Aldehydes from alkoxy radicals

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. 

Small alkylperoxy and alkoxy radicals can decompose uni-molecularly, though their rate constants are often in the second-order region. They abstract hydrogen atoms from alkanes, aldehydes, esters, and acids, add to olefins, and may react with 02. Furthermore, interactions with other radicals can lead to disproportionation or combination. These reactions are reviewed, and particular attention is given to CH 02 and CH30 a number of rate constants are estimated. 

For the HO-substituted alkoxy radicals formed from the higher alkenes, Atkinson et al. [112], Akimoto et al. [111], and this group [109] have shown that unimoiecular dissociation, analogous to reaction (34), is dominant over reaction with 02, leading to the formation of aldehydes and HOO. For instance, the reaction sequence for the HO-initiated oxidation of 2-butene in the presence of NO is shown below. 

The hydroxyl groups may be formed along the polymer chain or on its end groups. The carbonyl and aldehyde groups are formed from the scission of alkoxy radicals or by the decomposition of hydroperoxy radicals. The reaction between two polymer alkoxy radicals also produces a carbonyl and hydroxyl group by disproportionation ... 

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. 

A detailed study of the oxidation of propane also led Knox and Norrish [4] to the conclusion that the main oxidation route was via the successive degradation of aldehydes. Their scheme differed from that shown above, however, since it did not include the intermediate formation of an alkylperoxy radical and hydroxyl radicals were thought to be the principal chain carriers rather than the alkyl, alkylperoxy and alkoxy radicals. 

The high oxidation rates of EPA and DHA and the instability of their hydroperoxides caused the rapid formation of secondary products such as volatile aldehydes and other compounds, which, in turn, impart flavor reversion in fish oils (56). The hydroperoxides produced from autoxidation of EPA (73) and DHA (74) have been identified but not quantified. They form eight and ten isomers, respectively. Noble and Nawar (75) analyzed the volatile compounds in autoxidized DHA and identified a number of aldehydes. Most of the aldehydes identified could be explained by the p-scission of alkoxy radicals generated by the homolytic cleavage of each isomer of the hydroperoxides as shown in Figure 9. 

The lowest excited state of simple ketones and aldehydes corresponds to excitation of an electron from the np lone pair to the 7t -MO. The transition is forbidden in compounds of C2v symmetry. The Tocal symmetry is the same in compounds such as acetaldehyde, so that n,7t transitions of ketones and aldehydes are generally weak, 20 50 m 1 cm and they are easily overlooked in absorption spectra or hidden by the red edge of stronger 71,71 absorption. The nature of the lowest excited state is, however, decisive for the photophysical properties and the photochemical reactivity of carbonyl compounds the reactivity of n,7t excited ketones is comparable to that of alkoxy radicals (see below). 

In the same sense that a-alkoxy radicals derived from carbohydrates can be generated and utilized in intramolecular addition reactions, a-amino radicals can be employed by the reduction of A-(a-benzotriazolylalkyl)alkenylamines in syntheses of amino-substituted carbocycles and C-substituted pyrrolidines [20]. In the carbo-cycle synthesis, cu-unsaturated aldehydes are condensed with benzotriazole and secondary amines to provide (a-benzotriazolylalkyl)alkenylamines (Scheme 1). These intermediates rapidly ionize in solution to the corresponding iminium ions... 

Alkoxy-radicals in ring b, generated from hydroxy-derivatives of B-homo-cholestanes with Pb(OAc)4, attack suitably placed C—H bonds to form trans-annular ether bridges. The main reactions are illustrated in Scheme 16. The 7a/ -alcohol gave only a little of the 7a, 19-oxide, the main product being an unsaturated aldehyde of uncertain structure. All the reactions in Scheme 16 occur through favourable conformations of the seven-membered ring. ... 

Mechanism. Degradation of a-linolenic acid (a-lin) as proposed by (29,50,21) is demonstrated in Figure 6. The initial step is a hydrogen abstraction from an a-linolenic molecule by a radical species that was formed as a result of herbicidal action. In the following radical-chain reaction the w-3 alkyl peroxide is formed via the peroxy radical. Subsequently, this peroxide is decomposed in a Fenton-type reaction to the oj-3 alkoxy radical in the presence of transition metals that can undergo one-electron transfer reaction, e.g., Cu(I/II), Fe(Il/III), Ti(IIl/IV), or Ce(III/IV). The w-3 alkoxy radical can split by 8-sclssion to an unsaturated aldehyde and the ethyl radical. The latter is either oxidized to ethylene or reduced to ethane. 

In an analogous manner to the development of decomposition pathways for nitramines, estimates can be made for the decomposition pathways of other classes of energetic materials. For instance, nitrate esters will first break off an NO2 group. From Tables IV and V, one concludes that the resulting alkoxy radical will readily decompose to form an aldehyde, with the subsequent radical eliminating a nitro group, forming another aldehyde. 

The lowest-lying excited state of ketones most often corresponds to a o 7t c=o transition. The maximum of this band is around 280 nm with simple aldehydes or ketones and is shifted to the red for conjugated or aryl derivatives. As hinted above, the unpaired electron on the hq orbital gives to these states electrophilic properties similar to those of alkoxy radicals, and indeed the observed chemistry is similar in the two cases. Typical reactions are a-fragmentation, inter- or intramolecular (from the easily accessible y position) hydrogen abstraction and attack of alkenes (finally resulting in a formal 2h-2 cycloaddition to give an oxetane, the Paterno-Btichi reaction). 

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