Ozone alkane oxidation

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. 

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. 

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... 

Here again, the OH radical initiating the reaction is regenerated. In the mechanism shown, three molecules of NO are oxidized to N02 and an equivalent number of ozone molecules is subsequently formed. This raises the total number of product ozone molecules to six. Additional N02 and ozone arises from the photoxidation of formaldehyde, HCHO, produced from acetaldehyde, and from the photooxidation of the ketones occurring as products of alkane oxidation. 

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. 

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

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. °... 

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... 

Although isobutane does not give any oxidation products in the absence of Magic Acid under the same low-temperature ozonization conditions, it was not possible for the authors to determine642 whether formation of intermediate oxidation products, such as alcohols, plays any role in the ozonization of alkanes in Magic Acid. There is no experimental evidence for reactions proceeding via the intermediacy of carbenium ions whether the initial oxidation step of alkanes to alcohols is important. This oxidation, indeed, was found to be extremely slow in the acidic media studied. 

Straight-chain alkanes also efficiently react with ozone in Magic Acid at —78°C in SO2CIF solution. Ethane gave protonated acetaldehyde as the major reaction product together with some acetylium ion (Scheme 5.62). Reaction of methane, however, is rather complex and involves oxidative oligocondensation to terf-butyl cation, which reacts with ozone to give methylated acetone (Scheme 5.63). 

The mechanism and kinetics of the atmospheric oxidation of alkynes, initiated by OH radicals, have been studied particularly to determine the role of alkyne oxidation in tropospheric ozone formation. A general mechanism for OH-initiated oxidation of alkynes has been developed with the aid of thermodynamic calculations. In general, the significance of atmospheric alkynes to the formation of tropospheric ozone was found to be smaller than for alkanes and alkenes, due to the absence of the hydroxy peroxy-forming product channel in the OH-initiated atmospheric oxidation of alkynes.227... 

Photolysis of atmospheric pollutants by solar radiation results in an increase of ozone concentration in certain urban areas and is the cause of a sequence of oxidation reactions with polymers. Ozone reacts with practically all organic materials especially with alkenes. The rate of its reaction with alkene is several orders of magnitude higher than that with alkane. The ratio of the rate constants of ozone with ethene/ethane is 1.5 x 105, with propene/propane 1.6 x 106, and with butene- 1/butane 1.1 x 106, at room temperature [5],... 

By this method (Z)-monounsaturated fatty acids and esters could be obtained with an ( )-isomer content of less than 10% this stereoselectivity being however inferior to that of the commonly used acetylenic approach 55,56). However, the salt-free techniques used today in Wittig reactions allow (Z)-alkenoic acids to be synthesized with less than 2% of the ( )-isomers. Thus, Bestmann et al. prepared methyl and ethyl esters of (Z)-4,5,6,7,8,9,ll- and 13-alkenoic acids of different chain lengths 35,57 62), which served as intermediates in the synthesis of insect pheromones, both by reaction of co-alkoxycarbonyl-substituted alkyl-triphenyl-phosphonium salts with simple alkanals and of co-formylalkanoic esters with alkylidenephosphoranes. As the starting material for the synthesis of -substituted alkyl-phosphonium salts co-chloro- and -bromocarboxylic esters were used. The corresponding -substituted aldehydes can usually be obtained by ozone cleavage of suitable olefin derivatives or by oxidation of alkohols 57,58). 

Gasoline hydrocarbons volatilized to the atmosphere quickly undergo photochemical oxidation. The hydrocarbons are oxidized by reaction with molecular oxygen (which attacks the ring structure of aromatics), ozone (which reacts rapidly with alkenes but slowly with aromatics), and hydroxyl and nitrate radicals (which initiate side-chain oxidation reactions) (Stephens 1973). Alkanes, isoalkanes, and cycloalkanes have half-lives on the order of 1-10 days, whereas alkenes, cycloalkenes, and substituted benzenes have half- lives of less than 1 day (EPA 1979a). Photochemical oxidation products include aldehydes, hydroxy compounds, nitro compounds, and peroxyacyl nitrates (Cupitt 1980 EPA 1979a Stephens 1973). 

Li12[Mnn2ZnW (ZnW90 34)2] Alkanes, alkylarenes Ketones, a-oxidation products Ozone h2o Likely metal-ozonide intermediate 461... 

Since the relative rate of formation of dimethyhnethylcarboxonium ion from isobutane is considerably faster than that of rert-butyl cation from isobutane in the absence of ozone under the same conditions,it is, however, likely that protonated ozone inserts into the C-H bond of the alkane to form a pentacoordinate peroxonium ion 70 that decomposes to a very reactive tert-alkyloxenium ion (71) that subsequently undergoes Baeyer-Villiger oxidation [Eq. (6.52)]. 

When the dry ozonation technique is employed (dry silica gel with a preadsorbed organic substrate is saturated with ozone at -78°C then warmed up to room temperature), ozone exhibits an enhanced reactivity, presumably due to the slightly acidic nature of silica gel. Regioselectivities and stereoselectivities under these conditions are very similar to those in superacidic liquid-phase ozonation. Straight-chain alkanes are oxidized at the secondary carbon to yield a mixture of alcohols and ketones, Branched acyclic alkanes give tertiary alcohols and yield ketones via C-C bond cleavage, Insertion of ozone into the C-H and C-C a bonds was evoked to account for the observations (Scheme 6.27). 

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