Hydroxyl radical saturated hydrocarbons

Moreover, the slight differences in reactivity of C—H bonds in alkanes to free radicals lead to indiscriminate attack of the hydrocarbon chains. Improvements in the efficiency and selectivity in the conversion of saturated hydrocarbons under relatively mild conditions is a desirable goal. This objective may be achieved by the selective activation of C—H bonds in alkanes by the use of suitable metal catalysts. The latter condition obtains in the selective microbiological hydroxylation of saturated hydrocarbons (see Section V.A), in which the enzyme probably interacts with the hydrocarbon via metal-catalyzed redox reactions. 

NMHC. A large number of hydrocarbons are present in petroleum deposits, and their release during refining or use of fuels and solvents, or during the combustion of fuels, results in the presence of more than a hundred different hydrocarbons in polluted air (43,44). These unnatural hydrocarbons join the natural terpenes such as isoprene and the pinenes in their reactions with tropospheric hydroxyl radical. In saturated hydrocarbons (containing all single carbon-carbon bonds) abstraction of a hydrogen (e,g, R4) is the sole tropospheric reaction, but in unsaturated hydrocarbons HO-addition to a carbon-carbon double bond is usually the dominant reaction pathway. 

While it is well established that HO—ONO can be involved in such two-electron processes as alkene epoxidation and the oxidation of amines, sulfides and phosphines, the controversy remains concerning the mechanism of HO-ONO oxidation of saturated hydrocarbons. Rank and coworkers advanced the hypothesis that the reactive species in hydrocarbon oxidations by peroxynitrous acid, and in lipid peroxidation in the presence of air, is the discrete hydroxyl radical formed in the homolysis of HO—ONO. The HO—ONO oxidation of methane (equation 7) on the restricted surface with the B3LYP and QCISD methods gave about the same activation energy (31 3 kcalmol" ) irrespective of basis set size . ... 

Halide-saturated hydrocarbons such as carbon tetrachloride degrade very slowly, if at all, when exposed to solar detoxification treatment. Bicarbonate, a common constituent of groundwater, acts as a scavenger of hydroxyl radicals and can significantly hinder solar detoxification treatment. The presence of nontargeted contaminants in process infiuent can lower process efficiency. 

In addition, the hydrated electron acts as a nucleophile, especially with organic molecules that contain halogen atoms (Eq. 6-16). This reaction results in rapid elimination of a halide ion from the initially formed negatively charged organic species. The reaction of Eq. 6-16 is of special interest for the degradation of per-halogenated saturated hydrocarbons that are usually not affected by hydroxyl radicals (Sun et al, 2000). 

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... 

Here the intervention of the hydrocarbon radical cation seems possible. Hydrocarbon photocatalyzed oxidations seem to depend significantly on the relative positions of the valence-band edge of the active photocatalyst and the oxidation potential of the substrate. For example, in contrast to the photocatalytic oxidation of toluene described above, lower activity was observed in neat benzene, despite the fact that its oxidation potential lies at or slightly below the valence-band edge. This observation implies the importance of radical cation formation (by photoinduced electron transfer across the irradiated interface) as a preliminary step to hydrocarbon radical formation. If the benzene is dispersed into a benzene-saturated aqueous solution into which the semiconductor is suspended, complete mineralization is attained [158]. Thus, to observe selective photoelectrochemistry, it is necessary to avoid primary formation of the highly reactive, nonselective hydroxyl radical (formed by water oxidation) by the use of an unreactive, but polar, organic solvent. 

Alkanes. In most of the chemical reactions observed in irradiation chambers, saturated hydrocarbons—even highly-branched ones such as p-menthane (l-isopropyl-4-methylcyclohexane)—have been quite unreac-tive. Since attack of alkanes by hydroxyl radical (26), atomic oxygen (27, 28), or ozone (29) follows the C-H reactivity order, tertiary > secondary > primary, the chemical measurements with alkanes would be expected to follow a clear pattern. However some alkanes (e.g., p-menthane) with tertiary hydrogens do not react more rapidly than those (e.g., n-octane) with only secondary and primary hydrogens, and hydrogen abstraction reactions often do not appear to be rate-determining steps. 

Figure 6.34 A synthetic porphyrin containing manganese (V) which can hydroxylate saturated hydrocarbons through a free-radical mechanism at room temperature. Manganese (III) is oxidized to manganese (V) with iodosylbenzene...

Another metallocene, namely, decamethylosmocene, (Mc5C5)20s (catalyst 1.2), turned out to be a good precatalyst in a very efficient oxidation of alkanes with hydrogen peroxide in acetonitrile at 20 — 60 °C [9]. The reaction proceeds with a substantial lag period that can be reduced by the addition of pyridine in a small concentration. Alkanes, RH, are oxidized primarily to the corresponding alkyl hydroperoxides, ROOH. TONs attain 51,000 in the case of cyclohexane (maximum turnover frequency was 6000 h ) and 3600 in the case of ethane. The oxidation of benzene and styrene afforded phenol and benzaldehyde, respectively. A kinetic study of cyclohexane oxidation catalyzed by 1.2 and selectivity parameters (measured in the oxidation of n-heptane, methylcyclohexane, isooctane, c -dimethylcyclohexane, and trans-dimethylcyclohexane) indicated that the oxidation of saturated, olefinic, and aromatic hydrocarbons proceeds with the participation of hydroxyl radicals. 

Formation of hydroxyl quinoline must have been initiated by ionic reactions that involved and OH ions. Hydrogenation by Ha enables the heterocyclic ring to be saturated, and this can be followed by hydrogenolysis of C-N bonds that first open the hetero-ring and then convert the resultant aliphatic and aromatic amine intermediates to hydrocarbons and ammonia (24). Also direct attack of OH on quinoline may have led to formation of hydroxyl quinoline. It has been suggested that both ionic reaction and free radical capping are possible under supercritical conditions (25). 

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