Ozonization, aldehydes from mechanism

The complex 100 is calculated to be more stable than the separated aldehyde and carbonyl oxide by 9 kcal mol-1 and the formation of complex 100 from ethylene and ozone is endothermic by only 3.1 kcal mol-1 <1991CPL(187) 491 >. Cycloaddition then leads to the secondary ozonide 101 (1,2,4-trioxolane). subsequent study of the stereochemistry of ozonation reactions using the AMI method provides further support for the modified Criegee mechanism C1997JOC2757, CHEC-III(6.06.2)193>. 

The ozone concentration in the troposphere during the daytime is typically about 1 pphm (parts per hundred million parts of air by volume) [20], Values up to 100 pphm were measured in some photochemical smog areas. The molecular mechanism of the ozone aging of diene based elastomers was studied in detail and is well understood [19,21], Products or intermediates different from those arising in autoxidation or photo-oxidation of polymers were identified ozonides (3), zwitterions (4), diperoxides (5), polyperoxides (6), polymeric ozonides (7) and terminal aldehydes (8). Reactivity of aminic antiozonants (AOZ) with these species accounts for the protection of rubbers against atmospheric 03. AOZ must also possess antioxidant properties, because the free radical processes are concerted with ozonation due to the permanent presence of oxygen. 

Formation of the protective film from ozonized rubber and AOZ creates a barrier against penetration of 03 into rubber [3,4]. Reactions of AOZ with ozonides 3, zwitterions 4 and aldehydic fragments 8 are envisaged. The respective reactivities are lower than the direct ozonation and the relevant contribution to the antiozonant mechanism is therefore inferior to the ozone scavenging. The reactivity of the ozonized rubber with AOZ accounts for rubber chain repairing, classified also as chain relinking or selfhealing mechanisms [4,21,247-249]. This contributes to formation of relaxed surface films. 

These compounds, exemplified by acrolein, crotonaldehyde, and methyl vinyl ketone, are known to react with ozone and with OH radicals. Photolysis and N03 radical reaction are of minor importance. Under atmospheric conditions the 03 reactions are also of minor significance (Atkinson and Carter, 1984), leaving the OH radical reaction as the major loss process. For the aldehydes, OH radical reaction can proceed via two reaction pathways OH radical addition to the double bond and H-atom abstraction from the -CHO group (Atkinson, 1989). For crotonaldehyde, for example, the OH reaction mechanism is given in Fig. 3. As can be noted from Fig. 3, these a,/3-unsaturated aldehydes are expected to ultimately give rise to a-dicarbonyls such as glyoxal and methylglyoxal. For the a,/3-unsaturated ketones such as methyl vinyl ketone, the major... 

---