# Autoxidation

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{{Short description|Spontaneous oxidation by oxygen at normal temperature}}
'''Autoxidation''' (sometimes '''auto-oxidation''') refers to [oxidations](/source/redox) brought about by reactions with oxygen at normal temperatures, without the intervention of flame or electric spark.<ref>{{cite book |last1=Foote |first1=Christopher S. |title=Active Oxygen in Chemistry |date=1996 |publisher=Springer Netherlands |location=Dordrecht |isbn=978-94-007-0874-7 |pages=24–65 |chapter=2. Autoxidation}} {{doi|10.1007/978-94-007-0874-7_2}}</ref> The term is usually used to describe the gradual degradation of [organic compound](/source/organic_compound)s in air at ambient temperatures. Many common phenomena can be attributed to autoxidation, such as food going [rancid](/source/Rancidification),<ref>{{cite journal |last1=Holman |first1=Ralph T. |title=Autoxidation of fats and related substances |journal=Progress in the Chemistry of Fats and Other Lipids |date=January 1954 |volume=2 |pages=51–98 |doi=10.1016/0079-6832(54)90004-X}}</ref> the '[drying](/source/Drying_oil)' of varnishes and paints, and the perishing of rubber.<ref>{{cite journal |last1=Helberg |first1=Julian |last2=Pratt |first2=Derek A. |title=Autoxidation vs. antioxidants – the fight for forever |journal=Chemical Society Reviews |date=2021 |volume=50 |issue=13 |pages=7343–7358 |doi=10.1039/D1CS00265A|pmid=34037013 }}</ref> It is also an important concept in both industrial chemistry and biology.<ref>{{cite journal |last1=Frank |first1=Charles E. |title=Hydrocarbon Autoxidation. |journal=Chemical Reviews |date=February 1950 |volume=46 |issue=1 |pages=155–169 |doi=10.1021/cr60143a003|pmid=24537520 }}</ref> Autoxidation is therefore a fairly broad term and can encompass examples of [photooxygenation](/source/photooxygenation) and [catalytic oxidation](/source/catalytic_oxidation).

The common mechanism is a [free radical](/source/free_radical) [chain reaction](/source/chain_reaction), where the addition of oxygen gives rise to [hydroperoxide](/source/hydroperoxide)s and their associated peroxy radicals (ROO•).<ref>{{cite journal |last1=Simic |first1=Michael G. |title=Free radical mechanisms in autoxidation processes |journal=Journal of Chemical Education |date=February 1981 |volume=58 |issue=2 |page=125 |doi=10.1021/ed058p125 |bibcode=1981JChEd..58..125S |doi-access=free }}</ref> Typically, an [induction period](/source/induction_period) is seen at the start where there is little activity; this is followed by a gradually accelerating take-up of oxygen, giving an [autocatalytic](/source/Autocatalysis) reaction which can only be kept in check by the use of [antioxidants](/source/antioxidants). [Unsaturated](/source/Saturated_and_unsaturated_compounds) compounds are the most strongly affected but many organic materials will oxidise in this way given time.

Although autoxidation is usually undesirable, it has been exploited in chemical synthesis. In these cases the term 'autoxidation' is often used more broadly to include spontaneous reactions with oxygen at elevated temperatures, such as in the [Cumene process](/source/Cumene_process).

==Mechanism==
The free radical chain reaction is sometimes referred to as the Bolland-Gee mechanism<ref>Named after Geoffrey Gee and John Lawson Bolland</ref><ref>{{cite journal |last1=Hammond |first1=Earl G.|author2-link=Pamela J. White |last2=White |first2=Pamela J. |title=A Brief History of Lipid Oxidation |journal=Journal of the American Oil Chemists' Society |date=July 2011 |volume=88 |issue=7 |pages=891–897 |doi=10.1007/s11746-011-1761-8|s2cid=84637577 }}</ref> or the basic autoxidation scheme (BAS)<ref name="Fate">{{cite journal |last1=Smith |first1=Leesa M. |last2=Aitken |first2=Heather M. |last3=Coote |first3=Michelle L. |title=The Fate of the Peroxyl Radical in Autoxidation: How Does Polymer Degradation Really Occur? |journal=Accounts of Chemical Research |date=18 September 2018 |volume=51 |issue=9 |pages=2006–2013 |doi=10.1021/acs.accounts.8b00250|pmid=30016062 |hdl=1885/209140 |s2cid=51679950 |hdl-access=free }}</ref> and was originally based on the oxidation of rubbers,<ref>{{cite journal |last1=Bolland |first1=J. L. |last2=Gee |first2=Geoffrey |title=Kinetic studies in the chemistry of rubber and related materials. II. The kinetics of oxidation of unconjugated olefins |journal=Transactions of the Faraday Society |date=1946 |volume=42 |page=236 |doi=10.1039/TF9464200236}}</ref> but remains generally accurate for many materials. It can be divided into three stages: initiation, propagation, and termination.<ref>{{cite journal|title=Inhibition of the Autoxidation of Organic Substances in the Liquid Phase.
|author=K. U. Ingold|journal=Chem. Rev.|year=1961|volume=61|issue=6|pages=563–589|doi=10.1021/cr60214a002}}</ref> The initiation step is often ill-defined and many agents have been proposed as [radical initiator](/source/radical_initiator)s.<ref>{{cite book |title=Atmospheric oxidation and antioxidants |date=1993 |publisher=Elsevier |location=Amsterdam |isbn=0-444-89615-5}}</ref> The autoxidation of unsaturated compounds may be initiated by reactions with [singlet oxygen](/source/singlet_oxygen)<ref>{{cite journal |last1=Choe |first1=Eunok |last2=Min |first2=David B. |title=Mechanisms and Factors for Edible Oil Oxidation |journal=Comprehensive Reviews in Food Science and Food Safety |date=September 2006 |volume=5 |issue=4 |pages=169–186 |doi=10.1111/j.1541-4337.2006.00009.x}}</ref> or environmental pollutants such as [ozone](/source/ozone) and [NO<sub>2</sub>](/source/Nitrogen_dioxide).<ref>{{cite book |title=Autoxidation in food and biological systems |date=1980 |publisher=Plenum Press |location=New York |isbn=978-1-4757-9351-2 |pages=1–16 |chapter=Initiation of the Autoxidation of Polyunsaturated Fatty Acids (PUFA) by Ozone and Nitrogen Dioxide}} {{doi|10.1007/978-1-4757-9351-2_1}}</ref> Saturated polymers, such as [polyolefin](/source/polyolefin)s would be expected to resist autoxidation, however in practise they contain hydroperoxides formed by thermal oxidation during their high temperature moulding and casting, which can act as initiators.<ref>{{cite journal |last1=Grause |first1=Guido |last2=Chien |first2=Mei-Fang |last3=Inoue |first3=Chihiro |title=Changes during the weathering of polyolefins |journal=Polymer Degradation and Stability |date=November 2020 |volume=181 |article-number=109364 |doi=10.1016/j.polymdegradstab.2020.109364|s2cid=225243217 }}</ref><ref>{{cite book |last1=David |first1=C. |title=Degradation of polymers |date=1975 |publisher=Elsevier Scientific Pub. Co |location=Amsterdam |isbn=978-0-444-41155-6 |pages=425–538 |chapter=Chapter 4 Oxidative Degradation of Polymers}} {{doi|10.1016/S0069-8040(08)70336-4}}</ref> In biological systems [reactive oxygen species](/source/reactive_oxygen_species) are important. For industrial reactions a radical initiator, such as [benzoyl peroxide](/source/benzoyl_peroxide), will be intentionally added.

All of these processes lead to the generation of carbon centred radicals on the polymer chain (R•), typically by abstraction of H from labile C-H bonds. Once the carbon-centred radical has formed, it reacts rapidly with O<sub>2</sub> to give a peroxy radical (ROO•). This in turn abstracts an H atom from a weak C-H bond give a hydroperoxide (ROOH) and a fresh carbon-centred radical. The hydroperoxides can then undergo a number of possible [homolytic](/source/Homolysis_(chemistry)) reactions to generate more radicals,<ref name="Fate" /> giving an accelerating reaction. As the concentration of radicals increases chain termination reactions become more important, these reduce the number of radicals by [radical disproportionation](/source/radical_disproportionation) or combination, leading to a [sigmoid](/source/Sigmoid_function) reaction plot.

330px|right|thumb|The cyclic mechanism of autoxidation

'''Chain initiation'''
:: <chem>Polymer -> P\bullet +\  P\bullet </chem>

'''Chain propagation'''
::<chem>P\bullet +\ O2 -> POO\bullet </chem>
::<chem>POO\bullet +\ PH -> {POOH} +\ P\bullet </chem>

'''Chain branching'''
::<chem>POOH -> PO\bullet +\  OH\bullet </chem>
::<chem>{PH} + OH\bullet -> P\bullet +\ H2O</chem>
::<chem>PO\bullet -> Chain\ scission\ reactions</chem>

'''Termination'''
::<chem>POO\bullet +\ POO\bullet -> cross\ linking\ reaction\ to\ non-radical\ product</chem>
::<chem>POO\bullet +\ P\bullet -> cross\ linking\ reaction\ to\ non-radical\ product</chem>
::<chem> P\bullet +\ P\bullet -> cross\ linking\ reaction\ to\ non-radical\ product</chem>

==In oils and polymers==
The autoxidation of unsaturated [fatty acids](/source/fatty_acids) causes them to [crosslink](/source/crosslink) to form [polymer](/source/polymer)s.<ref>{{cite journal |last1=Wexler |first1=Herman |title=Polymerization of Drying Oils |journal=Chemical Reviews |date=1 December 1964 |volume=64 |issue=6 |pages=591–611 |doi=10.1021/cr60232a001}}</ref> This phenomenon has been known since antiquity and forms the basis of [drying oil](/source/drying_oil)s, which were traditionally used to make many varnishes and paints.<ref>{{cite journal |last1=Honzíček |first1=Jan |title=Curing of Air-Drying Paints: A Critical Review |journal=Industrial & Engineering Chemistry Research |date=17 July 2019 |volume=58 |issue=28 |pages=12485–12505 |doi=10.1021/acs.iecr.9b02567|hdl=10195/74955 |hdl-access=free }}</ref> [Linseed oil](/source/Linseed_oil), which is rich in [polyunsaturated fats](/source/polyunsaturated_fats), is a prime example.

Conversely, autoxidation can also cause polymers such as plastics to deteriorate.<ref>{{cite book |last1=Grassie |first1=Norman |title=Polymer degradation & stabilisation |year=1988 |orig-date=1985 |publisher=Cambridge University Press |location=Cambridge [England] |isbn=978-0-521-35797-5 |edition=1st pbk.}}</ref> Sensitivity varies depending in the polymer backbone, in general structures containing unsaturated groups, [allyl](/source/allyl)ic and [benzyl](/source/benzyl)ic C−H bonds  and [tertiary carbon](/source/tertiary_carbon) centres are more susceptible, [rubber](/source/rubber)s are therefore particularly sensitive. Autoxidation can be inhibited by a wide range of [polymer stabilizers](/source/polymer_stabilizers), or accelerated by [biodegradable additives](/source/biodegradable_additives).
Similarly, antioxidant [oil additive](/source/oil_additive)s and [fuel additive](/source/fuel_additive)s are used to inhibit autoxidation.

==In food==
The prevention of autoxidation is important in the food and drink industry and is achieved both by both chemical [preservative](/source/preservative)s and a range of oxygen excluding [food preservation](/source/food_preservation) techniques such as [canning](/source/canning). It is well known that fats, especially [polyunsaturated fats](/source/polyunsaturated_fats), become rancid, even when kept at low temperatures,<ref>{{cite journal |last1=Prabhu |first1=H. Ramachandra |title=Lipid peroxidation in culinary oils subjected to thermal stress |journal=Indian Journal of Clinical Biochemistry |date=August 2000 |volume=15 |issue=1 |pages=1–5 |doi=10.1007/BF02873539|pmid=23105229 |pmc=3453543 }}</ref> however many other foods are susceptible to autoxidation.
The complex mixture of compounds found in wine, including [polyphenol](/source/polyphenols_in_wine)s, polysaccharides, and proteins, can undergo autoxidation during the [aging](/source/Aging_of_wine) process, leading to [wine fault](/source/wine_fault)s. 
The [browning](/source/Browning_(chemical_process)) of many foods, such as skinned apples, can be considered an autoxidation process, although it is generally an enzymatic process such as [lipid peroxidation](/source/lipid_peroxidation) which proceeds via a different mechanism to the one shown above.

==In industry==
In the [chemical industry](/source/chemical_industry), many organic chemicals are produced by autoxidation:

* in the [cumene process](/source/cumene_process), isopropylbenzene undergoes autoxidation to give [cumene hydroperoxide](/source/cumene_hydroperoxide).  This compound is then converted to [phenol](/source/phenol) and [acetone](/source/acetone), both [commodity chemical](/source/commodity_chemical)s.   are made from [benzene](/source/benzene) and [propylene](/source/propylene).  Many variations of this reaction have been developed, e.g. use of diisopropylbenzene as a substrate.
* the autoxidation of [cyclohexane](/source/cyclohexane) yields [cyclohexanol](/source/cyclohexanol) and [cyclohexanone](/source/cyclohexanone).<ref>I.V. Berezin, E.T. Denisov, ''The Oxidation of Cyclohexane'', Pergamon Press, New York, '''1996'''.</ref>
* [''p''-xylene](/source/P-Xylene) undergoes auoxidation to [terephthalic acid](/source/terephthalic_acid).
* [ethylbenzene](/source/ethylbenzene) is oxidized to [ethylbenzene hydroperoxide](/source/ethylbenzene_hydroperoxide), an epoxidizing agent in the propylene oxide/styrene process [POSM](/source/Styrene)

In the '''Bashkirov process''', the autoxidation is conducted in the presence of boric acid, yielding an intermediate borate ester.  The process is more selective with the boric acid, but the conversion to the alcohol requires hydrolysis of the ester.  This approach continues to be used in the production of [cyclododecanol](/source/cyclododecanol) from [cyclododecane](/source/cyclododecane).  Cyclododecanol is a precursor to [cyclododecanone](/source/cyclododecanone), which is used to make [nylon-12](/source/nylon-12).<ref>{{cite book |doi=10.1002/14356007.a18_261.pub2 |chapter=Oxidation |title=Ullmann's Encyclopedia of Industrial Chemistry |date=2015 |last1=Teles |first1=J. Henrique |last2=Hermans |first2=Ive |last3=Franz |first3=Gerhard |last4=Sheldon |first4=Roger A. |pages=1–103 |isbn=978-3-527-30385-4 }}</ref>

==See also==
* [Photodegradation](/source/Photodegradation) - this often involves autoxidation processes which are accelerated by UV energy

==Further reading==
An old review that provides a lucid summary of qualitative and practical aspects: {{cite journal |doi=10.1021/cr60143a003 |title=Hydrocarbon Autoxidation |date=1950 |last1=Frank |first1=Charles E. |journal=Chemical Reviews |volume=46 |issue=1 |pages=155–169 |pmid=24537520 }}

== References ==
<references />

Category:Organic redox reactions

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Adapted from the Wikipedia article [Autoxidation](https://en.wikipedia.org/wiki/Autoxidation) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Autoxidation?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.