Abstraction selectivity from hydrocarbons

From the different contributions, it may be concluded that, in the oxidation of ammonia, the same type of redox mechanism is operative for metal oxides as in the selective oxidation of hydrocarbons. As a consequence, the hydrogen atoms will be abstracted successively from the NH3 molecule by a stepwise mechanism. 

A reaction mechanism and the description of the active site as a molecular ensemble of atoms is shown in Scheme 8.1, as proposed by Haber and coworkers [8-10] and established by Grasselli et al. [11,12] for the (amm)oxidation of propene to (acrylonitrile) acrolein. A selective oxidation reaction involves one (or several) H-atom abstraction(s) from the hydrocarbon molecule, one (or several)... 

In the same vein, with propane, the primary C-H bond reacted twice as fast as the secondary despite its high bond dissociation energy, meaning antiradical reactivity. Whereas radical H-abstraction occurs from the weakest C-H bond of hydrocarbons (selectivity tertiary C-H > secondary C-H > primary C-H), this new activation mode proceeding with the opposite selectivity using C-H oxidative addition by... 

The factors that influence the diastereoselectivity for the class of monocyclic hydrocarbons, from which monoterpenes are the most studied group, are mainly steric and conformational. Monoterpenes, in general, show relatively low overall syn/anti selectivity, usually attributed to the availability of allylic hydrogen atoms in the right alignment for abstraction. 

If the hydrocarbon radical cation has a definitive structure, proton loss occurs from one particular, well-defined position and these transformations are more selective than the alternative C-H abstractions from alkanes with radical reagents (Eq. 2). For example, C-H substitutions of the adamantane cage with radical reagents always give mixtures of 1 and 2-substituted adamantanes [2], As the adamantane radical cation (4) has one single structure, proton transfer from the radical cation to the solvent occurs highly selectively. Scheme 2 shows the geometry of 4 and the structure of the complex of the adamantane radical cation with acetonitrile (S) where the tertiary C-H bond is already half-broken. 

The IE growth with a temperature rise from 4.2 to 50 K was observed by Iwasaki and co-workers in the reactions of H- and D-atom abstraction from saturated hydrocarbon molecules in crystalline xenon matrices [95]. The temperature rise causes a growing selectivity of various CH-bond breaks under H-atom abstraction. The ratio of the accumulation rates of radicals formed under a break of primary, secondary, and tertiary CH bonds in isobutane is 1 1 1 at 4.2 K and 1 6 14 at 30 K [96-98], These data also indicate the existence of the pretunneling molecular shifts of the reagents within the nearest coordination spheres. It is assumed in ref. 82 that these shifts are quasi-reversible, and the kinetic pattern of H-atom transfer reactions has the form... 

Another interesting feature related to the a nature of the cyclopropyl and vinyl radicals is their reactivity. In general, a radicals are more reactive and less selective than n radicals. Phenyl, vinyl, and cyclopropyl a radicals will abstract hydrogen atoms from saturated hydrocarbons at 77 K, conditions under which k radicals are unreactive [23]. In Ruchardt s [24] radical reactivity classification, based on the reaction of a series of o and n radicals with BrCClj and CCl, the cyclopropyl radical, an inverting a radical in an sp -hybridized orbital, closely resembled the noninverting phenyl radical. Further support for this finding... 

The ODS of hydrocarbon fuels consists of the conversion of sulfur compounds in the fuels by oxidation to element sulfur, sulfur oxides, sulfoxides, and/or sulfones followed by adsorption or abstraction separation of the oxidized sulfur compounds from the hydrocarbon fuels. The potential advantages for ODS are (a) the process does not need to use H2 gas (b) ODS takes place at relatively mild operating conditions in comparison with HDS or even at ambient conditions and (c) ODS is able to remove some refractory sulfur compounds, such as 4,6-DMDBT, that are difficult to be removed by HDS or selective adsorption. 

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