Alkanes ozonation

The radicals produced undergo a similar reaction sequence to that of the alkanes. Ozone adds across the double bond forming an unstable ozonide that breaks... 

Ozone can be used to completely oxidize low concentrations of organics in aqueous streams or partially degrade compounds that are refractory or difficult to treat by other methods. Compounds that can be treated with ozone include alkanes, alcohols, ketones, aldehydes, phenols, benzene and its derivatives, and cyanide. Ozone readHy oxidizes cyanide to cyanate, however, further oxidation of the cyanate by ozone proceeds rather slowly and may require other oxidation treatment like alkaline chlorination to complete the degradation process. 

Alkenes are reduced by addition of H2 in the presence of a catalyst such as platinum or palladium to yield alkanes, a process called catalytic hydrogenation. Alkenes are also oxidized by reaction with a peroxyacid to give epoxides, which can be converted into lTans-l,2-diols by acid-catalyzed epoxide hydrolysis. The corresponding cis-l,2-diols can be made directly from alkenes by hydroxylation with 0s04. Alkenes can also be cleaved to produce carbonyl compounds by reaction with ozone, followed by reduction with zinc metal. 

The haloalkanes (also called alkyl halides) are alkanes in which at least one hydrogen atom has been replaced by a halogen atom. Although they have important uses, many haloalkanes are highly toxic and a threat to the environment. The haloalkane 1,2-dichlorofluoroethane, CHC1FCH2C1, is an example of a chlorofluorocarbon (CFC), one of the compounds held responsible for the depletion of the ozone layer (see Box 13.3). Many pesticides are aromatic compounds with several halogen atoms. 

The concentration of NO determines the relative importance of reaction 3, and the formation of NO2 is particularly significant since this is readily photolyzed to produce 0( P) that reacts with oxygen to produce ozone. This alkane-NO reaction may produce O3 at the troposphere-stratosphere interface ... 

Clearly, whether or not ozone is formed depends also on the rate at which, for example, unsaturated hydrocarbons react with it. Rates of reactions of ozone with alkanes are, as noted above, much slower than for reaction with OH radicals, and reactions with ozone are of the greatest significance with unsaturated aliphatic compounds. The pathways plausibly follow those involved in chemical ozonization (Hudlicky 1990). 

TS-1 is a material that perfectly fits the definition of single-site catalyst discussed in the previous Section. It is an active and selective catalyst in a number of low-temperature oxidation reactions with aqueous H2O2 as the oxidant. Such reactions include phenol hydroxylation [9,17], olefin epoxida-tion [9,10,14,17,40], alkane oxidation [11,17,20], oxidation of ammonia to hydroxylamine [14,17,18], cyclohexanone ammoximation [8,17,18,41], conversion of secondary amines to dialkylhydroxylamines [8,17], and conversion of secondary alcohols to ketones [9,17], (see Fig. 1). Few oxidation reactions with ozone and oxygen as oxidants have been investigated. 

TABLE 2.1 Oxidation of Alkanes with Ozone Catalyzed by Li12[Mnn2-ZnW(ZnW9034)2] in 40 percent t-BuOH-Water ... 

Nolting F, Behnke W, Zetzsch C. 1988. A smog chamber for studies of the reactions of terpenes and alkanes with ozone and OH. Journal of Atmospheric Chemistry 6 47-59. 

Nirmalakhandan, N.N., Speece, R.E. (1988b) QSAR model for predicting Henry s law constant. Environ. Sci. Technol. 22,1349-1357. Nolting, F., Behnke, W., Zetzsch C. (1988) A smog chamber for studies of the reactions of terpenes and alkanes with ozone and OH. J. Atmos Chem. 6, 47-59. 

As the reaction temperature is increased, chemiluminescence is observed in the reactions of ozone with aromatic hydrocarbons and even alkanes. Variation of temperature has been used to control the selectivity in a gas chromatography (GC) detector [35], At room temperature, only olefins are detected at a temperature of 150°C, aromatic compounds begin to exhibit a chemiluminescent response and at 250°C alkanes respond, giving the detector a nearly universal response similar to a flame ionization detector (FID). The mechanisms of these reactions are complex and unknown. However, it seems likely that oxygen atoms produced in the thermal decomposition of ozone may play a significant role, as may surface reactions with 03 and O atoms. 

Ozone also reacts with ethane in the gas phase at room temperature. Rather than a direct molecular reaction, however, evidence points to the initiation of radical-chain reactions by the very small O-atom concentrations present in ozone at room temperature. Added oxygen scavenges the radicals and slows the build-up, leading to induction periods which may be in excess of 3 h. Recent advances in mechanistic investigations of gas-phase ozonolysis of alkanes have been reviewed. Oligomeric peroxides dominate the products of oxidation of nitrotoluenes with ozone in acetic acid. °... 

The first thing that stands out in Table 6.2 is that the OH-CH4 rate constant, 6.2 X 10 15 cm3 molecule 1 s-1, is much smaller than those for the higher alkanes, a factor of 40 below that for ethane. This relatively slow reaction between OH and CH4 is the reason that the focus is on non-methane hydrocarbons (NMHC) in terms of ozone control in urban areas. Thus, even at a typical peak OH concentration of 5 X 106 molecules cm 3, the calculated lifetime of CH4 at 298 K is 373 days, far too long to play a significant role on urban and even regional scales. Clearly, however, this reaction is important in the global troposphere (see Chapter 14.B.2b). 

Because CNG is primarily methane, it is expected to have relatively low reactivity, with the small amounts of reactive impurities such as small olefins and alkanes being responsible for most of its reactivity (see Table 16.14). Emissions of CO are smaller than from gasoline-powered vehicles, while the effect on NOx emissions is not clear (National Research Council, 1991). As seen in Tables 16.10 and 16.11, CNG shows the highest promise for low-reactivity exhaust emissions, and this appears to be the case for its use in real vehicles (Gabele, 1995). Figure 16.40, for example, shows the estimated ozone production per mile traveled for a vehicle fueled on CNG compared to vehicles fueled on reformulated gasoline (RFG) or the alcohol fuels M85 or E85 (vide infra). These measurements and estimates based on them include the contributions from both exhaust (including CO) and evaporative emissions (Black et al., 1998). Clearly, the reactivity of the CNG-powered vehicle emissions was substantially smaller than for the other vehicle-fuel combinations. 

CFCs gained a lot of attention in the 1980s for their role in damaging the ozone layer. CFCs are synthetic alkanes with chlorine and fluorine attached. They were widely used as refrigerants and propellants (in spray cans) for many decades until scientists realized they were harming the planets atmosphere and possibly changing its climate. 

Grosjean, E., J. Bittercourt de Andrade, and D. Grosjean, Carbonyl products of the gas-phase reaction of ozone with simple alkanes , Environ. Sci. Technol., 30, 975-983 (1996a). 

Ozone reacts slowly with saturated hydrocarbons usually to give alcohols.92 93 The reactivity of alkanes toward ozone is several orders of magnitude less than that of alkenes. Oxidation of saturated hydrocarbons takes place preferentially at the tertiary carbon. In liquid-phase ozonation94 the order of reactivity of the primary, secondary and tertiary C—H bonds is 1 13 110. The formation of tertiary alcohols occurs with high degree (60-94%) of stereoselectivity.94-96... 

Branched acyclic alkanes also exhibit a slightly different behavior toward ozone on silica gel.105 Although tertiary alcohols are usually the main products, C—C bond cleavage to yield ketones always occurs and may become the predominant reaction. Atomic oxygen generated by microwave discharge of a C02/He mixture is a more selective reagent in the transformation of these compounds to tertiary alcohols.106... 

The cyclopropane ring is known to exhibit high stability toward ozone, and oxidation of the secondary C—H bond in the a position to the ring takes place.104 In contrast, highly strained bicyclo[n.l.O]alkanes undergo C—C bond cleavage in the cyclopropane ring 208... 

Carbon—carbon bond cleavage is also characteristic of branched saturated hydrocarbons reacting with ozone in superacid media.1 Depending on the structure of the reacting alkanes, different mechanisms can be operative. 

Chlorinated alkanes were used in the production of chlorofluoroalkanes (CFCs).188 Because of their atmospheric ozone-depleting ability, the use of CFCs is being phased out. Hydrochlorofluorocarbons, containing at least one hydrogen atom (HCFCs), are introduced as safer alternatives.189 190 In the long run, however, hydrofluorocarbons (HFCs) will be the ideal substitutes to CFCs.190... 

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