Remote oxidation hydrocarbons

Similar regioselectivities have been achieved with intramolecular oxidation of long-chain esters. Benzophenone forms biradicals upon UV irradiation, which eliminate hydrogen atoms from saturated hydrocarbons. Benzophenone esters prefer intramolecular attack, and a cetylester is oxidized at C14 in 66% yield (Scheme 2.4.4). The general scheme of this remote oxidation procedure has been applied even more successfully to rigid steroid skeletons (see Scheme 3.4.3). 

When NMHC are significant in concentration, differences in their oxidation mechanisms such as how the NMHC chemistry was parameterized, details of R02-/R02 recombination (95), and heterogenous chemistry also contribute to differences in computed [HO ]. Recently, the sensitivity of [HO ] to non-methane hydrocarbon oxidation was studied in the context of the remote marine boundary-layer (156). It was concluded that differences in radical-radical recombination mechanisms (R02 /R02 ) can cause significant differences in computed [HO ] in regions of low NO and NMHC levels. The effect of cloud chemistry in the troposphere has also recently been studied (151,180). The rapid aqueous-phase breakdown of formaldehyde in the presence of clouds reduces the source of HOj due to RIO. In addition, the dissolution in clouds of a NO reservoir (N2O5) at night reduces the formation of HO and CH2O due to R6-RIO and R13. Predictions for HO and HO2 concentrations with cloud chemistry considered compared to predictions without cloud chemistry are 10-40% lower for HO and 10-45% lower for HO2. 

Attempts to achieve selective oxidations of hydrocarbons or other compounds when the desired site of attack is remote from an activating functional group are faced with several difficulties. With powerful transition-metal oxidants, the initial oxidation products are almost always more susceptible to oxidation than the starting material. When a hydrocarbon is oxidized, it is likely to be oxidized to a carboxylic acid, with chain cleavage by successive oxidation of alcohol and carbonyl intermediates. There are a few circumstances under which oxidations of hydrocarbons can be synthetically useful processes. One group involves catalytic industrial processes. Much effort has been expended on the development of selective catalytic oxidation processes and several have economic importance. We focus on several reactions that are used on a laboratory scale. 

For the anodic substitution of unactivated CH-bonds, some fairly selective reactions for tertiary CH-bonds in hydrocarbons and y—CH-bonds in esters or ketones are available [85-87]. However, in some cases, a better control of follow-up oxidations remains to be developed. Chemically, a number of selective reactions are available, such as the ozone on silica gel for tertiary CH-bonds [88], the Barton or Hoffmann-LoefHer-Freytag reaction for y-CH-bonds [89], and for remote CH-bonds, Cprop)2NCl/H [90, 91], photochlorination of fatty acids adsorbed on alumina [92] or template-directed oxidations [93]. 

In remote tropospheric air, where NO concentrations can be quite low (17), the HO + CO oxidation mechanism can follow other pathways, leading to net ozone destruction rather than formation (18, 19). Reactions 1 through 5 typify the more complex catalytic reactivity of HO with hydrocarbons, which produce a complex array of oxidation products while generating ozone pho-tochemically (11-13). 

Althou the evidence presented in support of such reaction is not entirely convincing, in its favour is the existence of a lower oxidation state of antimony which can be readily reached by smooth reduction. In the case of other Lewis acids a lower oxidation state is either unavailable or less easily obtained and the likelyhood of hydride abstraction seems therefore more remote, at least from saturated hydrocarbons. Kennedy s scheme implies however the removal of the allylic hydride ion from an olefin. Although plausible in certain cases (but never proved), this mechanism is obviously impossible with styrene and 1,1 -diphenylethylene With 3-phenylindene it would yield an aryl-substituted allylic carbenium ion which would not be expected to be in equilibrium with its precursor yet, this equilibrium was observed With 2,3-dimethylindene in the same conditions initiation did not take place yet Kennedy s mechanism shouldhave operated without impediments. Finally, with 1,1-diphenylpropene hydride abstraction would have produced an allylic ion incapable of giving back the precursor by reacting with methanol yet Bywater and Worsfold showed that this reversible reaction takes place. 

Figure 2 Schematic showing principle oxidation processes in the troposphere in NO -rich air (after Prinn, 1994). In NOj.-poor air (e.g., remote marine air), recychng of HO2 to OH is achieved hy reactions of O3 with HO2 or hy conversion of 2HO2 to H2O2 followed hy photodissociation of H2O2. In a more complete schematic, nonmethane hydrocarbons (RH) would also react with OH to form acids, aldehydes and ketones in...

Formaldehyde is released to the atmosphere in large amounts and is formed in the atmosphere by the oxidation of hydrocarbons. However, the input is counterbalanced by several removal paths (Howard 1989). Because of its high solubility, there will be efficient transfer into rain and surface water, which may be important sinks (NRC 1981). One model has predicted dry deposition and wet removal half-lives of 19 and 50 hours, respectively (Lowe et al. 1980). Although formaldehyde is found in remote areas, it probably is not transported there, but is generated from longer-lived precursors that have been transported (NRC 1981). 

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