Criegee radicals

The percentages for the ultimate products are taken from the product distribution reported by Herron and Huie (1977). They refer to comparatively low pressures, but other authors, specifically Scott et al. (1957), have found a similarly low yield of formic acid also at higher pressures. Thus, it appears that in the atmosphere decomposition is the main fate of the Criegee radical. 

The reaction of propene with ozone leads to the formation of the Criegee radicals H2COO- and CH3CHOO In this case the applicability of the Criegee mechanism is supported by the formation of secondary ozonides in a reaction of Criegee radicals with aldehydes, such as... 

Another pathway in the formation of HO2 and probably direct H2O2 is via the ozonolysis of alkenes, which has been investigated for many years. However, several open questions remain about the initial and product relationship. It is well known that the excited Criegee radical can produce OH and HO2 radicals (Neeb et al. 1998). The stabilized Criegee radical can react with water vapor to form a-hydroxy-alkyl peroxides (RR C(OH)OOH) Gab et al. 1985, Becker et al. 1993). Recently, it... 

The produced OH and HO2 (as a subsequent product from H) react additionally with the alkenes and provide a huge spectrum of products. Most important is SOA formation (known as a blue haze from biogenic emissions). The stabilized Criegee radical reacts with major species (H2O, SO2, NO, NO2, CO, RCHO and ketones). In the reaction with water vapor direct H2O2 can also be formed (Sauer et al. 1999). 

The reaction of CH3O2 with halogen atoms X (X = F, Cl, Br) constitutes an interesting source of Criegee radical formation in flow systems. 

Alkenes can also be oxidized by ozone. The addition of O3 to the double bond forms an unstable ozonide, which decomposes to yield an aldehyde and a Criegee radical, for example... 

The Criegee radicals contain excess energy and decompose to yield a variety of fragments unless the radicals are stabilized by collisions. At atmospheric pressure, about 50% of the radicals are stabilized and react predominantly with water to yield acids, for example... 

The kinetics of the reactions of acrylic acid (AcA) and methacrylic acid (MAcA) with ozone have been studied by Neeb et al. (1998), see table VI-D-2. The studies were carried out in excess ozone, and the pseudo first-order decay of the acids was monitored using FTIR spectroscopy. Final kinetic data were obtained in the presence of added cyclohexane (an OH scavenger) and formic acid (a Criegee radical scavenger). Values reported are as follows (03 AcA) = (0.65 0.13)x 10 cm molecule" s and... 

Peroxyeslers may undergo non-radical decomposition via ibe Criegee rearrangement (Scheme 3.35). This process is analogous to the earboxy inversion process described for diacyl peroxides (see 3.3.2.1,3) and probably involves ionic intermediates. 

The ozonolysis of ethylene in the liquid phase (without a solvent) was shown to take place by the Criegee mechanism.This reaction has been used to study the structure of the intermediate 16 or 17. The compound dioxirane (21) was identified in the reaetion mixture at low temperatures and is probably in equilibrium with the biradical 17 (R = H). Dioxirane has been produced in solution but it oxidatively cleaves dialky] ethers (such as Et—O—Et) via a chain radical process, so the choice of solvent is important. 

The mechanism of decomposition of the Criegee intermediates is believed to occur via several reaction channels shown for the [(R,CH2)(R2)CHOO] Criegee intermediate in Fig. 6.4. The oxygen-atom elimination channel for simple alkenes is not believed to be important. However, the ester and hydroperoxide channels are important and explain the production of free radicals such as OH. Theoretical calculations have shed some light on this (e.g., Gutbrod et al., 1996, 1997a ... 

One can estimate the relative contribution of the Criegee intermediate (Cl) to SOz oxidation in the gas phase in the troposphere. The absolute value of the rate constant for the reaction of the Cl with SOz is not known, with estimates ranging from 1.7 X 10 " to 3 X 10 15 cm2 molecule-1 s-1 (Hatakeyama and Akimoto, 1994). Using the highest value and a concentration of the Cl of 1 X 105 molecules cm"3, one obtains 10"6 s"1 for the first-order rate of removal of S02 by this reaction. This can be compared to the rate of removal of SOz by reaction with 1 X 106 OH radicals cm3, which is also 10 6 s-1 using the effective bimolecular rate constant cited earlier. Using the lower estimates for the CI-S02 rate constant, which is more reasonable, would lower its contribution proportionately. 

Artifacts can also arise from reaction of the sorbent with components of the airstream, such as 03 and N02. For example, Clausen and Wolkoff (1997) observed a number of products from the reactions of 03 with Tenax, such as acetophenone, decanal, and benozic acid. Interestingly, 2,6-diphenyl-p-hydroquinone was generated when limonene was also present and was suggested to arise from the reaction with the Tenax of the radicals or the Criegee biradical generated in the ozone-limonene reaction. They also observed reactions of NOz with the Tenax sorbent. 

The H atoms formed in reaction 15a can react with 02 (reaction 11) to form H02. The stabilized Criegee intermediate (CH200) can participate in further reactions, some of which will result in the formation of peroxy radicals. Larger alkenes react with ozone to produce organic peroxy radicals. 

The selective oxidation of cellulose to dialdehyde by sodium periodate is well known. It has been postulated by Criegee (74) and by Waters (73) that this reaction proceeds by a free radical mechanism. Toda (76) and Morimoto, Okada, Okada, and Nakagawa (77) have concluded that sodium periodate oxidation should initiate graft polymerization. They succeeded in grafting methyl methacrylate and acrylonitrile onto cellulose substrates, such as rayon and paper. A similar procedure is recommended in a patent of Chemische Werke Huels (78) to graft vinyl monomers onto cotton, polyethylene oxide, copolymers of vinyl chloride-vinyl acetate, and others. 

In 1944 Criegee observed that trans-9-decalyl peroxyesters rearrange on standing to 1,6-epoxycyclodecyl esters.172 Further study of the reaction showed that it has the characteristics of an ionic rather than a radical pathway The rate is... 

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