Mechanisms of alkanes

Callaghan AV, LM Gieg, KG Kropp, JM Suflita, LY Young (2006) Comparison of mechanisms of alkane metabolism under sulfate-reducing conditions among two bacterial isolates and a bacterial consortium. Appl Environ Microbiol 72 4274-4282. 

The mechanism of alkane sulfoxidation includes the following elementary steps [13,21 24] initiation, three steps of chain propagation, and a few steps of chain termination. 

Scheme 1 Proposed mechanism of alkane dehydrogenation with acceptors...

Dual Function Mechanism of Alkane Aromatization over H-ZSM-5 Supported Ga, Zn, Pt Catalysts Respective Role of Acidity and Additive... 

The role of adsorbed oxygen species in the mechanism of alkane transformation, on the contrary, is more questionable. The effect induced by the substitution of O2 with N2O and IR indications are in agreement with this interpretation, but, on the other hand, activated electrophilic oxygen species form on reduced sites, preferably in tetrahedral coordination (79). The partial reduction of tetrahedral V =0 with formation of tetrahedral v after propane oxidative dehydrogenation can be observed using UV-Visible diffuse reflectance, ESR and V-NMR spectroscopies. It is thus not possible to assign unequivocally the active species in propane selective activation to a tetrahedral V =0 species or to or V -0-0 species formed in the... 

The second major theory of the mechanism of alkane oxidation in the gas phase is the aldehyde theory. The products of gaseous oxidation almost invariably contain aldehydes, and it has been proposed (41) that these are formed by decomposition of alkylperoxy radicals. 

In the cases of mesoporous silica, AMS and quartz chip, the 0.5 C3/C4 ratio being close to unity means that two reactions proceed with almost equal probability to each other. This is in accordance with the classical radical mechanism of alkane cracking supposing that the energy required to form tertiary radical is not so different from that required for secondary radical and that both radicals are cracked by P-scission mechanism shown below [13]. Thus, the results shown in Fig. 4 strongly suggest that isohexane is cracked via the radical mechanism on the mesoporous silica catalysts, or, in other words, MCM-41, both with and without aluminum impurity, and FSM-16 exhibit radical type catalytic function. 

On the other hand, above 20mol% SbF5, a small but increasing amount of unionized SbF5 can be observed, which may rationalize the change in the mechanism of alkane activation from the protolytic to the oxidative pathway, when the concentration of SbF5 increases over 20mol% (see Section 5.1.1). 

Both normal and inverse KIEs have played a major role in unraveling the mechanisms of alkane activation with transition metal complexes. Alkyl hydride complexes are typical intermediates in such reactions. The loss of alkane in well-defined alkyl hydrides frequently exhibits an inverse KIE and involves a O-alkane complex.86 87 As a result, an inverse isotope effect is now taken as evidence for the intermediacy of o-alkane complexes in reductive eliminations (Scheme 8.14). 

The use of deuterium as an isotopic tracer in a study of mechanisms of alkane conversions is generally unrealistic, because at a comparable temperature a D2/alkane exchange is considerably faster than an alkane... 

Filatov, M., Harris, N., and Shaik, S. (1999) On the "rebound" mechanism of alkane hydroxylation by cytochrome P450 electronic structure of the intermediate and the electron transfer character in the rebound step. Angew. Chem., Int. Ed. 38, 3510-3512. 

The reaction of alkyl peroxy radicals with NO is very important in the atmospheric degradation mechanism of alkanes. The reaction proceeds via two pathways. 

Racemization. Optically active 1-bromo-l-methyl-2,2-diphenylcy-clopropane, l-iodo-l-methyl-2,2-diphenylcyclopropane, and l-bromo-2,2-diphenylcyclopropane carboxylic acid were prepared to study the mechanism of alkane formation by hydrido complex. While the first two substrates could not be reduced, the a-bromo acid absorbed 87 mole % of hydrogen, being converted into optically inactive acid (Reaction 18). A sample of the optically active acid retained its configuration under reaction conditions, indicating that a symmetrical intermediate was formed at some stage of the reduction. 

Shaik and co-workers have carried out a number studies using density functional theory based quantum chemical and QM/MM techniques to examine various aspects of the mechanism of alkane hydroxylation by cytochrome P450.178 181 These studies included, for example, calculation of the potential energy surface for the so-called rebound mechanism with methane as a substrate for two spin states, the high spin (HS) quartet state and low spin (LS) doublet state. In the rebound mechanism, Compound I initially abstracts a... 

Radiolysis of mixed alkane crystals consisting of a lower alkane (RiH), such as pentane, to which relatively low concentrations (about 1 mol%) of a higher alkane (RnH) and a suitable electron acceptor (for instance CO2 or a chloroalkane) have been added, has proven very suitable for the study of asymmetric proton transfer from alkane radical cations to alkane molecules. Radiation-chemical mechanisms of alkanes are inherently very complex and the discussion will be limited to processes that have some bearing on the concepts discussed. 

In this work we have studied the mechanism of alkane oligomerization, establishing a network for the primary and secondary reactions occurring on Ga ZSM-5 catalysts. Normal C3 and normal and branched C4 alkanes and alkenes and a LSR have... 

The mechanism of alkane hydroxylation is called the rebound mechanism. It transpires via an initial hydrogen abstraction, followed by rebound of the alkyl radical onto the oxygen of the iron-hydroxo species. Figure 2.15(a) . This mechanism accounts for two key observations ... 

Yoshizawa, K., Y. Shiota, and Y. Kagawa (2000). Energetics for the oxygen rebound mechanism of alkane hydroxylation by the iron-oxo species of... 

Most of the complexes discussed above contain electron-donating Cp, Cp, or phosphine ligands, but the mechanism of alkane RE from an octahedral alkylhydrido Rh complex containing the Tp ligand and a -acid ligand has been studied. 120 The... 

Peroxyacids are more reactive than alkyl peroxides [45]. The mechanism of alkane hydroxylation by peroxyacids is believed to involve an electrophilic attack of the oxygen atom, for example, in the reaction with trifluoroperoxyacetic acid [45c] ... 

A large number of very recent publications have been devoted to the mechanism of alkane C-H bond functionalization by dioxiranes [48] which remains a matter of debate. Two alternative mechanisms have been proposed ... 

Possible mechanisms of alkane oxidation in the presence of heterogeneous catalysts are discussed in publications [44], The rate-determining step in the selective oxidation of methane is the rupture of the C-H bond. Methane activation can occur by a homolytic mechanism [44aji] ... 

In a series of recent publications, Bergman, Harris, Frei, Bromberg and their coworkers investigated the mechanism of alkane activation by complexes of rhodium and iridium [37, 38]. The study of ultrafast (femtosecond-picosecond)... 

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