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| '''Autoxidation''' is any [[oxidation]] that occurs in open air or in presence of [[oxygen]] and/or [[UV]] radiation and forms [[peroxide]]s and [[hydroperoxide]]s. A classic example of autoxidation is that of simple [[ether]]s like [[diethyl ether]], whose peroxides can be dangerously explosive. It can be considered to be a slow, [[Flameless oxidation|flameless combustion]] of materials by reaction with oxygen. Autoxidation is important because it is a useful reaction for converting compounds to oxygenated derivatives, and also because it occurs in situations where it is not desired (as in the destructive cracking of the rubber in automobile tires).
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| Although virtually all types of organic materials can undergo air oxidation, certain types are particularly prone to autoxidation, including unsaturated compounds that have [[allylic]] or [[benzylic]] hydrogen atoms; these materials are converted to hydroperoxides by autoxidation.
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| ==Mechanism==
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| Autoxidation is a [[free radical]] chain process. Such reactions can be divided into three stages: chain initiation, propagation, and termination. In the initiation process, some event causes free radicals to be formed. For example, free radicals can be produced purposefully by the decomposition of a [[radical initiator]], such as [[benzoyl peroxide]]. In some cases, initiation occurs by a process that is not well understood but is thought to be the spontaneous reaction of oxygen with a material with a readily abstractable hydrogen. Destructive autoxidation processes also are initiated by pollutants such as those in smog.
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| Once free radicals are formed, they react in a chain to convert the material to a hydroperoxide. The chain is ended by termination reactions in which free radicals collide and combine their odd electrons to form a new bond.
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| '''Chain initiation'''
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| :<math>\mathrm{ROOH + RH \ \xrightarrow {energy} \ RO{\cdot} + {\cdot}OH + RH \ \longrightarrow {} \ RO{\cdot} + H_2O + R{\cdot} \quad}</math>
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| :<math>\mathrm{RO{\cdot} + RH \ \xrightarrow {H-abstraction} \ R{\cdot} + ROH \quad}</math>
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| '''Chain propagation'''<ref>I.V. Berezin, E.T. Denisov, ''The Oxidation of Cyclohexane'', Pergamon Press, New York, '''1996'''.</ref>
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| :<math>\mathrm{R{^{\cdot}} + O_2 \ \xrightarrow {fast} \ ROO{^{\cdot}}}</math> | |
| :<math>\mathrm{ROO{^{\cdot}} + RH \ \xrightarrow {H-abstraction} \ ROOH + {^{\cdot}}R}</math>
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| '''Chain termination'''
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| :<math>\mathrm{2 ROO{^{\cdot}} \ \xrightarrow {} \ 2 RO{^{\cdot}} + O_2 \ \longrightarrow {} \ ROH + QO + O_2}</math>
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| '''Source of alcohol and ketone'''<ref>I. Hermans, T.L. Nguyen, P.A. Jacobs, J. Peeters, ''ChemPhysChem'' '''2005''', ''6'', 637-645.</ref>
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| :<math>\mathrm{ROOH + ROO{^{\cdot}} \ \longrightarrow {} \ ROOH + Q{^{\cdot}}OOH \ \longrightarrow {} \ ROOH + QO + ^{\cdot}OH }</math>
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| :<math>\mathrm{ROOH + QO + ^{\cdot}OH + RH \ \longrightarrow {} \ ROOH + QO + H_2O + R^{\cdot} \ \longrightarrow {} \ RO^{\cdot} + ROH + QO + H_2O }</math>
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| ==Reaction rate==
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| In steady state, the concentration of chain-carrying radicals is constant, thus the rate of initiation equals the rate of termination.
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| :<math>\mathrm{r_{init} = k_{init} \cdot [ROOH] = k_{term} \cdot [ROO^{\cdot}]^2}</math>
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| :<math>\mathrm{r_{prop} = k_{prop} \cdot [RH] \cdot [ROO^{\cdot}] = k_{prop}\cdot [RH] \cdot \sqrt[\,]{\frac{k_{init}}{k_{term}}}\cdot \sqrt[\,]{[ROOH]}}</math>
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| ==Autoxidations in industry==
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| Autoxidation is a process of enormous economic impact, since all foods, plastics, gasolines, oils, rubber, and other materials that must be exposed to air undergo continuous destructive reactions of this type. All plastics and rubber and most processed foods contain [[antioxidants]] to protect them against the attack of oxygen.
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| In the [[chemical industry]] many chemicals are produced by autoxidation:
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| * in the [[cumene process]] phenol and acetone are made from [[benzene]] and [[propylene]]
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| * the autoxidation of [[cyclohexane]] yields [[cyclohexanol]] and [[cyclohexanone]]
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| * [[P-Xylene|''p''-xylene]] is oxidized to [[terephthalic acid]]
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| * [[ethylbenzene]] is oxidized to ethylbenzene hydroperoxide, an epoxidizing agent in the propylene oxide/styrene process [[POSM]].
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| ==Autoxidation in food==
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| It is well known that fats [[lipid peroxidation|become]] rancid, even when kept at low temperatures. This is especially true for [[polyunsaturated fat]]s.<ref>[http://www.springerlink.com/content/350458182q7jl86t/ Lipid peroxidation in culinary oils subjected to thermal stress. H. Ramachandra Prabhu, Indian Journal of Clinical Biochemistry, 2000, Volume 15, Number 1, 1-5, ] {{doi|10.1007/BF02873539}}</ref>
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| The complex mixture of compounds found in wine, including [[polyphenols in wine|polyphenol]]s, polysaccharides, and proteins, can undergo autoxidation during the [[Aging of wine|aging]] process. Simple polyphenols can lead to the formation of [[B type proanthocyanidin|B-type procyanidin]]s in wines<ref>[http://cat.inist.fr/?aModele=afficheN&cpsidt=14708334 Tandem mass spectrometry of the B-type procyanidins in wine and B-type dehydrodicatechins in an autoxidation mixture of (+)-catechin and (-)-epicatechin. Weixing Sun, Miller Jack M., Journal of mass spectrometry, 2003, vol. 38, no4, pp. 438-446]</ref> or in model solutions.<ref>[http://www3.interscience.wiley.com/journal/121543604/abstract Identification of autoxidation oligomers of flavan-3-ols in model solutions by HPLC-MS/MS. Fei He, Qiu-Hong Pan, Ying Shi, Xue-Ting Zhang, Chang-Qing Duan, Journal of Mass Spectrometry, Volume 44 Issue 5, Pages 633 - 640, 2008]</ref> This is correlated to the [[Browning (chemical process)|browning]] color change characteristic of this process.<ref>[http://www.ajevonline.org/cgi/content/abstract/41/1/84 Nonenzymic Autoxidative Reactions of Caffeic Acid in Wine. Johannes J. L. Cilliers 1 and Vernon L. Singleton, Am. J. Enol. Vitic. 41:1:84-86, 1990.]</ref>
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| This phenomenon is also observed in carrot puree.<ref>[http://pubs.acs.org/doi/abs/10.1021/jf981134n Phenolic Autoxidation Is Responsible for Color Degradation in Processed Carrot Puree. Talcott S. T. and Howard L. R., J. Agric. Food Chem., 1999, 47 (5), pp 2109–2115.]</ref>
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| == References ==
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| <references />
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| [[Category:Organic redox reactions]]
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