture of AB and ABA type block copolymers of ethylene oxide (EO) and isoprene (Is) are studied, and the effects of the casting solvents and the fractional compositions of each block segment are determined. The domain structures of EO-Is copolymers are essentially identical to those of EO-Is-EO copolymers, but they strongly depend on the fractional compositions and the casting solvents. The role of casting solvent in the different domain formation mechanisms is interpreted in terms of an interrelation of two binodal surfaces that represent the critical concentration for crystallization of the EO segment and the critical concentration for micelle formation of the incompatible EO and Is segments. 

Proton abstraction is a common ion formation mechanism in the negative ion (NI) FD-MS mode. It is not often reported in the literature because little NI-FD-MS work is done. (Field electron emission has to be contended with in NI-FD-MS.) Polar organics in the NI mode will often show (M - H) ions, but polymers to date have only been analyzed in the positive ion mode. It is interesting to note tiiat mixtures of poly(ethylene glycol) (PEG) and water are typically used as a viscous solvent in NI-FD-MS studies of organic molecules. In this case, however, no NI-FD signals are observed from the PEG. 

Several gas-phase diagnostic studies have been performed especially on the hot-filament CVD process in an attempt to analyze the spatial distribution of stable and reactive species in the region between filament and substrate and to obtain information about the decisive steps in the diamond formation mechanism. One of the first quantitative investigations of this kind was performed by Celii et al. using tunable infrared-laser detection of methane, acetylene, ethylene and methyl [54,551. Aside from a considerable conversion of methane (starting concentration 8 X 10 " cm ) to acetylene (2 x lO cm" ) near the hot filament (tungsten, 2200°C), methyl radicals were found with a concentration of 2 x lO cm". ... 

As to the formation mechanism of carbon black, there are indications that the carbon particles are formed by recombination of smaller hydrocarbons (acetylene, ethylene and their radicals as well as aromatic cracking products). 

Wreford and his coworkers [94] have reported that (n-C4H5)2Ti-dmpe (dmpe = l,2-bis(dimethylphosphino)ethane) catalyzes the dimerization of ethylene. A mechanism involving formation of a metallacyclopentane complex is proposed ... 

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