Ethylene formation

Ince JE, CJ Knowles (1986) Ethylene formation by cell-free extracts of Escherichia coli. Arch Microbiol 146 151-158. 

Since we do not observe any evidence for ethylene formation, we rule out reaction R7. Similarly, we mle out reaction R9, which should also produce at least some ethylene. 

This method was first reported by Winston and others (1998) and it is based on the oxidation of alpha-kclo-y-mcthiolbutyric acid (KMBA) to ethylene by peroxyl radicals produced from AAPH. The ethylene formation, which is partially inhibited... 

The suggested scheme for ethylene formation is dimerization of methylene-metal complexes ... 

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. 

In order to study the repression of ethylene formation when the ethane is cracked along with other hydrocarbons, a test is proposed in which 20 mols of ethylene is to be charged along with 100 mols of ethane. If Vr/V is to be kept the same in both cases, find how much less ethylene is formed in the recycle operation. 

In attempts to cany out cyclisation from butadiene 1,3 in the gaseous phase and under photochemical conditions R Srinivasan (J.Amer.Chem.Soc. 88, 3765 (1966) showed that irradiation yields a complex mixture of ethylene, acetylene, but-1 ene, hydrogen and polymers. The quantum yields of acetylene and ethylene formation increase with pressure. 

It was observed that the water content does not influence ethylene formation. When 5% Rh is added to alumina, the main steam reforming reaction occurs above 460 °C and the products include hydrogen, carbon dioxide, carbon monoxide and methane. 

The chief evidence for reaction (38) was that upon addition of 02 to a few millimeters pressure to the reaction mixture ethane formation was almost entirely eliminated whereas methane and ethylene formation were much less drastically reduced, the assumption being that 02 would suppress methyl reactions but not methylene reactions (see Sec. IV-C). 

Comparing the effects of alkali cations of various sizes applied in reduction of C02 in HCOJ solution with a Cu cathode, Na+, K+, and Cs+ were shown to favor the formation of hydrocarbons.138 The selectivity of ethylene formation surpasses that of methane with increasing cation size. Deactivation of the Cu cathode... 

Many investigators have actively studied the electrochemical reduction of C02 using various metal electrodes in organic solvents because these solvents dissolve much more C02 than water. With the exception of methanol, however, no hydrocarbons were obtained. The solubility of C02 in methanol is approximately 5 times that in water at ambient temperature, and 8-15 times that in water at temperatures below 0°C. Thus, studies of electrochemical reduction of C02 in methanol at —30°C have been conducted.148-150 In methanol-based electrolytes using Cs+ salts the main products were methane, ethane, ethylene, formic acid, and CO.151 This system is effective for the formation of C2 compounds, mainly ethylene. In the LiOH-methanol system, the efficiency of hydrogen formation, a competing reaction of C02 reduction, was depressed to below 2% at relatively negative potentials.152 The maximum current efficiency for hydrocarbon (methane and ethylene) formation was of 78%. 

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. 

Alkanes. The chlorination of ethane known to produce more 1,1-dichloroethane than 1,2-dichloroethane is explained by the so-called vicinal effect.115 One study revealed285 that this observation may be explained by the precursor 1,2-dichloroethane radical (the 11 2-chloroethyl radical) thermally dissociating into ethylene and a chlorine atom [Eq. (10.54)]. Indeed, this radical is the major source of ethylene under the conditions studied. At temperatures above 300°C, the dissociation dominates over the chlorination reaction [Eq. (10.55)], resulting in a high rate of ethylene formation with little 1,2-dichloroethane ... 

The rate of ethylene formation, v, assuming step (6) to be rate-determining, is given by... 

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 is a major hydrocarbon product in the vacuum ultraviolet flash photolysis of methane (137). Braun et al. suggest that the following processes are responsible for ethylene formation... 

Two significant studies in support of the proposed importance of /x-methylene intermediates in nonstoichiometric CO/H2 reactions must be mentioned here. First, coordinated carbon monoxide has been reduced with H3A1(OR2)x to give high yields of ethylene. The intermediacy of dinuclear /x-methylene complexes was postulated, and they are thought to account for clean carbene dimerization, as shown in Scheme 32 (245). Note, however, that the isomeric ethanediyl alternative (CO)xM—CH2— CH2—M(CO)x has not been excluded as a possible key intermediate, which, alternatively, could equally well account for ethylene formation. 

The production of ethylene in fruit tissue and in small amounts in leaves may justify its consideration as a hormone, functioning in the gaseous state, Cherimoyas and some varieties of pear produce 1000 times the effective physiological concentration. Ethylene formation is closely linked to oxidation and may be centered in the mitochondria. Its effects are to promote cell-wall softening, starch hydrolysis, and organic add disappearance in fruits—the syndrome known as ripening. Ethylene also decreases the geotiopic responses of stems and petioles. 

A recent photochemical study of cyclopropanone has demonstrated that two reactions take place at certain wavelengths. 75) Although the quantum yield for decomposition of 2 ( Cp) is approximately unity over the range A =2920—3650 A, the quantum yield for ethylene formation... 

While copper favours ethylene formation, iron and cobalt, when a sufficently high temperature is reached, favour ethane formation accompanied in one case by benzene hydrocarbons and in the other case by liquid paraffins. 

The total oxygen-scavenging capacity (TOSC) assay is based on the oxidation of a-keto-y -methiolbutyric acid (KMBA) to ethylene. Ethylene formation is monitored by gas chromatography in the course of reaction and areas below the kinetic curves for control sample and analyzed sample are compared. The oxidant is usually ABAP, but other oxidants were also used and compared (R5, W12). 

Assuming carbon-carbon bond cleavage occurs in the route leading to ethylene formation, unsuccessful attempts have been made to isolate the bis(ethylene) intermediate by reacting ethylene with titanocene obtained in a way similar to that described above (41). However, car-bonylation of the reaction mixture yielded cyclopentanone, inferring that titanocene intermediates are produced (43). 

Stimulate fig enlargement and ethylene formation, and increase respiration... 

Because of the potential commercial significance of this work, we are presently developing kinetic expressions for the rate of ethylene formation in the SC water environment. We are also measuring the rate of ethanol dehydration in the vicinity of the critical point of water to determine if the properties of the fluid near the critical point have any influence on the reaction rate. In the near future we plan to begin studies of the reaction chemistry of glucose and related model compounds (levulinic acid) in SC water. 

Salicylic acid at 25 pM strongly inhibited ethylene formation from ACC in cell suspension cultures, and a number of other benzoic acid derivatives suppress ethylene production at higher concentrations.44 The complex interrelationship of phenolic acids with phytohormones deserves further investigation. The early stage of seedling growth is very sensitive to phenolic acids, and this is a prime time in hormone-mediated growth responses. 

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