Oxidation Reaction Mechanism of Alkenes by OH

The initial reactions of alkenes and OH are the addition, and they are mostly in the high-pressure limit under atmospheric conditions including ethylene as seen in Chap. 5, Sect. 5.2.8. The addition reaction forms p-hydroxyalkyl radicals which have an OH group on the carbon adjacent to the carbon atom with an unpaired electron. 

For asymmetric alkenes with carbon number C3, there is a possibility that the OH adds to either end of the double bond as shown above. It is known that the formation of a secondary radical by the addition of OH to the end carbon like reaction (7.26) predominates, and the ratios of the reaction type (7.26) and (7.27) is ca. 65 and 35 %, respectively in case of propylene (Finlayson-Pitts and Pitts 2000 Calvert et al. 2000). 

CH3CH2CH-CH2OH + N02 CHjCHsCHtONOstCHsOH CH3CH2CH(OH)-CH2 + NOj OOCHjCHsCHtOHj-CHjOH 0 I i I 

The hydroxyalkoxy radicals formed in pathway (k) can take place isomerization by intramolecular shift of H-atom via a six-membered ring as in the case of the alkoxy radical in the previous paragraph. Dihydroxyl aldehyde is then formed in pathways (q), (r), (s) though dihydroxyl radicals which has two OH groups in a molecule. The formation of dihydroxyl aldehyde has been confirmed in the laboratory experiment, and the yield of dihydroxyl aldehyde (3,4-dihydroxyl butanal) is 0.04 for 1-butene but it is as high as 0.6 for 1-octene (Kwok et al. 1996b). Under the low NOx concentrations, a part of hydroxyperoxy radicals react with HO2, and is known to produce hydroxyhydroperoxy butane (pathways (f), (m)) (Hatakeyama et al. 1995 Tuazon et al. 1998). 

As for the pathways of O3 and alkene reactions, detailed explanation has been made for C2H4 in Chap. 5, Sect. 5.4.3, and the similar reaction mechanisms are basically considered for other atkenes. In the case of asymmetric chain alkenes, two reaction pathways can be considered depending on either end of double bond forms carbonyl oxide (Criegee intermediate) when the primary ozonides rupture corresponding to reaction (5.56). 

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