Alkanes Alkene" hydrogenation mechanism

However, ammonia adsorption experiments in our laboratories ( 5) demonstrated an enhanced acidity after reduction (Figure 1). So a dual function mechanism (9, 10), in which metal sites are responsible for the (de)hydrogenation of (alkanes) alkenes and acid sites isomerize the alkenes via a carbocation mechanism, may also explain the high isomerization activity. 

The radical mechanism of thermal degradation of pristine PE has been widely discussed in a framework of random scission type reactions [14-22], It is known that PE decomposition products comprise a wide range of alkanes, alkenes and dienes. Branching of PE chains causes enhanced intermolecular hydrogen transfer and results in lowering thermal stability. The polymer matrix transformations, usually observed at lower temperatures and involving... 

The mechanism of alkene hydrogenation by [HCo(CN)5]3 was worked out in very fine details [62, 63]. Activated alkenes add across the Co H bond with formation of an intermediate Co-alkyl species or with generation of a radical pair. Both pathways require further reaction with an additional [HCo(CN)5]3 to yield the product alkane. Reaction of [Co(CN)5]3 with H2 completes the catalytic cycle. 

Interesting kinetic information of the importance of adsorption effects to overall kinetics of a zeolitic reaction is obtained by studying the reaction at conditions where pore filling varies . Experimental data indicate that especially the orders of alkane and hydrogen are sensitively dependent on this. Whereas at high pressures pores are completely filled and hence in a bifimctional mechanism alkane and alkene are equilibrated within the micropore , this is not necessarily the case at low pressures. 

It will not have escaped notice that adsorbed species implicated in the exchange of alkanes with deuterium are formally the same as those invoked in the hydrogenation of alkenes indeed the reiteration of the alkyl-alkene transformation (process 6.J) held responsible for multiple exchange in linear and branched alkanes, and designated the afi exchange mechanism, is on the face it of identical with the old and well-tried Horiuti-Polanyi mechanism for alkene hydrogenation. This will be discussed further in the next chapter (sections 7.1 and 7.21), but briefly it supposes the sequential addition of two hydrogen atoms to some adsorbed form of the alkene, e.g. 

Similar mechanisms are postulated for commercial alkene/arene, carbonyl and nitrile hydrogenations on metal surfaces in particular, individual metal atoms are involved. In contrast hydrogenolysis, the cleavage of C—C or C—O (N, S, etc.) bonds, appears to need two or more adjacent sites and can sometimes be reduced by alloying the main component (addition of copper to nickel, for example). The stability of supported metal (especially platinum) catalysts permits their use at high temperatures, to promote hydrogen transfers between alkanes, alkenes and arenes or dehydrogenation processes. 

As mentioned earUer the mechanism of alkane metathesis occurs via (i) a C-H bond activation, followed by (ii) metal alkylidene and olefin formation, and finally (iii) the olefin metathesis step and alkene hydrogenation. Thus, it was of interest to commence this transformation with an alkylidene precursor rather than metal... 

For the complexes Cp3ThR (R= Pr or "Bu), (MeC5H4)3Th("Bu), and (indenyl)Th("Bu), UV photolysis in aromatic solvents produces 1 1 mixtures of alkane alkene and Cp3Th, and a photoinduced p-hydrogen elimination mechanism is proposed. In the corresponding... 

When NBS is used to brominate non-alkenyl substrates such as alkanes, another mechanism, involving abstraction of the hydrogen of the substrate by the succinimidyl radical " 14 can operate. " This mechanism is facilitated by solvents (such as CH2CI2, CHCI3, or MeCN) in which NBS is more soluble, and by the presence of small amounts of an alkene that lacks an allylic hydrogen (e.g., ethene). 

Hence, the rate depends only on the ratio of the partial pressures of hydrogen and n-pentane. Support for the mechanism is provided by the fact that the rate of n-pentene isomerization on a platinum-free catalyst is very similar to that of the above reaction. The essence of the bifunctional mechanism is that the metal converts alkanes into alkenes and vice versa, enabling isomerization via the carbenium ion mechanism which allows a lower temperature than reactions involving a carbo-nium-ion formation step from an alkane. 

It has been generally accepted that the thermal decomposition of paraffinic hydrocarbons proceeds via a free radical chain mechanism [2], In order to explain the different product distributions obtained in terms of experimental conditions (temperature, pressure), two mechanisms were proposed. The first one was by Kossiakoff and Rice [3], This R-K model comes from the studies of low molecular weight alkanes at high temperature (> 600 °C) and atmospheric pressure. In these conditions, the unimolecular reactions are favoured. The alkyl radicals undergo successive decomposition by [3-scission, the main primary products are methane, ethane and 1-alkenes [4], The second one was proposed by Fabuss, Smith and Satterfield [5]. It is adapted to low temperature (< 450 °C) but high pressure (> 100 bar). In this case, the bimolecular reactions are favoured (radical addition, hydrogen abstraction). Thus, an equimolar distribution ofn-alkanes and 1-alkenes is obtained. 

The chromatograms of the liquid phase show the presence of smaller and larger hydrocarbons than the parent one. Nevertheless, the main products are n-alkanes and 1-alkenes with a carbon number between 3 to 9 and an equimolar distribution is obtained. The product distribution can be explained by the F-S-S mechanism. Between the peaks of these hydrocarbons, it is possible to observe numerous smaller peaks. They have been identified by mass spectrometry as X-alkenes, dienes and also cyclic compounds (saturated, partially saturated and aromatic). These secondary products start to appear at 400 °C. Of course, their quantities increase at 425 °C. As these hydrocarbons are not seen for the lower temperature, it is possible to imagine that they are secondary reaction products. The analysis of the gaseous phase shows the presence of hydrogen, light alkanes and 1-alkenes. 

In this chapter, we will study the elementary reaction steps of these mechanisms focusing primarily on the anthraphos systems. This chapter begins with a description of the impact of different methods (coupled cluster, configuration interaction and various DFT functionals), different basis sets, and phosphine substituents on the oxidative addition of methane to a related Ir system, [CpIr(III)(PH3)Me]+. Then, it compares the elementary reaction steps, including the effect of reaction conditions such as temperature, hydrogen pressure, alkane and alkene concentration, phosphine substituents and alternative metals (Rh). Finally, it considers how these elementary steps constitute the reaction mechanisms. Additional computational details are provided at the end of the chapter. 

Casey has suggested that the hydrogenation of alkenes by Shvo s catalyst may proceed by a mechanism involving loss of CO from the Ru-hydride complex, and coordination of the alkene. Insertion of the alkene into the Ru-H bond would give a ruthenium alkyl complex that can be cleaved by H2 to produce the alkane [75], If this is correct, it adds further to the remarkable chemistry of this series of Shvo complexes, if the same complex hydrogenates ketones by an ionic mechanism but hydrogenates alkenes by a conventional insertion pathway. 

Chemical catalysts for transfer hydrogenation have been known for many decades [2e]. The most commonly used are heterogeneous catalysts such as Pd/C, or Raney Ni, which are able to mediate for example the reduction of alkenes by dehydrogenation of an alkane present in high concentration. Cyclohexene, cyclo-hexadiene and dihydronaphthalene are commonly used as hydrogen donors since the byproducts are aromatic and therefore more difficult to reduce. The heterogeneous reaction is useful for simple non-chiral reductions, but attempts at the enantioselective reaction have failed because the mechanism seems to occur via a radical (two-proton and two-electron) mechanism that makes it unsuitable for enantioselective reactions [2 c]. 

With propene, n-butene, and n-pentene, the alkanes formed are propane, n-butane, and n-pentane (plus isopentane), respectively. The production of considerable amounts of light -alkanes is a disadvantage of this reaction route. Furthermore, the yield of the desired alkylate is reduced relative to isobutane and alkene consumption (8). For example, propene alkylation with HF can give more than 15 vol% yield of propane (21). Aluminum chloride-ether complexes also catalyze self-alkylation. However, when acidity is moderated with metal chlorides, the self-alkylation activity is drastically reduced. Intuitively, the formation of isobutylene via proton transfer from an isobutyl cation should be more pronounced at a weaker acidity, but the opposite has been found (92). Other properties besides acidity may contribute to the self-alkylation activity. Earlier publications concerned with zeolites claimed this mechanism to be a source of hydrogen for saturating cracking products or dimerization products (69,93). However, as shown in reaction (10), only the feed alkene will be saturated, and dehydrogenation does not take place. 

Apparently, the exchange patterns can be explained qualitatively by reference to either structure for the adsorbed olefin, the eclipsed 1,2-diadsorbed alkane or the olefin tt complex. This argument should, of course, refer to the transition state for the formation of chemisorbed olefin from monoadsorbed alkane, the critical step in the a,)3 exchange mechanism however the revised argument would be much the same. Nevertheless we are provided with two alternative descriptions of the chemisorbed alkene under conditions closely related to those employed in hydrogenation studies. 

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