Ethyne to ethene

Scheme 109 Catalytic cycle for the iron-complex-mediated reduction of ethyne to ethene.

The ability of K. pneumoniae Mo nitrogenase to convert ethyne to ethene has been described [77] in terms of the now familiar scheme (Figure 13). C2H2 can... 

The vnf genes have also been reported for other bacteria, and in the cyanobacteria Anabaena variabilis and A. azotica, the latter from Chinese rice fields. These cyanobacteria express a vanadium nitrogenase which, as the bacterial VNases, also reduces ethyne to ethene and further to ethane.I ]... 

The formation of hydrazine in the case of VNase may hint at a different activation mechanism of the N2 molecule, i.e. protonation prior to bond dissociation (VNase) versus bond dissociation prior to protonation (MoNase). However, the intermediate formation of hydrazine has also been detected in shock-frozen MoNase. The reduction of ethyne to ethene (followed by further reduction to ethane in the case of VNase), when carried out in D2O, leads to the formation of the Z (cis) isomer (Z)-l,2-deuteroethene, [Equation (4.27)], very much suggesting activation of ethyne by side-on coordination to the metal... 

In the cracking of benzene to acetylene over alumina and silica-supported nickel catalysts it was observed that the selectivity of the reaction, expressed as the ethyne-to-ethene ratio, was dramatically affected (from 1 9 to 9 1) by controlling the micro-wave energy input (i.e. 90% selectivity could be achieved) [109]. 

It will also become clear that the behaviour of alkynes closely resembles that of the 1,2- and 1,3-alkadienes already discussed. Their strong chemisorption was due to the simultaneous interaction of both double bonds in the latter case, and probably to the large release of strain that accompanied the opening of one of the double-bonds (or at least the disengagement of one set of tt orbitals) in the latter case. The heat of hydrogenation reflects the magnitude of the release of strain and hence the strength of the interaction with surface atoms for ethyne to ethene it is 172 kJ moU compared to 137 kJ moU for ethene to ethane. These reactions are thus quite exothermic, and in industrial use care must be taken to avoid temperature excursions, as these would lead to loss of selectivity. 

Write a balanced chemical equation for (a) the hydrogenation of ethyne (acetylene, C2H2) to ethene (C2H4) by hydrogen (give the oxidation number of the carbon atoms in the reactant and product) (b) the shift reaction (sometimes called the water gas shift reaction, WGSR) (c) the reaction of barium hydride with water. 

Reductive dechlorination in combination with the elimination of chloride has been demonstrated in a strain of Clostridium rectum (Ohisa et al. 1982) y-hexachlorocyclohexene formed 1,2,4-trichlorobenzene and y-l,3,4,5,6-pentachlorocyclohexene formed 1,4-dichlorobenzene (Figure 7.69). It was suggested that this reductive dechlorination is coupled to the synthesis of ATP, and this possibility has been clearly demonstrated during the dehalogenation of 3-chlorobenzoate coupled to the oxidation of formate in Desulfomonile tiedjei (Mohn and Tiedje 1991). Combined reduction and elimination has also been demonstrated in methanogenic cultures that transform 1,2-dibromoethane to ethene and 1,2-dibromoethene to ethyne (Belay and Daniels 1987). 

Vitamin B12 catalyzed also the dechlorination of tetrachloroethene (PCE) to tri-chloroethene (TCE) and 1,2-dichloroethene (DCE) in the presence of dithiothreitol or Ti(III) citrate [137-141], but zero-valent metals have also been used as bulk electron donors [142, 143]. With vitamin B12, carbon mass recoveries were 81-84% for PCE reduction and 89% for TCE reduction cis-l,2-DCE, ethene, and ethyne were the main products [138, 139]. Using Ni(II) humic acid complexes, TCE reduction was more rapid, leading to ethane and ethene as the primary products [144, 145]. Angst, Schwarzenbach and colleagues [140, 141] have shown that the corrinoid-catalyzed dechlorinations of the DCE isomers and vinyl chloride (VC) to ethene and ethyne were pH-dependent, and showed the reactivity order 1,1-DCE>VC> trans-DCE>cis-DCE. Similar results have been obtained by Lesage and colleagues [146]. Dror and Schlautmann [147, 148] have demonstrated the importance of specific core metals and their solubility for the reactivity of a porphyrin complex. 

Ethyne, ethene and ethane contain 10. 12 and 14 valence electrons, respectively. They are isoelectronic with N2, 02 and F., respectively. Along both series of molecules the central link becomes progressively weaker and the bond lengths increase. The carbon-carbon bond length increases from 121 pm in ethyne to 133 pm in ethene and to 155 pm in ethane. The corresponding bond enthalpy terms are 837, 612 and 348 kJ mol Data for the diatomic molecules is contained in the text. Discuss these data in terms of the VSEPR and MO treatments of the molecules. 

Electronic and steric factors seem to determine both the site and the orientation of addition of diazoalkanes to conjugated enynes. Although rates of addition of DPD to ethene and ethyne bearing identical single substituents are approximately the same,48 addition to butenyne occurs almost exclusively at the double bond.46... 

Although this does not seem to offer particular problems with ethene, an explosive decomposition of ethyne to carbon and hydrogen may occur if the gas is compressed to 10-20 kg cm-2. Even liquid ethyne (bp—83°) must be handled with care. Ethyne is not used commercially under pressure unless it is mixed with an inert gas and handled in rugged equipment. Ethyne burns with pure oxygen to give a very hot flame that is widely used for welding. For this purpose, the gas is dissolved under about 15 kg cm 2 in 2-propanone (acetone,... 

Neither ethene nor ethyne is a very good dienophile but L4 + 2] cycloadditions of either with 1,3-cyclopentadiene go well at temperatures of 160-180° because 1,3-cyclopentadiene is a very reactive diene. Achieving the overall result of addition of ethene or ethyne to a less reactive diene could necessitate a synthetic sequence wherein one of the reactive dienophiles listed in Table 13-1 is used to introduce the desired two carbons, and the activating groups are subsequently removed. An example follows ... 

There have been two recent studies of the hydrogenation of ethyne on gold catalysts. With 1% Au/TiC>2 made by thermally decomposing the supported complex [Au2(PPh3)6](BF4)2 and having particles of average size 4.6 nm, reaction occurred at 453 K with about 90% selectivity to ethene,... 

While the unsaturation displayed by 1-6 is clearly different in nature from that in ethyne and ethene, writing three or two lines, as appropriate, to connect the metals is nevertheless a good formalism. They are a useful pointer to the nature of their chemical reactivity and are not intended as an indication of the nature of the bonding. 

Consideration of the degree of metal substitution at the carbons of the C2 fragment and the separations of the two carbon atoms, has led to the formulation of polymetallic species as permetallated ethyne (E), ethene (F), or ethane (G) (Chart 2).35 Addition of metal fragments to the unsaturated species E leads to sequential formation of F and G and progressive saturation of the C=C triple bond. These ideas are broadly consistent with the results of computational studies (Section IX.D). Compounds of type E are described below, while the more highly metallated species are discussed in Sections VII, VIII, and IX. 

Ess and Houk applied this model to 1,3-dipolar cycloadditions to ethene and ethyne. B3LYP/6-31G(d) and CBS-QB3 computations were carried out for the reactions of nine 1,3-dipoles shown in Scheme 4.3. The activation energies of these 18 reactions do not correlate with the reaction energies thus, there is no correlation to the effect that the more exothermic is the reaction, the lower will be its activation barrier. Rather, the activation energies correlate extremely well with the distortion energy (r = 0.97). Ess and Houk argue that the TS is achieved when the t-orbitals of the dipole align well with the jc-orbitals of the dipolarophile. This... 

Kirmse and Streu have used the photocycloaddition of ethene to cyclopen-tenone and 3-methylcyclopentenone to yield the [2 + 2] adducts (32, R = H or Me) as a route to the synthesis of the norbornenones (33). Cyano Dewar heptalene (34) has been successfuUy synthesized using a multi-stage reaction in which the photocycloaddition of ethyne to the enone (35), yielding (36), played an important part. The Dewar heptalene (34) is photochemically labile and is converted into (37) on irradiation with 254 nm light, although it is... 

Borodzinski A, Bond GC (2006) Selective hydrogenation of ethyne in ethene-rich streams on palladium catalysts. Part 1. Effect of changes to the catalyst during reaction. Catal Rev - Sci Eng 48 91... 

The reverse reaction of a cyclometallation is of importance for the construction of catalytic cycles. The simple retrocyclometallations of a and b are not productive, since they would again lead to ethene and ethyne, and additional reactions have to be invoked for a productive catalytic cycle. For c-e the retro reactions f-g can be envisaged, leading to new products see Fig. 4.28. 

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