Autoxidation (sometimesauto-oxidation) refers tooxidations brought about by reactions with oxygen at normal temperatures, without the intervention of flame or electric spark.^[1] The term is usually used to describe the gradual degradation oforganic compounds in air at ambient temperatures. Many common phenomena can be attributed to autoxidation, such as food goingrancid,^[2] the 'drying' of varnishes and paints, and the perishing of rubber.^[3] It is also an important concept in both industrial chemistry and biology.^[4] Autoxidation is therefore a fairly broad term and can encompass examples ofphotooxygenation andcatalytic oxidation.

The common mechanism is afree radicalchain reaction, where the addition of oxygen gives rise tohydroperoxides and their associated peroxy radicals (ROO•).^[5] Typically, aninduction period is seen at the start where there is little activity; this is followed by a gradually accelerating take-up of oxygen, giving anautocatalytic reaction which can only be kept in check by the use ofantioxidants.Unsaturated 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 theCumene process.

The free radical chain reaction is sometimes referred to as the Bolland-Gee mechanism^[6][7] or the basic autoxidation scheme (BAS)^[8] and was originally based on the oxidation of rubbers,^[9] but remains generally accurate for many materials. It can be divided into three stages: initiation, propagation, and termination.^[10] The initiation step is often ill-defined and many agents have been proposed asradical initiators.^[11] The autoxidation of unsaturated compounds may be initiated by reactions withsinglet oxygen^[12] or environmental pollutants such asozone andNO₂.^[13] Saturated polymers, such aspolyolefins 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.^[14][15] In biological systemsreactive oxygen species are important. For industrial reactions a radical initiator, such asbenzoyl 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 O₂ 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 possiblehomolytic reactions to generate more radicals,^[8] giving an accelerating reaction. As the concentration of radicals increases chain termination reactions become more important, these reduce the number of radicals byradical disproportionation or combination, leading to asigmoid reaction plot.

Chain initiation

${\displaystyle \text{Polymer}⟶\text{P}\bullet +\text{P}\bullet }$

Chain propagation

${\displaystyle \text{P}\bullet +{\text{O}}_2^{}⟶\text{POO}\bullet }$

${\displaystyle \text{POO}\bullet +\text{PH}⟶\text{POOH}+\text{P}\bullet }$

Chain branching

${\displaystyle \text{POOH}⟶\text{PO}\bullet +\text{OH}\bullet }$

${\displaystyle \text{PH}+\text{OH}\bullet ⟶\text{P}\bullet +{\text{H}}_2^{}\text{O}}$

${\displaystyle \text{PO}\bullet ⟶\text{Chain}\text{scission}\text{reactions}}$

Termination

${\displaystyle \text{POO}\bullet +\text{POO}\bullet ⟶\text{cross}\text{linking}\text{reaction}\text{to}\text{non}-\text{radical}\text{product}}$

${\displaystyle \text{POO}\bullet +\text{P}\bullet ⟶\text{cross}\text{linking}\text{reaction}\text{to}\text{non}-\text{radical}\text{product}}$

${\displaystyle \text{P}\bullet +\text{P}\bullet ⟶\text{cross}\text{linking}\text{reaction}\text{to}\text{non}-\text{radical}\text{product}}$

The autoxidation of unsaturatedfatty acids causes them tocrosslink to formpolymers.^[16] This phenomenon has been known since antiquity and forms the basis ofdrying oils, which were traditionally used to make many varnishes and paints.^[17]Linseed oil, which is rich inpolyunsaturated fats, is a prime example.

Conversely, autoxidation can also cause polymers such as plastics to deteriorate.^[18] Sensitivity varies depending in the polymer backbone, in general structures containing unsaturated groups,allylic andbenzylic C−H bonds andtertiary carbon centres are more susceptible,rubbers are therefore particularly sensitive. Autoxidation can be inhibited by a wide range ofpolymer stabilizers, or accelerated bybiodegradable additives.Similarly, antioxidantoil additives andfuel additives are used to inhibit autoxidation.

The prevention of autoxidation is important in the food and drink industry and is achieved both by both chemicalpreservatives and a range of oxygen excludingfood preservation techniques such ascanning. It is well known that fats, especiallypolyunsaturated fats, become rancid, even when kept at low temperatures,^[19] however many other foods are susceptible to autoxidation.The complex mixture of compounds found in wine, includingpolyphenols, polysaccharides, and proteins, can undergo autoxidation during theaging process, leading towine faults. Thebrowning of many foods, such as skinned apples, can be considered an autoxidation process, although it is generally an enzymatic process such aslipid peroxidation which proceeds via a different mechanism to the one shown above.

In thechemical industry, many organic chemicals are produced by autoxidation:

• in thecumene process, isopropylbenzene undergoes autoxidation to givecumene hydroperoxide. This compound is then converted tophenol andacetone, bothcommodity chemicals. are made frombenzene andpropylene. Many variations of this reaction have been developed, e.g. use of diisopropylbenzene as a substrate.
• the autoxidation ofcyclohexane yieldscyclohexanol andcyclohexanone.^[20]
• p-xylene undergoes auoxidation toterephthalic acid.
• ethylbenzene is oxidized toethylbenzene hydroperoxide, an epoxidizing agent in the propylene oxide/styrene processPOSM

In theBashkirov 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 ofcyclododecanol fromcyclododecane. Cyclododecanol is a precursor tocyclododecanone, which is used to makenylon-12.^[21]

• Photodegradation - this often involves autoxidation processes which are accelerated by UV energy

An old review that provides a lucid summary of qualitative and practical aspects:Frank, Charles E. (1950). "Hydrocarbon Autoxidation".Chemical Reviews.46 (1):155–169.doi:10.1021/cr60143a003.PMID 24537520.

• Organic redox reactions

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