Autoxidation

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Autoxidation (sometimes auto-oxidation) refers to oxidations 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 of organic compounds in air at ambient temperatures. Many common phenomena can be attributed to autoxidation, such as food going rancid, [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 of photooxygenation and catalytic oxidation.

Contents

The common mechanism is a free radical chain reaction, where the addition of oxygen gives rise to hydroperoxides and their associated peroxy radicals (ROO•). [5] Typically, an 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 reaction which can only be kept in check by the use of antioxidants. 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 the Cumene process.

Mechanism

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 as radical initiators. [11] The autoxidation of unsaturated compounds may be initiated by reactions with singlet oxygen [12] or environmental pollutants such as ozone and NO2. [13] Saturated polymers, such as polyolefins 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 systems reactive oxygen species are important. For industrial reactions a radical initiator, such as 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 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 possible homolytic 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 by radical disproportionation or combination, leading to a sigmoid reaction plot.

The cyclic mechanism of autoxidation Polymer auto-oxidation.png
The cyclic mechanism of autoxidation

Chain initiation

Chain propagation

Chain branching

Termination

In oils and polymers

The autoxidation of unsaturated fatty acids causes them to crosslink to form polymers. [16] This phenomenon has been known since antiquity and forms the basis of drying oils, which were traditionally used to make many varnishes and paints. [17] Linseed oil, which is rich in polyunsaturated 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 and benzylic C−H bonds and tertiary carbon centres are more susceptible, rubbers are therefore particularly sensitive. Autoxidation can be inhibited by a wide range of polymer stabilizers, or accelerated by biodegradable additives. Similarly, antioxidant oil additives and fuel additives 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 preservatives and a range of oxygen excluding food preservation techniques such as canning. It is well known that fats, especially polyunsaturated 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, including polyphenols, polysaccharides, and proteins, can undergo autoxidation during the aging process, leading to wine faults. The browning of many foods, such as skinned apples, can be considered an autoxidation process, although it is generally an enzymatic process such as lipid peroxidation which proceeds via a different mechanism to the one shown above.

In industry

In the chemical industry, many organic chemicals are produced by autoxidation:

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 from cyclododecane. Cyclododecanol is a precursor to cyclododecanone, which is used to make nylon-12. [21]

See also

Further reading

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.

Related Research Articles

Antioxidants are compounds that inhibit oxidation, a chemical reaction that can produce free radicals. Autoxidation leads to degradation of organic compounds, including living matter. Antioxidants are frequently added to industrial products, such as polymers, fuels, and lubricants, to extend their usable lifetimes. Foods are also treated with antioxidants to forestall spoilage, in particular the rancidification of oils and fats. In cells, antioxidants such as glutathione, mycothiol, or bacillithiol, and enzyme systems like superoxide dismutase, can prevent damage from oxidative stress.

Rancidification is the process of complete or incomplete autoxidation or hydrolysis of fats and oils when exposed to air, light, moisture, or bacterial action, producing short-chain aldehydes, ketones and free fatty acids.

<span class="mw-page-title-main">Cumene process</span> Industrial process

The cumene process is an industrial process for synthesizing phenol and acetone from benzene and propylene. The term stems from cumene, the intermediate material during the process. It was invented by R. Ūdris and P. Sergeyev in 1942 (USSR), and independently by Heinrich Hock in 1944.

<span class="mw-page-title-main">Epoxide</span> Organic compounds with a carbon-carbon-oxygen ring

In organic chemistry, an epoxide is a cyclic ether, where the ether forms a three-atom ring: two atoms of carbon and one atom of oxygen. This triangular structure has substantial ring strain, making epoxides highly reactive, more so than other ethers. They are produced on a large scale for many applications. In general, low molecular weight epoxides are colourless and nonpolar, and often volatile.

<span class="mw-page-title-main">Polymer degradation</span> Alteration in the polymer properties under the influence of environmental factors

Polymer degradation is the reduction in the physical properties of a polymer, such as strength, caused by changes in its chemical composition. Polymers and particularly plastics are subject to degradation at all stages of their product life cycle, including during their initial processing, use, disposal into the environment and recycling. The rate of this degradation varies significantly; biodegradation can take decades, whereas some industrial processes can completely decompose a polymer in hours.

<span class="mw-page-title-main">Drying oil</span> Oil that hardens after exposure to air

A drying oil is an oil that hardens to a tough, solid film after a period of exposure to air, at room temperature. The oil hardens through a chemical reaction in which the components crosslink by the action of oxygen. Drying oils are a key component of oil paint and some varnishes. Some commonly used drying oils include linseed oil, tung oil, poppy seed oil, perilla oil, castor oil and walnut oil. The use of natural drying oils has declined over the past several decades, as they have been replaced by alkyd resins and other binders.

<span class="mw-page-title-main">Allyl group</span> Chemical group (–CH₂–CH=CH₂)

In organic chemistry, an allyl group is a substituent with the structural formula −CH2−HC=CH2. It consists of a methylene bridge attached to a vinyl group. The name is derived from the scientific name for garlic, Allium sativum. In 1844, Theodor Wertheim isolated an allyl derivative from garlic oil and named it "Schwefelallyl". The term allyl applies to many compounds related to H2C=CH−CH2, some of which are of practical or of everyday importance, for example, allyl chloride.

Lipid peroxidation, or lipid oxidation, is a complex chemical process that leads to oxidative degradation of lipids, resulting in the formation of peroxide and hydroperoxide derivatives. It occurs when free radicals, specifically reactive oxygen species (ROS), interact with lipids within cell membranes, typically polyunsaturated fatty acids (PUFAs) as they have carbon–carbon double bonds. This reaction leads to the formation of lipid radicals, collectively referred to as lipid peroxides or lipid oxidation products (LOPs), which in turn react with other oxidizing agents, leading to a chain reaction that results in oxidative stress and cell damage.

<span class="mw-page-title-main">Diethyl ether peroxide</span> Chemical compound

Diethyl ether hydroperoxide is the organic compound with the formula C2H5OCH(OOH)CH3. It is a colorless liquid. Diethyl ether hydroperoxide and its condensation products are responsible for the explosive organic peroxides that slowly form upon exposure of diethyl ether to ambient air and temperature conditions.

<span class="mw-page-title-main">Hydroquinone</span> Chemical compound

Hydroquinone, also known as benzene-1,4-diol or quinol, is an aromatic organic compound that is a type of phenol, a derivative of benzene, having the chemical formula C6H4(OH)2. It has two hydroxyl groups bonded to a benzene ring in a para position. It is a white granular solid. Substituted derivatives of this parent compound are also referred to as hydroquinones. The name "hydroquinone" was coined by Friedrich Wöhler in 1843.

<span class="mw-page-title-main">Organic peroxides</span> Organic compounds of the form R–O–O–R’

In organic chemistry, organic peroxides are organic compounds containing the peroxide functional group. If the R′ is hydrogen, the compounds are called hydroperoxides, which are discussed in that article. The O−O bond of peroxides easily breaks, producing free radicals of the form RO•. Thus, organic peroxides are useful as initiators for some types of polymerization, such as the acrylic, unsaturated polyester, and vinyl ester resins used in glass-reinforced plastics. MEKP and benzoyl peroxide are commonly used for this purpose. However, the same property also means that organic peroxides can explosively combust. Organic peroxides, like their inorganic counterparts, are often powerful bleaching agents.

<span class="mw-page-title-main">Cumene</span> Chemical compound

Cumene (isopropylbenzene) is an organic compound that contains a benzene ring with an isopropyl substituent. It is a constituent of crude oil and refined fuels. It is a flammable colorless liquid that has a boiling point of 152 °C. Nearly all the cumene that is produced as a pure compound on an industrial scale is converted to cumene hydroperoxide, which is an intermediate in the synthesis of other industrially important chemicals, primarily phenol and acetone.

<span class="mw-page-title-main">Hydroperoxide</span> Class of chemical compounds

Hydroperoxides or peroxols are compounds of the form ROOH, where R stands for any group, typically organic, which contain the hydroperoxy functional group. Hydroperoxide also refers to the hydroperoxide anion and its salts, and the neutral hydroperoxyl radical (•OOH) consist of an unbond hydroperoxy group. When R is organic, the compounds are called organic hydroperoxides. Such compounds are a subset of organic peroxides, which have the formula ROOR. Organic hydroperoxides can either intentionally or unintentionally initiate explosive polymerisation in materials with unsaturated chemical bonds.

<span class="mw-page-title-main">Hindered amine light stabilizers</span>

Hindered amine light stabilizers (HALS) are chemical compounds containing an amine functional group that are used as stabilizers in plastics and polymers. These compounds are typically derivatives of tetramethylpiperidine and are primarily used to protect the polymers from the effects of photo-oxidation; as opposed to other forms of polymer degradation such as ozonolysis. They are also increasingly being used as thermal stabilizers, particularly for low and moderate level of heat, however during the high temperature processing of polymers they remain less effective than traditional phenolic antioxidants.

In polymers, such as plastics, thermal degradation refers to a type of polymer degradation where damaging chemical changes take place at elevated temperatures, without the simultaneous involvement of other compounds such as oxygen. Simply put, even in the absence of air, polymers will begin to degrade if heated high enough. It is distinct from thermal-oxidation, which can usually take place at less elevated temperatures.

Polymer stabilizers are chemical additives which may be added to polymeric materials to inhibit or retard their degradation. Mainly they protect plastic and rubber products against heat, oxidation, and UV light. The biggest quantity of stabilizers is used for polyvinyl chloride (PVC), as the production and processing of this type of plastic would not be possible without stablizing chemicals. Common polymer degradation processes include oxidation, UV-damage, thermal degradation, ozonolysis, combinations thereof such as photo-oxidation, as well as reactions with catalyst residues, dyes, or impurities. All of these degrade the polymer at a chemical level, via chain scission, uncontrolled recombination and cross-linking, which adversely affects many key properties such as strength, malleability, appearance and colour.

<span class="mw-page-title-main">Photo-oxidation of polymers</span>

In polymer chemistry photo-oxidation is the degradation of a polymer surface due to the combined action of light and oxygen. It is the most significant factor in the weathering of plastics. Photo-oxidation causes the polymer chains to break, resulting in the material becoming increasingly brittle. This leads to mechanical failure and, at an advanced stage, the formation of microplastics. In textiles the process is called phototendering.

<span class="mw-page-title-main">Radical (chemistry)</span> Atom, molecule, or ion that has an unpaired valence electron; typically highly reactive

In chemistry, a radical, also known as a free radical, is an atom, molecule, or ion that has at least one unpaired valence electron. With some exceptions, these unpaired electrons make radicals highly chemically reactive. Many radicals spontaneously dimerize. Most organic radicals have short lifetimes.

α-Methylstyrene (AMS) is an organic compound with the formula C6H5C(CH3)=CH2. It is a colorless oil.

<span class="mw-page-title-main">Cumene hydroperoxide</span> Aromatic organic chemical compound

Cumene hydroperoxide is the organic compound with the formula C6H5C(CH3)2OOH. An oily liquid, it is classified as an organic hydroperoxide. Products of decomposition of cumene hydroperoxide are methylstyrene, acetophenone, and 2-phenyl-2-propanol.

References

  1. ↑ Foote, Christopher S. (1996). "2. Autoxidation". Active Oxygen in Chemistry. Dordrecht: Springer Netherlands. pp. 24–65. ISBN   978-94-007-0874-7. doi : 10.1007/978-94-007-0874-7_2
  2. ↑ Holman, Ralph T. (January 1954). "Autoxidation of fats and related substances". Progress in the Chemistry of Fats and Other Lipids. 2: 51–98. doi:10.1016/0079-6832(54)90004-X.
  3. ↑ Helberg, Julian; Pratt, Derek A. (2021). "Autoxidation vs. antioxidants – the fight for forever". Chemical Society Reviews. 50 (13): 7343–7358. doi:10.1039/D1CS00265A. PMID   34037013.
  4. ↑ Frank, Charles E. (February 1950). "Hydrocarbon Autoxidation". Chemical Reviews. 46 (1): 155–169. doi:10.1021/cr60143a003. PMID   24537520.
  5. ↑ Simic, Michael G. (February 1981). "Free radical mechanisms in autoxidation processes". Journal of Chemical Education. 58 (2): 125. Bibcode:1981JChEd..58..125S. doi: 10.1021/ed058p125 .
  6. ↑ Named after Geoffrey Gee and John Lawson Bolland
  7. ↑ Hammond, Earl G.; White, Pamela J. (July 2011). "A Brief History of Lipid Oxidation". Journal of the American Oil Chemists' Society. 88 (7): 891–897. doi:10.1007/s11746-011-1761-8. S2CID   84637577.
  8. 1 2 Smith, Leesa M.; Aitken, Heather M.; Coote, Michelle L. (18 September 2018). "The Fate of the Peroxyl Radical in Autoxidation: How Does Polymer Degradation Really Occur?". Accounts of Chemical Research. 51 (9): 2006–2013. doi:10.1021/acs.accounts.8b00250. hdl: 1885/209140 . PMID   30016062. S2CID   51679950.
  9. ↑ Bolland, J. L.; Gee, Geoffrey (1946). "Kinetic studies in the chemistry of rubber and related materials. II. The kinetics of oxidation of unconjugated olefins". Transactions of the Faraday Society. 42: 236. doi:10.1039/TF9464200236.
  10. ↑ K. U. Ingold (1961). "Inhibition of the Autoxidation of Organic Substances in the Liquid Phase". Chem. Rev. 61 (6): 563–589. doi:10.1021/cr60214a002.
  11. ↑ Atmospheric oxidation and antioxidants. Amsterdam: Elsevier. 1993. ISBN   0-444-89615-5.
  12. ↑ Choe, Eunok; Min, David B. (September 2006). "Mechanisms and Factors for Edible Oil Oxidation". Comprehensive Reviews in Food Science and Food Safety. 5 (4): 169–186. doi:10.1111/j.1541-4337.2006.00009.x.
  13. ↑ "Initiation of the Autoxidation of Polyunsaturated Fatty Acids (PUFA) by Ozone and Nitrogen Dioxide". Autoxidation in food and biological systems. New York: Plenum Press. 1980. pp. 1–16. ISBN   978-1-4757-9351-2. doi : 10.1007/978-1-4757-9351-2_1
  14. ↑ Grause, Guido; Chien, Mei-Fang; Inoue, Chihiro (November 2020). "Changes during the weathering of polyolefins". Polymer Degradation and Stability. 181: 109364. doi:10.1016/j.polymdegradstab.2020.109364. S2CID   225243217.
  15. ↑ David, C. (1975). "Chapter 4 Oxidative Degradation of Polymers". Degradation of polymers. Amsterdam: Elsevier Scientific Pub. Co. pp. 425–538. ISBN   978-0-444-41155-6. doi : 10.1016/S0069-8040(08)70336-4
  16. ↑ Wexler, Herman (1 December 1964). "Polymerization of Drying Oils". Chemical Reviews. 64 (6): 591–611. doi:10.1021/cr60232a001.
  17. ↑ Honzíček, Jan (17 July 2019). "Curing of Air-Drying Paints: A Critical Review". Industrial & Engineering Chemistry Research. 58 (28): 12485–12505. doi:10.1021/acs.iecr.9b02567. hdl: 10195/74955 .
  18. ↑ Grassie, Norman (1988) [1985]. Polymer degradation & stabilisation (1st pbk. ed.). Cambridge [England]: Cambridge University Press. ISBN   9780521357975.
  19. ↑ Prabhu, H. Ramachandra (August 2000). "Lipid peroxidation in culinary oils subjected to thermal stress". Indian Journal of Clinical Biochemistry. 15 (1): 1–5. doi:10.1007/BF02873539. PMC   3453543 . PMID   23105229.
  20. ↑ I.V. Berezin, E.T. Denisov, The Oxidation of Cyclohexane, Pergamon Press, New York, 1996.
  21. ↑ Teles, J. Henrique; Hermans, Ive; Franz, Gerhard; Sheldon, Roger A. (2015). "Oxidation". Ullmann's Encyclopedia of Industrial Chemistry. pp. 1–103. doi:10.1002/14356007.a18_261.pub2. ISBN   978-3-527-30385-4.
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