Linolenic ethane

Figure 2. Mechanism for the Formation of 2-Pentenal, Propanal, 2-Propanone and Ethanal from the Thermal Degradation and Retro-Aldol Condensation of Linolenic Acid.

Alkanes, e.g., pentane, produced by this mechanism as an end-product of the oxidation of linoleic and arachidonic acid, and ethane from linolenic acid (Fig. 2.13) (Tappel and Dillard, 1981). 

Recent experiments with pea chloroplasts illuminated in the absence of an electron acceptor have shown that both chlorophyll and linolenic acid breakdown was retarded by the singlet quenchers DABCO and crocin (23), Further work showed that linolenic acid breakdown and ethane generation in isolated chloroplast thylakoids was promoted by the addition of the singlet oxygen generator rose bengal immobilized on DEAE-sepharose (24). 

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. 

The short-chain hydrocarbons include alkanes, alkenes and alkynes. Ethane, ethene, pentane and pentene are sometimes present in exhaled air and these represent oxidation products of linolenate (C2 compounds) and linoleate (C5 compounds). 

The first stage of autoxidation of a pure oil is easily traceable by applying conventional wet chemistry, for example determination of the peroxide value. Peroxides are known to be heat labile compounds, which undergo breakdown at elevated temperatures to form simple hydrocarbons. Thus ethane and pentane are the predominant breakdown products of linolenate and linoleate peroxides, respectively (Evans et al., 1967). Scholz and Ptak (1966) and Evans et al. (1969) proposed a gas chromatographic method to measure rancidity in edible oils whereby the undiluted oil is injected directly into the hot injector (250°C). At these temperatures lipid peroxides are... 

It should be noted that the breakdown of peroxidized linolenic acid (Lieberman and Mapson, 1964 Dumelin and Tappel, 1977) or propanal (Baur and Yang, 1969) by the free-radical mechanism is accompanied by the formation of ethane ... 

However, in ethylene-producing plant tissue no significant amount of ethane production is observed. Direct evidence that this reaction does not occur in plants came from the observation that neither linolenic acid nor propanal was converted to ethylene in plant tissues (Baur and Yang, 1969b Mapson... 

As observed with the monohydroperoxides (Section D), thermal decomposition of the hydroperoxy bicyclo-endoperoxides from methyl linolenate produced more complex mixtures of volatile compounds than acid decomposition with acidic methanol. The thermal decomposition products included methyl 9-oxononanoate, propanal, 2,4-heptadienal, methyl octanoate, methyl 13-0X0-9,11-tridecadienoate and ethane (Figure 4.25X The acid decomposition products, analysed as the di- and tetramethyl acetals, comprised only propanal, methyl 9-oxononaoate, and malonaldehyde. As with the monohydroperoxides, by thermal decomposition the bicyclo-endoperoxides are cleaved on either side of the hydroperoxide group, whereas by acid decomposition they are cleaved only between the hydroperoxide group and the... 

A large proportion of the volatiles identified in vegetable oils are derived from the cleavage reactions of the hydroperoxides of oleate, linoleate, and linolenate (Section D). A wide range of hydrocarbons (ethane, propane, pentane and hexane) appears to be formed in soybean oil oxidized to low peroxide values. A number of volatiles identified in vegetable oils that are not expected as primary cleavage products of monohydroperoxides include dialdehydes, ketones, ethyl esters, nonane, decane, undecane, 2-pentylfuran, lactone, benzene, benzaldehyde and acetophenone. Some of these volatiles may be derived from secondary oxidation products, but the origin of many volatiles still remains obscure. However, studies of volatile decomposition products should be interpreted with caution, because the conditions used for isolation and identification may cause artifacts, especially when fats are subjected to elevated temperatures. 

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