Hydrocarbons alkoxy radical reactions

With current estimates of the rates of alkoxy radical reactions [27], isomerization is likely to be the more important of these latter two processes. Clearly, a better understanding of these reactions is required before their role in the atmospheric degradation of aromatic hydrocarbons can be assessed. 

Using the preceding discussion of alkylperoxy and alkoxy radical reactions as a guide, we present now specific mechanisms for several individual hydrocarbons, starting with alkanes. 

The results of LACTOZ have provided an extended kinetic data base for the following classes of reactions reactions of OH with VOCs, reactions of NO3 with VOCs and peroxy radicals, reactions of O3 with alkenes, reactions of peroxy radicals (self reactions, reaction with HO2, other RO2, NO, NO2), reactions of alkoxy radicals (reactions with O2, decomposition, isomerisation), thermal decomposition of peroxynitrates. Photolysis parameters (absorption cross-section, quantum yields) have been refined or obtained for the first time for species which photolyse in the troposphere. Significantly new mechanistic information has also been obtained for the oxidation of aromatic compounds and biogenic compounds (especially isoprene). These different data allow the rates of the processes involved to be modelled, especially the ozone production from the oxidation of hydrocarbons. The data from LACTOZ are summarised in the tables given in this report and have been used in evaluations of chemical data for atmospheric chemistry conducted by international evaluation groups of NASA and lUPAC. 

An important side reaction in all free-radical nitrations is reaction 10, in which unstable alkyl nitrites are formed (eq. 10). They decompose to form nitric oxide and alkoxy radicals (eq. 11) which form oxygenated compounds and low molecular weight alkyl radicals which can form low molecular weight nitroparaffins by reactions 7 or 9. The oxygenated hydrocarbons often react further to produce even lighter oxygenated products, carbon oxides, and water. 

Alkyl radicals, R, react very rapidly with O2 to form alkylperoxy radicals. H reacts to form the hydroperoxy radical HO2. Alkoxy radicals, RO, react with O2 to form HO2 and R CHO, where R contains one less carbon. This formation of an aldehyde from an alkoxy radical ultimately leads to the process of hydrocarbon chain shortening or clipping upon subsequent reaction of the aldehyde. This aldehyde can undergo photodecomposition forming R, H, and CO or, after OH attack, forming CH(0)00, the peroxyacyi radical. 

The simplest hydrocarbon, methane, has posed a wealth of challenges to experimentalists and theoreticians seeking to discern its combustion mechanism. Methane s reactions have been explored in a wide variety of contexts over the past several decades. We have discussed these briefly the interested reader is referred to the reviews cited in our previous discussion for further details. Due to the scope of this review, we are primarily interested in these reactions insofar as they provide useful benchmarks for the reactions of larger alkylperoxy (RO2 ) and alkoxy (RO ) systems. With respect to the reactive intermediates present in methane combustion and their implications for larger systems, Lightfoot has published a review on the atmospheric role of these species, while Wallington et al. have provided multiple overviews of gas-phase peroxy radical chemistry. Lesclaux has provided multiple reviews of developments in peroxy radical chemistry. Batt published a review of the gas-phase decomposition reactions available to the alkoxy radicals. ... 

At the end of reaction the initial hydrocarbon concentration is still important, but there are also many other hydrogen donors. Thus, in the absence of oxygen, reactions of alkoxy radicals will lead to aldehydes by pyrolysis and to alcohols by abstraction. Let us examine these two possible reactions for each alkoxy radical. 

The decomposition of the alkoxy radical by Reaction 8 occurs by a-scission at the C—C bond attached to the largest hydrocarbon group (185, 229). Straight-chain paraffins produce aldehydes, while highly branched paraffins yield ketones (Reaction 11). The knock resistance of naphthenes may be caused by the stability of the naphthene ring to C—C scission (25). 

My mind was prepared from my knowledge of the pyrolysis of chlorinated hydrocarbons (Chapter 1) and related subjects. The work of the late Professor E. W. R. Steacie, who eventually became Director of the National Research Council of Canada, had showed that the pyrolysis of alkyl nitrites in the gas phase gave NO and an alkoxy radical in a unimolecular reaction. The temperatures required for this reaction were much too high for... 

The reaction of alkoxy radicals, as the intermediates of hydrocarbon oxidation, with molecular oxygen takes place in the case of primary and secondary radicals... 

Copper(I) can also be regenerated by electron transfer oxidation of radicals produced by hydrogen transfer from solvent by alkoxy radicals.101 Thus, reactions carried out in hydrocarbon solvents will produce alkyl radicals that are oxidized by Cu(II) at rates approaching diffusion control.95-100... 

Before moving on to consider the fate of the carbonyl products, it is appropriate to discuss the atmospheric fate of CF30 radicals. The usual modes of alkoxy radical loss are not possible for CF30 radicals. Reaction with O2 and decomposition via F atom elimination are both thermodynamically impossible under atmospheric conditions. Instead, CF3O radicals react with NO and hydrocarbons. 

The subsequent cycMsation of the alkoxy-radical depends upon its ability to attack a suitably placed C-H bond on the -carbon atom. The alternative to cyclisation under homolytic conditions is fragmentation of the alkoxy radical into a carbonyl compound (ii) and an alkyl radical (10), which affords a mixture of stable products by further transformations. Heusler [44] reached similar conclusions from a study of steroid reactions, and has demonstrated a close similarity between thermally and photolytically-induced homolytic reactions with lead tetraacetate in hydrocarbon solvents. 

In the Barton reaction, an alkyl nitrite is converted into an alcohol-oxime. The nitrite is photoexcited to a 1,2-diradical. It fragments to NO and an alkoxy radical (RO-), and the latter abstracts H- from the nearest C-H bond. The resulting alkyl radical then combines with NO to give a nitroso compound, which then tautomerizes to the oxime, probably by a polar stepwise mechanism. The Barton reaction has been used for the remote functionalization of hydrocarbons, especially steroids. 

In many cases both homolytic and heterolytic pathways afford the same products, e.g. alcohols and ketones from hydrocarbons, which means that results have to be interpreted with care. Certain elementary tests for homolytic pathways need to be performed, e.g. inhibition by a radical scavenger such as lonol indicates a free radical chain mechanism and loss of yield on flushing with an inert gas suggests the intermediacy of dioxygen in reactions with H2O2 or RO2H. More sophisticated reality tests can also be performed to demonstrate the intermediacy of alkoxy radicals in oxidations with RO2H [17]. 

When the reaction becomes diffusion controlled as a result of the increased viscosity of the oil, alkoxy radicals can initiate polymerisation of polycondensation products. This leads to sludge and deposit formation as well as to additional oil-soluble high molecular weight products which contribute to the viscosity increase. This process can be described as co-polymerisation of two different polycondensation species in which the alkyl groups R, R and R could represent 0x0- or hydroxy-functionalised long hydrocarbon chains Reaction (4.23) ... 

Step 2 in this sequence is exothermic by about 66 kcal. If any significant share of this is left as excitation energy of the alkoxy radical, it will probably dissociate further into a simple aldehyde or ketone, plus a smaller alkyl group. However, at dry ice temperatures or lower, the initiation step, i, is almost certainly too slow to account for the reaction rates which have been reported here. The suggestion—that ozone attacks alcohols and hydrocarbons by abstracting hydrogen to form the HO radical— appears equally unlikely. 

For longer chain alkanes ( 04) the reaction mechanism becomes more complex due to the IsomerIzatIons of the alkoxy radicals (47, 48), and to the fact that addition of NO to the alkyl-peroxy radicals (49) becomes more Important than NO to NO2 oxidation. For the alkenes and aromatic hydrocarbons the oxidation mechanisms In the atmosphere are more complex, and discussions of these systems, along with a more detailed treatment of the alkanes, are given later. 

Although a great number of different hydrocarbons occur in the troposphere and individual oxidation schemes can be quite complex, the mechanisms follow the same overall principle. The process is initiated by reaction with an OH radical, proceeds via organic peroxy and alkoxy radicals as intermediates, and leads to carbonyl compounds as oxidation products. In this respect the individual reaction steps are similar to those discussed above for methane. 

In general, alkyl-peroxy radicals are rather stable and do not undergo side reactions. In an oxygen-saturated polymer, alkyl radicals are rapidly converted to alkyl-peroxy radicals by reaction with oxygen, so that their other reactions are not important. The most important side reaction is that of the alkoxy radical. In liquid hydrocarbons, the main reaction of alkoxy radicals is hydrogen abstraction to give... 

In principle, termination of two alkoxy radicals would give a peroxide cross-link, where as termination of an alkoxy radical with an alkyl radical would give an ether cross-link. It is very difficult to get direct experimental evidence for either of these reactions, since both dialkyl peroxides and ethers are difficult to detect however, there is no doubt that chain scission is dominant in saturated hydrocarbon polymers. 

The mechanism of autoxidation of solid polyolefins RH has many common features with hydrocarbon oxidation polymers are oxidized by the chain route the chain develops as alternation of acts of R- with O2 and RO with RH in the absence of an inhibitor and at a sufficiently high [O2] in polymer, chain oxidation occurs with autoacceleration because the hydroperoxide groups that formed are a source of initiation. However, there are several substantial distinctions. As already mentioned in Chapter 6, reactions resulting in polymer destruction play an important role. In initiated oxidation the main source of destruction is reactions of peroxide radicals, whereas in autoxidation the contribution of alkoxy radicals, which decompose in the reaction of the type... 

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