Saturated hydrocarbons hydrogen abstraction from

The ab initio results suggest that it is unlikely that water-kerogen interactions occur by a hydrocarbon thermal radical reaction pathway. This conclusion is supported by experimental and natural observations. For example, at 330°C, P-scission of an alkyl radical is 300 times faster than hydrogen abstraction from water so olefin formation will greatly exceed saturates formation (Ross 1992). However, formation of large amounts of olefin in hydrous pyrolysis has not been reported (Larson 1999) and olefins are rare components of crude oils (Hunt 1996). 

It is well accepted that, in all cases the oxidation reaction of a saturated hydrocarbon starts with a hydrogen abstraction from the alkane molecule, whieh has been proposed to be the rate-limiting step 2,23 2,50 55,69-73 activation involving... 

The simplified mechanism which accounts for this phenomenon is that the very high-energy radiation is able to abstract atoms from molecules, which then become unsaturated and can react further. This phenomenon was first reported in 1924 by Lind and Bardwell (L13), who bombarded simple saturated hydrocarbons up to and including butane with alpha particles. They found that hydrogen and methane in a ratio of 5 to 1 were eliminated to give higher hydrocarbons, up to those heavy enough to be liquids and solids. Low yields of condensates are obtained for these endothermic reactions. 

Our initial experiments to prepare the radical by the reaction of sodium atoms with phenyl iodide were inconclusive because the resolution was very poor when the parent halide was used as the matrix. Attempts to increase the resolution by using camphane or adamantane as a matrix were unsuccessful as immediately after formation the phenyl radical abstracted hydrogen from the matrix to form benzene and a radical from the matrix. Complete reaction also occurred with matrices of other saturated hydrocarbons which possessed only secondary hydrogens... 

The great facility with which the phenyl radical abstracts hydrogen from saturated hydrocarbons at 77°K calls for some comment. This high reactivity has also been found for the other cr-radicals, vinyl and cyclopropyl, but not for methyl and other 77-radicals. This difference in reactivity may be explained by the fact that in 77-radicals such as methyl the density of the unpaired electron is equally spread over the two lobes of the p-orbital, whereas for a cr-radical such as phenyl the unpaired electron is located in a highly directional sp-orbital which projects away from the neighbouring atoms, so that repulsive interactions between the radical and a reactant molecule are small in the activated complex. 

Oxygen atom from Cpd I is inserted into the C-H bond of saturated hydrocarbons (Scheme la) by means of hydrogen atom abstraction followed by recombination of the transient hydroxyl with the carbon radical [the so-called oxygen rebound mechanism proposed by Groves in 1976 (8, 10)]. Another possibility can be the concerted oxygen insertion into the C-H bond. Both pathways are rationalized by the two-state mechanism developed by Shaik et al. (6, 9), which describes different reactivities... 

The model system comprising the reaction sequence from initial 0-atom attack to steady-state erosion of a hydrocarbon surface can serve as a benchmark for fundamental atom-surface interactions at hyperthermal collision energies and for etching mechanisms of materials. Within this model system, there is still much to learn. It is likely that when a hyperthermal oxygen atom strikes a saturated hydrocarbon surface, it will either abstract a hydrogen atom or it will scatter inelastically. The subsequent reaction sequence becomes murky. Very little is known about the mechanisms of oxidation, surface roughening, or material loss. In fact, even the sticking... 

In the photoaddition of a saturated hydrocarbon to ethyl propiolate (equation 19) it is likely that the excited state of the acetylenic ester initiates reaction by abstracting a hydrogen atom from the hydrocarbon. The addition of cyclic ethers to an alkyne seems similar (equation 20), although a ketone sensitizer is required for addition of tetrahydropyran or dioxan . When reactions of this type involve a conjugated acetylenic ester, the first-formed a,p-unsaturated ester can normally undergo further photochemical reaction to produce the p,y isomer (see equations 19 and 20). 

The monomer, NOg, an odd-electron molecule, has many of the characteristics of a free radical since it (i) associates with other radicals, (ii) abstracts hydrogen from saturated hydrocarbons, (iii) adds to unsaturated hydro-... 

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... 

Abstraction of a hydrogen atom together with a pair of electrons from a saturated hydrocarbon. This reaction has already been mentioned (p. 40) and will be discussed again under the Alkylation of Olefins (p. 143) and the Isomerization of Alkanes (p. 59). 

Such high sensitivity of amino radical cations to steric and inductive effects was also observed in hydrogen abstraction processes from saturated hydrocarbons 31,32) ... 

While rate coefficients for reactions with OH radicals with the lower members of the alkane and alkene series are well known from laboratory measurements, there exist only a few sporadic data for the higher homologues. It is possible, however, to estimate rate coefficients by an extrapolation of existing data where needed, and this allows us to derive lifetimes also for compounds for which OH rate coefficients have not been determined. Specifically, for the alkanes Greiner (1970a,b) has proposed a formula based on his studies of hydrogen abstraction by OH radicals from saturated hydrocarbons. The expression is... 

The obvious way saturated hydrocarbons such as propane, butane, and pentane can be formed in significant quantity would be if chain initiation were primarily from the polymer chain ends followed by hydrogen abstraction. For instance,... 

Chain scission produces hydrocarbons with terminal free radicals (structure II), which may be stabilized in several ways. If the free radical abstracts a hydrogen atom from a neighboring molecnle, it becomes a saturated end and creates another free radical in the neighboring molecule (structure III), which may stabilize in a number of ways. The most likely of these is beta scission, which accounts for most of the polymer backbone degradation by producing an unsaturated end and a new terminal free radical. 

All the information available on reactions of atomic hydrogen with saturated hydrocarbon molecules and their derivatives suggest that the dominant reaction here is the abstraction of a hydrogen atom from the molecule of the initial hydrocarbon (dehydrogenation) to form a hydrogen molecule and a radical R H + RH H2 + R- The activation energy for this process seldom exceeds 40 kJ. 

Alkyl radicals, which represent the active species in LDPE formation, are very reactive not only in incorporating ethylene into the growing PE chain but also in abstracting hydrogen from other saturated carbon atoms to transform themselves into saturated hydrocarbons. 

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