Hydrocarbon oxidation general mechanism

Abstract The basic principles of the oxidative carbonylation reaction together with its synthetic applications are reviewed. In the first section, an overview of oxidative carbonylation is presented, and the general mechanisms followed by different substrates (alkenes, dienes, allenes, alkynes, ketones, ketenes, aromatic hydrocarbons, aliphatic hydrocarbons, alcohols, phenols, amines) leading to a variety of carbonyl compounds are discussed. The second section is focused on processes catalyzed by Pdl2-based systems, and on their ability to promote different kind of oxidative carbonylations under mild conditions to afford important carbonyl derivatives with high selectivity and efficiency. In particular, the recent developments towards the one-step synthesis of new heterocyclic derivatives are described. 

Taking all these considerations into account, it is possible to postulate a general mechanism for the oxidation of aliphatic hydrocarbons namely,... 

Building on the foundation of the hydrocarbon oxidation mechanisms developed earlier, it is possible to characterize the flame as consisting of three zones [1] a preheat zone, a reaction zone, and a recombination zone. The general structure of the reaction zone is made up of early pyrolysis reactions and a zone in which the intermediates, CO and H2, are consumed. For a very stable... 

Aromatic hydrocarbons are subject to cytochrome P-450-catalyzed hydroxylation in a process that is similar to olefin epoxidation. As discussed in Section IV. G, halogen migration observed during the hydroxylation of 4-ClPhe and similar substrates, led to the discovery of a general mechanism of oxidation that invokes arene oxide intermediates and the NIH shift. Arene oxides and their oxepin tautomers have not been isolated as products of metabolism of benzenoid compounds, but their presence has been inferred by the isolation of phenols, dihydrodiols and dihydrophenolic GSH conjugates derived therefrom262. 

The chemiluminescent reaction of diphenoyl peroxide [26] with easily oxidized, aromatic hydrocarbons, reported by Koo and Schuster (1977b, 1978), was the first well-defined example of an electron-exchange chemiluminescent reaction of an organic peroxide. Its study led to the postulation of chemically initiated electron-exchange luminescence as a generalized mechanism for efficient chemical light formation (Schuster, 1979 Schuster et al., 1979). 

Volume 17 covers gas-phase combustion, which includes probably the most complex processes investigated by chemists. Chapter 1, about half the book, deals with the oxidation of hydrogen and carbon monoxide, with extensive consideration of all the individual reactions occurring. In Chapter 2, the combustion of hydrocarbons is discussed, with emphasis on the general mechanisms which have been suggested to account for the numerous products of partial oxidation. In Chapter 3, the oxidation of aldehydes, which are important intermediates in combustion of other compounds, is considered, and in Chapter 4, the oxidation of alcohols, ketones, oxirans, ethers, esters, peroxides, amines and halocarbons. 

Central nervous system (CNS) depression caused by acute inhalation exposure to volatile aliphatic and aromatic petroleum hydrocarbons is generally thought to occur when the lipophilic parent hydrocarbon dissolves in nerve cell membranes and disrupts the function of membrane proteins by disrupting their lipid environment or by directly altering protein conformation. Oxidative metabolism of CNS-depressing hydrocarbons reduces their lipophilicity and represents a process that counteracts CNS-depression toxicity. More detailed information on this mechanism of toxicity can be found in ATSDR profiles on toluene (ATSDR 1994), ethylbenzene (ATSDR 1999a), and xylene (ATSDR 1995d). 

The phenomenon of chemiluminescence is observed during the oxidation of hydrocarbons [Refs. 382, 533, 568, 677] and polymers [24, 36, 114, 115, 243, 244, 323, 526, 556]. The general mechanism for chemiluminescence with polymers does not differ from that for hydrocarbons. It has been shown [18, 472, 586, 611, 635—637] that chemiluminescence occurs when peroxyradicals react with each other by... 

Although considerable controversy has existed regarding the exact mechanism of hydrocarbon oxidation, one observation stands out from the mass of data that have accumulated, and this is that aldehydes appear early in the process and are prominent in the products. It is generally recognized that aldehydes are not the primary products, and it has been proposed that the most probable primary product is peroxidic in type. 

Atmospheric propagation reactions involve hydrocarbon oxidation steps and NO-to-NO. conversion reactions such as the HO.-NO reaction 5.25. The generalized mechanism pre.sented up to this point in this section is a propagation chain in which the hydrocarbon molecule RH is converted into the carbonyl RCHO and two NO-to-NO. conversions occur, with the OH radical recreated at the end of the set of reactions. As primary hydrocarbons are oxidized to carbonyls, the carbonyls join the primary hydrocarbons in the suite of compounds that can be attacked by OH. Aldehydes formed as intermediate products can themselves photolyze to create fresh HO species this is actually a branching step since more radicals are produced in the step than consumed. This type of branching is essential to sustain the photochemical cycle. 

The mechanism for the catalytic oxidation of methane by Pt(II) was first put forth by Shilov. The first step of the catalytic cycle involves the formation of a methylplatinum(II) intermediate 16 via the C-H activation of hydrocarbon methane with Pt(II) 15. A methylplatinum(IV) species 17 is obtained by the oxidation of methylplatinum(II) intermediate 16 (second step). The formation of the product takes place through reductive elimination from the methylplatinum(IV) 17 either via coordination to water or by the nucleophilic attack at the carbon by an external nucleophile such as water or chloride ion. Considerable amount of experimental data supports has been offered for supporting the given general mechanism (Fig. 6). 

The implicit numerical solution of the time-dependent conservation equations provides the most powerful general method of solving premixed laminar flame problems in systems of (in principle) arbitrary chemical complexity. Indeed, with the simultaneous development of improved diagnostic techniques for the measurement of flame profiles, the possibility of obtaining such solutions has opened the way to realistic studies of reaction mechanisms even in hydrocarbon flames. The choice of solution method and transport flux formulation involves compromise between precision and cost, which becomes a matter of considerable import when modeling hydrocarbon oxidation in flames, which may involve some 25 chemical species and 80 or so elementary reactions. 

A few further general examples of zinc catalytic activity or reactivity include the following. Other zinc-containing systems include a zinc phenoxide/nickel(0) catalytic system that can be used to carry out the chemo- and regioselective cyclotrimerization of monoynes.934 Zinc homoenolates have been used as novel nucleophiles in acylation and addition reactions and shown to have general utility.935,936 Iron/zinc species have been used in the oxidation of hydrocarbons, and the selectivity and conditions examined.362 There are implications for the mechanism of metal-catalyzed iodosylbenzene reactions with olefins from the observation that zinc triflate and a dizinc complex catalyze these reactions.937... 

---