Alkoxy radical isomerization

Kwok, E. S. C., J. Arey, and R. Atkinson, Alkoxy Radical Isomerization in the OH Radical-Initiated Reactions of C4-C8 n-Alkanes, J. Phys. Chem., 100, 214-219 (1996a). 

Carter, W. P. L., K. R. Darnall, A. C. Lloyd, A. M. Winer, and J. N. Pitts, Jr. (1976). Evidence for alkoxy radical isomerization in photooxidations of C4-C6 alkenes under simulated atmospheric conditions. Chem. Phys. Lett. 42, 22-27. 

To illustrate alkoxy radical isomerization, let us consider the OH reaction of -pentane. The n-pentane-OH reaction proceeds as follows to produce the 2-pentoxy radical ... 

Rate constants for alkoxy radical isomerizations can be combined with rate constants for alkoxy radical decomposition and reaction with O2 to predict the relative importance of the three pathways (Atkinson 1994). Alkoxy radicals can also react with NO and N02, but under ambient tropospheric conditions these reactions are generally of negligible importance. 

Kwok ESC, Arey J, Atkinson R. Alkoxy radical isomerization in the OH radical initiated reaction of C4—Cs n-alkanes. J Phys Chem 1996 100 214-9. 

Much of the remaining chemistry involves attack at the isobutyl CH group leading, via alkoxy radical isomerization, to 2-hydroxy-2-methylpropyl acetate ... 

The reactions of R02 with NO and with R02 generate alkoxy radicals (RO). Alkoxy radicals have several possible atmospheric fates, depending on their particular structure. These include reaction with 02, decomposition, and isomerization as we shall see, reactions with NO and N02 are unlikely to be important under most tropospheric conditions. Atkinson et al. (1995b) and Atkinson (1997b) have reviewed reactions of alkoxy radicals and /3-hydroxyalkyl radicals ... 

If the alkoxy radical has four or more carbon atoms and can form a 6-membered transition state via the alkoxy oxygen abstracting H from a C-H bond, an intramolecular isomerization may occur ... 

Table 6.7 compares the estimated rates of reaction with 02, decomposition, and isomerization for some alkoxy radicals with different structures. It is important to recognize that there is a great deal of uncertainty in many of these estimates, and this is an area that clearly requires more research. However, given these caveats, it is clear that where isomerization is possible, it usually predominates at room temperature. (Note, however, that this will be slower at the lower temperatures found at higher altitudes.) When isomerization is not feasible, e.g., for the smaller alkoxy radicals or for branched species, reaction with 02 is always significant and usually predominates. 

The alkoxy radical, RO, generated by R02 reactions can react with 02, decompose, or undergo an isomerization, depending on the structure of the radical. At very high NO, concentrations for a few alkoxy radicals, reactions with NO and N02 may also occur to a small extent. Figure 6.2 summarizes these possibilities. 

For the /3-hydroxyalkoxy radicals formed from alkenes C5 and larger, there is experimental evidence that isomerization starts to dominate (e.g., see Atkinson et al., 1995d Kwok et al., 1996b). Thus, isomerization followed by reaction with 02, NO, etc., ultimately leads to the formation of dihydroxycarbonyl compounds. For the reaction of OH with 1-butene, for example, isomerization of one of the alkoxy radicals ultimately leads to 3,4-dihydroxybutanal in competition with its decomposition and reaction with 02 ... 

The yields of such dihydroxycarbonyl products have been measured to increase continuously from 0.04 for 1-butene to 0.6 for the 1-octene reaction (Kwok et al., 1996b). The low yield for 1-butene reflects the fact that only one of the two possible alkoxy radicals formed can undergo isomerization via a 6-membered transition state. 

Eberhard, J., C. Muller, D. W. Stocker, and J. A. Kerr, Isomerization of Alkoxy Radicals under Atmospheric Conditions, Environ. Sci. Technol., 29, 232-241 (1995). 

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. 

Table 9.2 presents the results of experiments with the 9-decalyl radical, generated as indicated in Scheme 2 from two isomeric precursors.27 In this reaction, the alkyl radicals, which form by fragmentation of the alkoxy radical,... 

The alkoxy radicals formed in pathway Eq. 26a have very interesting atmospheric chemistry. The atmospheric fate of alkoxy radicals differs with the nature of the R group. Some alkoxy radicals (e.g., CH3O) are lost solely via reaction with 02, others undergo rapid decomposition via C-C bond scission. Long chain alkoxy radicals can undergo isomerization via intramolecular H-atom abstraction ... 

The resulting hydroxy alkyl radical then adds 02 to become a hydroxy alkyl peroxy radical which reacts with more NO to give more N02 and another alkoxy radical capable of undergoing further isomerization ... 

The complexity associated with unraveling the precise degradation mechanism of any given alkane can be appreciated by considering the case of n-hexane. Initial OH radical attack leads to the formation of three different alkoxy radicals, each of which can either react with O2, decompose, isomerize (via several possible pathways), or undergo a combination of these possible loss processes. Our understanding of the atmospheric chemistry of alkoxy radicals is rather crude at present and this is an area of active research [49]. 

As discussed in section 4, reaction of the peroxy radicals with N02 gives thermally unstable peroxy nitrates. Reaction with H02 gives hydroperoxides and possibly carbonyl compounds. Reaction with other peroxy radicals (R 02) gives alkoxy radicals, carbonyls, and alcohols. The alkoxy radicals will then either isomerize, react with 02, or decompose (see Sect. 3). Thus, the NO3 radical-initiated atmospheric degradation of alkenes leads to oxiranes (generally in small yield), nitrooxy hydroperoxides, nitrooxy carbonyls, and nitrooxyalcohols. For a detailed listing of products from individual alkenes the reader should consult Calvert et al. [55]. 

By -scission, the alkoxy radical may give a chain ketone (detected at 1725 cm-1) and an end-chain aromatic ketone (1690 cm-1). The -scissions are accompanied by the formation of a benzyl and an alkyl radical. The former is the precursor of benzene and the latter isomerizes to a tertiary radical. This last radical is the precursor, after several reactions, of acetophenone (1690 cm-1), end-chain aliphatic ketone (1725 cm-1), end-chain aliphatic acid (1710-1753cm-1) and acetic and formic acids (1710cm-1). 

Two different cases may occur. If this radical is formed in a succession of styrene units (1), it reacts in the same way as in PS. If it is formed on a styrene unit linked to an acrylonitrile unit (2), three reaction pathways may be envisaged. The alkoxy radical resulting from the decomposition of the hydroperoxide formed on this polystyryl radical may react by 3-scission. Scissions (a) and (b) yield chain ketones, acetophenone end-groups and phenyl and alkyl radicals as previously observed in the case of PS photooxidation mechanism. Scission (c) leads to the formation of an aromatic ketone and an alkyl radical. This alkyl radical may be the precursor of acrylonitrile units (identified by IR spectroscopy at 2220 cm-1), or may react directly with oxygen and after several reactions generates acid groups, or finally this radical may isomerize to a more... 

A similar application of a vinylcyclopropane in a stereoselective photoreaction has been repotted by Dtirr and coworkers. Scheme S shows the addition of a bicyclo[3.1.0]hexenol derivative and acetophenone, which afforded the strained 7-oxatricyclo[4.2.0.0]octane and a comparable amount of tetraphe-nylcyclopentadiene. Since the vinylcyclopropane is known to undergo photochemical isomerization and photofragmentation to the cyclopentadiene, it is conceivable that these reactions precede photocycloaddition. If this is the case, the major oxetane (4 1 selectivity) arises from addition of the alkoxy radical to... 

The alkyl radical initially formed reacts readily with oxygen to give the corresponding alkylperoxy radical, which may abstract hydrogen from a fuel molecule to form the alkylhydroperoxide or alternatively decompose to yield an aldehyde and an alkoxy radical. Some workers thought that this decomposition was preceded by an isomerization of the alkylperoxy radical, the activation energy of which had been estimated by Semenov [3] to be ca. 20 kcal. mole. Shtern was of the opinion that the major, if not the only, fate of the alkylperoxy radical was decomposition, but in contrast to other workers he believed that it must involve scission of a C—C bond and could not lead to the formation of a carbonyl compound and hydroxyl radical. 

The pyridinethiones (197) are a useful source of alkoxy radicals upon irradiation. The bond fission processes are brought about by irradiation at 350 nm and the resultant radicals have been demonstrated to react effectively with guanine. A study of the isomeric iV-hydroxypyridine-4-thione has also been reported. N-O bond fission has been studied in iV-phenylhydroxy-lamine. Derivatives of this type have been used frequently as sources of free radicals. For example, the Barton esters (198) have been used to form a-amido radicals. The kinetics of rotation around the C-N bond within these have been measured. Other studies have examined cyclisations within radicals produced from Barton esters. ... 

The resulting alkoxy radicals react under tropospheric conditions via a variety of processes unimolecular decomposition, unimolecular isomerization, or reaction with 02. Alkoxy radicals with fewer than five carbon atoms do not undergo isomerization for these, the competitive processes are unimolecular decomposition versus reaction with 02. The general alkoxy radical—02 reaction is... 

Alkoxy radicals that arise from the reaction of NO with alkylperoxy radicals also may enter into several competing processes. Four such reactions must be considered thermal decomposition, isomerization, reaction with oxygen, and addition to either NO or N02. Falls and Seinfeld (1978) have presented a brief review of the various possibilities. Table 6-13 summarizes current information on rate coefficients and projected rates in the atmosphere for several small alkoxy radicals. 

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