Autoxidation of alkanes

Equations (9.4) and (9.5) illustrate termination reactions. Depending on the structure of the alkyl group, disproportionation to alcohols and carbonyl compounds may take place  

Autoxidation without added initiator may also occur. This process is generally characterized by an induction period, and a much lower reaction rate since formation of alkyl radicals according to Eq. (9.6) is thermodynamically and kinetically unfavorable  

At elevated temperature alkyl hydroperoxides undergo thermal decomposition to alcohols [Eqs. (9.9)—(9.11)]. This decomposition serves as a major source of free radicals in autoxidation. Because of side reactions, such as p scission of alkylperoxy radicals, this process is difficult to control. Further transformation of the alkoxy 

Selectivity of alcohol formation can be substantially increased by carrying out the autoxidation in the presence of a stoichiometric quantity of boric acid that reacts with the intermediate hydroperoxide to form alkyl borate. This observation gained practical importance in the commercial oxidation of alkanes (see Section 9.5.1). 

Alkanes possessing two tertiary C—H bonds in p or y position may form dihydroperoxides. The product is formed in an intramolecular peroxy radical attack that is highly efficient in the transformation of 2,4-dimethylpentane 15 16 

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Hydroperoxides have been obtained from the autoxidation of alkanes, aralkanes, alkenes, ketones, enols, hydrazones, aromatic amines, amides, ethers, acetals, alcohols, and organomineral compounds, eg, Grignard reagents (10,45). In autoxidations involving hydrazones, double-bond migration occurs with the formation of hydroperoxy—azo compounds via free-radical chain processes (10,59) (eq. 20). 

Autoxidation of alkanes generally promotes the formation of alkyl hydroperoxides, but d4-tert-huty peroxide has been obtained in >30% yield by the bromine-catalyzed oxidation of isobutane (66). In the presence of iodine, styrene also has been oxidized to the corresponding peroxide (44). 

Autoxidation of alkanes may be carried out by metal catalysis.2,14 17 Although metal ions participate in all oxidation steps, their main role in autoxidation is not in their ability to generate free radicals directly by one-electron oxidation [Eq. (9.14)] but rather their activity to catalyze the homolytic decomposition of the intermediate hydroperoxide according to Eqs. (9.15) and (9.16). As a result of this decomposition, metal ions generate chain-initiating radicals. The overall reaction is given in Eq. (9.17) ... 

Although alkyl peroxides are usually intermediates and products autoxidation of alkanes, the reactions between alkanes and alkyl hydroperoxides need relatively high temperature or a metal complex as a catalyst. Sometimes other compounds can induce such reactions. Eor example, -pentane and n-hcxane are oxidized by tcrf-butyl hydroperoxide in benzene solution if (t-Bu)jAl is present in the system [44]. 

In some cases metalloporphyrins have been found to catalyze the autoxidation of alkanes or the alkyl groups of other saturated hydrocarbons (alkylaromatics, cycloalkanes, etc.) [85]. Typically,... 

We can suggest only one phase effect that might offset these effects. In the autoxidation of several hydrocarbons, Howard and Ingold (21) found solvent effects on kp and kt consistent with previously reported solvent effects on over-all rates (10, 19). We might then expect (25) that kp would be slightly larger and/or kt slightly smaller in the gas phase than in alkanes. 

Heteroatoms or functional groups can either increase or diminish the rate of autoxidation of alkyl groups. Haloalkanes and alkanes substituted with electron-withdrawing groups are usually more resistant toward homolytic C-H bond cleav-... 

Polymers with chelated Co have also been used as catalysts for alkane autoxidation. Kulkarni et al. (166) employed a tyrosine-based polymer for autoxidation in pure cyclohexane, but very different conditions were used by Shen and Weng (167,168) in the autoxidation of cyclohexane or cyclohexanone. The latter authors, used glacial acetic acid as a solvent and a Co-exchanged weak acid resin as the catalyst. At high conversions, adipic acid is formed ... 

The use of mixed-metal catalysts can also dramatically affect the products of autoxidations. An example mentioned earlier is the selective oxidation of acetaldehyde to acetic anhydride in the presence of a mixture of cobalt and copper acetates. Another example is the co-oxidation of alkanes and olefins in the presence of both an autoxidation and an epoxidation catalyst (see Section III.B) ... 

As a further example the four hydroperoxides obtained in the autoxidation of oleate would be expected to give either the aldehydes and radical esters shown in the following equation or, alternatively, the oo-oxoesters and alkane and alkene radicals if the fi scission takes place on the other C-C bond. The free radicals can then react with neutral molecules or inactivate one another (Figure 2.12). 

Systematic examination of the catalytic properties of dimeric complexes was initiated shortly after the identification of dinuclear iron sites in metalloenzymes. The first report of a reactive dimeric system came from Tabushi et al. in 1980, who examined the catalytic chemistry of [Fe3+(salen)]20, 1 (salen is N,N -(salicylaldehydo)-l,2-ethylenediamine) (12). They reported interesting stereoselectivity in the oxidation of unsaturated hydrocarbons with molecular oxygen in the presence of mercaptoethanol or ascorbic acid and pyridine as a solvent ([l]<<[alkane]<<[2-mercaptoethanol]). With adamantane as substrate, they observed the formation of a mixture of (1- and 2-) adamantols and adamantanone (Table I) (12). Both the relative reactivity between tertiary and secondary carbons (maximum value is 1.05) and final yield ( 12 turnovers per 12 hr) were dependent on the quantity of added 2-mercaptoethanol. Because autoxidation of adamantane gave a ratio of 3°/2° carbon oxidation of 0.18-0.42, the authors proposed two coexisting processes autooxidation and alkane activation. 

Classical autoxidation of tertiary C-H bonds in alkanes can afford the corresponding hydroperoxides in high selectivities. This is applied industrially in the conversion of pinane to the corresponding hydroperoxide, an intermediate in the manufacture of pinanol (Fig. 4.43). 

Air oxidation of /i-butane to maleic anhydride is possible over vanadium phos(4tate and, remaiicably, a 60% selectivity is obtained at 85% conversion. In the gas phase oxidation, in conffast to the situation found in the liquid, n-allcanes are oxidized more rapidly than branched chain alkanes. This is because secondary radicals are more readily able to sustain a chain for branched alkanes the relatively stable tertiary radical is preferentially formed but fails to continue the chain process. Vanadium(V)/ manganese(II)/AcOH has been used as a catalyst for the autoxidation of cyclohexane to adipic acid, giving 25-30% yields after only 4 h. ... 

Several interesting variations on the above radical chemistry have been described recently. One such system is copper salt catalyzed alkane oxidation by dioxygen in the presence of an aldehyde [17]. The proposed mechanism involves the initial autoxidation of the aldehyde to the corresponding peracid, which is the real oxidant for the Cu"-mediated oxidation of the alkane (eqs. (3)-(5)). The ratio of alkane oxidized to aldehyde converted is relatively low because much of the peracid formed reacts with the aldehyde to form two molecules of carboxylic acid. 

It is a well-known product of autoxidation of linoleic acid (Ci8 2), but it is not a flavor substance (Ullrich and Grosch, 1987). As the other volatile alkanes, it can eventually be present as a residual trace of solvent after a petroleum ether extraction of ground coffee samples. 

Rust, F. (1957). Intramolecular oxidation. The autoxidation of some dimethyl alkanes. J. Am. Chem. Soc. 79, 4000-4003. 

If there is a CH2 or a CH group in a-position to an aromatic system, it is attacked preferentially by oxygen with formation of a substituted benzyl hydroperoxide, 317 examples being tetralin, ind ne, fluorene, cumene, p-xylene, and ethylbenzene. Temperatures required for autoxidation of such compounds are lower than for alkanes. 

Although liquid-phase oxidations of alkanes can be carried out even in the absence of any metal derivative (the role of an inihator of chain radical process can be played by a non-metal compound), derivahves of transition metals are often used in these reactions. Metal-catalyzed autoxidation will be considered in Chapter IX. 

Cobalt compounds are among the most efficient catalysts of alkane and aryl alkane autoxidation [16], Toluene oxidation by dioxygen at 60 °C in acetic acid in the presence of Co(III) at the initial stages gives exclusively benzaldehyde [16d]. The reaction proceeds without an induction period, reaching the maximum rate at the begiiming of the reaction. The kinetic equation for the oxidation is... 

Scheme DC.3. The tilleniative mechanism for the air oxidation of alkanes assuming a radical-chain autoxidation.

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