Oxidation potentials of alkanes

The high oxidation potentials of alkanes, however, make it difficult to carry out the oxidation in solvents such as acetonitrile since the first intermediates generated in these oxidations are carbonium ions, as illustrated by equations (4) and (S), Their stabilization with strongly acidic solvents like anhydrous fluorosulfonic acid often lowers the oxidation potentials of these hydrocarbons. ... 

In spite of significant fundamental studies and its significant economic potential as an alternate route to alkenes, the oxidative dehydrogenation of alkanes to alkenes is not currently practiced.383 The main reason is that the secondary oxidation of the primary alkene products limits severely alkene yields, which becomes more significant with increasing conversion. This is due mainly to the higher energies of the C—H bonds in the reactant alkanes compared to those of the product alkenes. This leads to the rapid combustion of alkenes, that is, the formation of carbon oxides, at the temperatures required for C—H bond activation in alkanes. 

The protolytic oxidation of alkanes is also strongly supported by electrochemical studies. In 1973, Fleischmann, Plechter, and co-workers78 showed that the anodic oxidation potential of several alkanes in HSO3F was dependent on the proton donor ability of the medium. This acidity dependence shows that there is a rapid protonation equilibrium before the electron transfer step and it is the protonated alkane that undergoes oxidation (Scheme 5.9). 

The development of new, selective, energy-efficient chemistry for the direct, oxidative conversion of alkanes at temperatures below 250 °C is one of the most challenging and potentially important problems in contemporary catalytic science... 

For non-electrophilic strong oxidants, the reaction with an alkane typically follows an outer-sphere ET mechanism. Photoexcited aromatic compounds are among the most powerful outer-sphere oxidants (e.g., the oxidation potential of the excited singlet state of 1,2,4,5-tetracyanobenzene (TCB) is 3.44 V relative to the SCE) [14, 15]. Photoexcited TCB (TCB ) can generate radical cations even from straight-chain alkanes through an SET oxidation. The reaction involves formation of ion-radical pairs between the alkane radical cation and the reduced oxidant (Eq. 5). Proton loss from the radical cation to the solvent (Eq. 6) is followed by aromatic substitution (Eq. 7) to form alkylaromatic compounds. 

Short-contact-time reactions, defined as reactions occurring on a timescale of milliseconds, offer potential for conversion of hydrocarbons in one step into valuable products. Examples are the selective oxidation of methane in s)mgas without the formation of b)q5roducts (CO2, H2O, and coke) and the oxidative dehydrogenation of alkanes to give olefins or oxygenates. 

The equilibrium (1) at the electrode surface will lie to the right, i.e. the reduction of O will occur if the electrode potential is set at a value more cathodic than E. Conversely, the oxidation of R would require the potential to be more anodic than F/ . Since the potential range in certain solvents can extend from — 3-0 V to + 3-5 V, the driving force for an oxidation or a reduction is of the order of 3 eV or 260 kJ moR and experience shows that this is sufficient for the oxidation and reduction of most organic compounds, including many which are resistant to chemical redox reagents. For example, the electrochemical oxidation of alkanes and alkenes to carbonium ions is possible in several systems... 

Controlled potential electrolysis (cpe) of alkanes in acetonitrile/0.1 M TBABF4 yields acetamidoalkanes (Table 3). Voltammetry and coulometry indicate a 2e-oxidation to a carbenium ion that subsequently reacts with the nitrogen atom of acetonitrile in a Ritter reaction (Eq. 6) [20]. 

In acetonitrile, carbonium ions combine with the solvent to form a nitrillium ion. The latter reacts with added water to form the N-substituted acetamide, often in good yield [5, 6, 7]. Thus electrochemical oxidation of alkanes in acetonitrile is a route for the introduction of an amino-substituent. Some carbonium ions are inefficiently quenched by acetonitrile and eliminate a proton to form an alkene. This alkene is readily oxidised at the anode potentials used and oxidation products contribute to electrode fouling. Pulsing of the anode potential to +0.3 V vs, see helps to... 

This paper summarized our current understanding of the factors that determine selectivity for dehydrogenation versus formation of oxygen-containing products in the oxidation of light alkanes. From the patterns of product distribution in the oxidation of C2 to C6 alkanes obtained with supported vanadium oxide, orthovanadates of cations of different reduction potentials, and vanadates of different bonding units of VO in the active sites, it was shown that the selectivities can be explained by the probability of the surface alkyl species (or the... 

The anodic oxidation of alkanes on a platinum anode in anhydrous hydrogen fluoride19 at low potentials is accompanied by exhaustive fluorination, with all of the hydrogen atoms being replaced by fluorine. Thus, the major fluorination products of methane and propane are carbon tetrafluoride and octafluoropropane, respectively. 

The low cost of light alkanes and the fact that they are generally environmentally acceptable because of their low chemical reactivity have provided incentives to use them as feedstock for chemical production. A notable example of the successful use of alkane is the production of maleic anhydride by the selective oxidation of butane instead of benzene (7). However, except for this example, no other successful processes have been reported in recent years. A potential area for alkane utilization is the conversion to unsaturated hydrocarbons. Since the current chemical industry depends heavily on the use of unsaturated hydrocarbons as starting material, if alkanes can be dehydrogenated with high yields, they could become alternate feedstock. 

This potential-acidity diagram (Pourbaix s type) has been determined for a large series of alkanes.79 All of these results indicate two types of oxidation mechanism of the C—H bond (i) oxidation of alkanes into carbenium ion at high acidity levels and (ii) oxidation of alkanes into radicals at low acidity levels. 

Table 5.2. Standard Potential of Redox Couples of Alkanes in HF (at H0 22.1) and Acidity Levels of Oxidation (pHR in HF).79...

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