Olefin ozonation

FIGURE 3-6 Mass concentration of aerosol formation from olefin-ozone reaction as a function of olefin concentration, r, residence time in the flow reactor. Reprinted with permission from Burton et ai. 

The same kinetic scheme can be used for the most general situation involving multicompetitive olefin-ozone reactions ... 

Synthetic operations involving ozonolysis lead to formation of aldehydes, ketones or carboxylic acids, as shown in Scheme 16, or to various peroxide compounds, as depicted in Scheme 7 (Section V.B.5), depending on the nature of the R to R substituents and the prevalent conditions of reaction no effort is usually made to isolate either type of ozonide, but only the final products. This notwithstanding, intermediates 276 and 278 are prone to qualitative, quantitative and structural analysis. The appearance of a red-brown discoloration during ozonization of an olefin below — 180°C was postulated as due to formation of an olefin-ozone complex, in analogy to the jr-complexes formed with aromatic compounds however, this contention was contested (see also Section V1I.C.2). 

Formation of a jr-complex between 1-hexene or 2,4,4-trimethylpent-l-ene and ozone was tentatively proposed based on the appearance of a sharp band at ca 980 cm , assigned to one of the ozone vibrations . However, the existence of olefin-ozone jt-complexes has been contested . 

The chemistry of the ozonization of olefins has been reviewed (I, 2, 5). Although the exact details of the mechanism(s) have not been elucidated, the Criegee zwitterion mechanism plays an important role in olefin ozonization. According to this mechanism, ozonization occurs in the manner shown in Figure 2. 

T he role of olefin-ozone (O3) reactions in generating aerosols from organic vapors and the influence of oxygen (O2) and water vapor on aerosol production in these reactions have been studied here. Kinetic data were examined to try to derive a mechanism for forming aerosols from organic vapors. 

The yield data in Figures 3 and 4 can be rationalized on similar grounds. In the cis case the initial olefin-ozone adduct can be diverted to 3-heptene ozonide by butyraldehyde, and by doing so can eliminate a precursor to 3-hexene ozonide. This process continues to operate throughout a wide range of aldehyde concentrations with the total ozonide... 

The fate of 0 (if it is ever produced in natural waters corresponding to OH) is transformation into the ozonide anion under oxide conditions (5.127), which is an important intermediate in alkaline solution with a lifetime of about 10 s. Ozonide anion (not to be mixed up with the olefin-ozone adduct. Chapter 5.7.4) is easily produced through electron transfer onto dissolved ozone the electron affinity of O3 is several times (2.1 eV) that of O2 (0.44 eV Pichat et al. 2000, Addamo et al. 2005). 

More precisely, the rate of ozone formation depends closely on the chemical nature of the hydrocarbons present in the atmosphere. A reactivity scale has been proposed by Lowi and Carter (1990) and is largely utilized today in ozone prediction models. Thus the values indicated in Table 5.26 express the potential ozone formation as O3 formed per gram of organic material initially present. The most reactive compounds are light olefins, cycloparaffins, substituted aromatic hydrocarbons notably the xylenes, formaldehyde and acetaldehyde. Inversely, normal or substituted paraffins. 

Oxidation. Olefins in general can be oxidized by a variety of reagents ranging from oxygen itself to ozone (qv), hydroperoxides, nitric acid (qv), etc. In some sequences, oxidation is carried out to create a stable product such as 1,2-diols or glycols, aldehydes, ketones, or carboxyUc acids. In other... 

Ozonation ofAlkenes. The most common ozone reaction involves the cleavage of olefinic carbon—carbon double bonds. Electrophilic attack by ozone on carbon—carbon double bonds is concerted and stereospecific (54). The modified three-step Criegee mechanism involves a 1,3-dipolar cycloaddition of ozone to an olefinic double bond via a transitory TT-complex (3) to form an initial unstable ozonide, a 1,2,3-trioxolane or molozonide (4), where R is hydrogen or alkyl. The molozonide rearranges via a 1,3-cycloreversion to a carbonyl fragment (5) and a peroxidic dipolar ion or zwitterion (6). 

Ozonation of Aromatics. Aromatic ring unsaturation is attacked much slower than olefinic double bonds, but behaves as if the double bonds in the classical Kekule stmctures really do exist. Thus, benzene yields three moles of glyoxal, which can be oxidized further to glyoxyUc acid and then to oxahc acid. Substituted aromatics give mixtures of aUphatic acids. Ring substituents such as amino, nitro, and sulfonate are cleaved during ozonation. 

Thermally unstable cycHc trioxides, 1,2,3-trioxolanes or primary o2onides are prepared by reaction of olefins with o2one (64) (see Ozone). Dialkyl trioxides, ROOOR, have been obtained by coupling of alkoxy radicals, RO , with alkylperoxy radicals, ROO , at low temperatures. DiaLkyl trioxides are unstable above —30° C (63). Dialkyl tetraoxides, ROOOOR, have been similarly produced by coupling of two alkylperoxy radicals, ROO , at low temperatures. Dialkyl tetraoxides are unstable above —80°C (63). 

Oxidation of Straight-Chain 1-Olefins. Oxidation of a-olefins has been thoroughly studied using ozone, peracids, nitric acid, chromic acid, and others. 

Polyisobutylene has the chemical properties of a saturated hydrocarbon. The unsaturated end groups undergo reactions typical of a hindered olefin and are used, particularly in the case of low mol wt materials, as a route to modification eg, the introduction of amine groups to produce dispersants for lubricating oils. The in-chain unsaturation in butyl mbber is attacked by atmospheric ozone, and unless protected can lead to cracking of strained vulcanizates. Oxidative degradation, which leads to chain cleavage, is slow, and the polymers are protected by antioxidants (75). 

Like NR, SBR is an unsaturated hydrocarbon polymer. Hence unvulcanised compounds will dissolve in most hydrocarbon solvents and other liquids of similar solubility parameter, whilst vulcanised stocks will swell extensively. Both materials will also undergo many olefinic-type reactions such as oxidation, ozone attack, halogenation, hydrohalogenation and so on, although the activity and detailed reactions differ because of the presence of the adjacent methyl group to the double bond in the natural rubber molecule. Both rubbers may be reinforced by carbon black and neither can be classed as heat-resisting rubbers. 

Ozone cracking is a physicochemical phenomenon. Ozone attack on olefinic double bonds causes chain scission and the formation of decomposition products. The first step in the reaction is the formation of a relatively unstable primary ozonide, which cleaves to an aldehyde or ketone and a carbonyl. Subsequent recombination of the aldehyde and the carbonyl groups produces a second ozonide [58]. Cross-linking products may also be formed, especially with rubbers containing disubstituted carbon-carbon double bonds (e.g. butyl rubber, styrene-butadiene rubber), due to the attack of the carbonyl groups (produced by cleavage of primary ozonides) on the rubber carbon-carbon double bonds. 

Among the more exotic methods which have been used for the direct epoxidation of steroid olefins are chromic acid, ozone, e.g., (84), and photochemical oxygenation. Ozone is useful for the epoxidation of the unreactive 8,9-olefin, but the results of the other unusual methods can usually be duplicated by the methods of epoxidation discussed above. 

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