Criegee biradical

Grosjean, D. Gas phase reaction of ozone with 2-methyl-2-butene dicarbonyl formation from Criegee biradicals, Environ. ScL Technol, 24(9) 1428-1432, 1990. 

In the liquid phase, the Criegee intermediates have been assumed to be zwitterions and hence the term Criegee zwitterion is commonly used. In the gas phase, the structure is usually written as a biradical (although it may really be more zwitterionic in character e.g., see Cremer et al., 1993). Hence Criegee biradical is frequently used for this gaseous intermediate. Sander (1990), Bunnelle (1991), and Cremer et al. (1993) give a more detailed discussion of the structure and properties of the Criegee intermediate. 

While OH is the major gas-phase oxidant for S02, Criegee biradicals may also contribute. This is so particularly at night when OH concentrations are small but significant concentrations of 03 and alkenes may exist, generating the Criegee intermediate (see Chapter 6.E.2). 

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 mechanism of the reaction of 03 with alkenes was discussed in detail in Chapter 6.E.2. It was seen there that these reactions serve as an indoor source of OH through decomposition of the Criegee biradical. Weschler and Shields (1996b) proposed that the indoor reaction of 03 with alkenes could serve as a source of OH and calculated that at 20 ppb 03 and average indoor alkene concentrations, a steady-state OH concentration of about 2 X 105 molecules cm 3 might be expected. 

Criegee biradical (H2COO ) CRIG Isoprene OH-adduct IS03... 

Methyl Criegee biradical (CH3(H)C60 ) MCRG Dinitrate of isoprene DISN... 

The reaction of ozone with alkenes proceeds via addition to the double bond giving a short lived energy-rich ozonide, which decomposes via C - C bond scission to give carbonyl compounds and biradical species known as Criegee biradicals ... 

Criegee biradicals formed in the reaction of ozone with alkenes carry significant internal excitation and the majority of them decompose before they can be collisionally stabilized. The biradicals are sufficiently excited that many different rearrangements and decomposition pathways are energetically allowed. For example, in the case of the [CH3C( )H00( )] species formed during propene oxidation the IUPAC panel has recommended the following channels (and yields) [24] ... 

In summary, the reaction of ozone with alkenes is important in the atmospheric degradation of alkenes. In all cases the reaction leads to rupture of the > C = C < double bond. The double bond is replaced by a carbonyl group on one side and a Criegee biradical on the other. The Criegee biradical is formed energetically excited and decomposes by a variety of different routes to give a complex mixture of oxygenated products (mainly carbonyls). 

The major uncertainty concerns the fate under atmospheric conditions of the initially energy-rich Criegee biradical, which can be collisionally deactivated or can undergo unimolecular decomposition ... 

The decomposition pathways of Criegee biradicals are generally well established for the first three compounds in the series (Horie and Moortgat, 1992) ... 

The kinetics and products of the gas-phase alkene-03 reaction have been studied extensively (Atkinson et al. 2000) and are reasonably well understood for a large number of the smaller alkenes. The major mechanistic issue concerns the fate, under atmospheric conditions, of the initially energy-rich Criegee biradical, which can be collisionally stabilized or can undergo unimolecular decomposition ... 

Let us consider as an example the reaction of cyclohexene with ozone in the atmosphere. This reaction has been studied in laboratory chamber experiments by Kalberer et al. (2000). A potential reaction mechanism is depicted in Figure 14.11. The first steps of the reaction are the formation of an initial molozonide M, its transformation to a peroxy radical intermediate, and then to a dioxyrane-type intermediate, and finally to an excited Criegee biradical [CHO(CH2)4CHO O ]. A series of reactions then lead from the Criegee biradical to stable products, some with five and some with six carbon atoms (Figure 14.11). This rather complicated series of reactions leads to the stable gas-phase products listed in Table 14.10. 

The knowledge of the chemistry of the Criegee biradicals R R COO is very poor the branching ratios kjjkn, the rate constants 4, and the product distributions of reactions (2) and (4) are largely uncertain. 

Followed by the reactions of hydroxyethyl-substituted Criegee biradical ... 

HO2 addition to CH2O. In the atmosphere it can also be formed through the reaction of the Criegee biradical (CH2OO) with water vapor (Neeb et al., 1997). Ethene and all alkenes of the structure, CH2=CR2, form the CH2OO biradical on reaction with ozone. Hellpointner and Gab (1989) have reported the detection of this compound and other peroxides in the atmosphere. 

The MCM mechanism for isoprene also includes reactions of NO3 and of ozone. With NO3, a single site of addition is included (the 1-position, associated with formation of the peroxy radical in the 4-position). Four channels are included for reaction with ozone, which lead to methacrolein (0.3), methyl vinyl ketone (0.2) and the associated Criegee biradical CH2OO, and CH2O and the associated Criegee biradicals derived from methacrolein (0.3) and methyl vinyl ketone (0.2), where the numbers in brackets refer to the channel yields. 

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