Autoxidation

The free radical chain reaction is sometimes referred to as the Bolland-Gee mechanismNamed after Geoffrey Gee and John Lawson Bolland{{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 }} or the basic autoxidation scheme (BAS){{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 }} and was originally based on the oxidation of rubbers,{{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 |pages=236 |doi=10.1039/TF9464200236}} but remains generally accurate for many materials. It can be divided into three stages: initiation, propagation, and termination.{{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}} The initiation step is often ill-defined and many agents have been proposed as radical initiators.{{cite book |title=Atmospheric oxidation and antioxidants |date=1993 |publisher=Elsevier |location=Amsterdam |isbn=0-444-89615-5}} The autoxidation of unsaturated compounds may be initiated by reactions with singlet oxygen{{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}} or environmental pollutants such as ozone and NO₂.{{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}} 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.{{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 |pages=109364 |doi=10.1016/j.polymdegradstab.2020.109364|s2cid=225243217 }}{{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}} 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, 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.

File:Polymer auto-oxidation.png

Chain initiation

:: Polymer -> P\bullet +\ P\bullet

Chain propagation

::P\bullet +\ O2 -> POO\bullet

::POO\bullet +\ PH -> {POOH} +\ P\bullet

Chain branching

::POOH -> PO\bullet +\ OH\bullet

::{PH} + OH\bullet -> P\bullet +\ H2O

::PO\bullet -> Chain\ scission\ reactions

Termination

::POO\bullet +\ POO\bullet -> cross\ linking\ reaction\ to\ non-radical\ product

::POO\bullet +\ P\bullet -> cross\ linking\ reaction\ to\ non-radical\ product

:: P\bullet +\ P\bullet -> cross\ linking\ reaction\ to\ non-radical\ product

The autoxidation of unsaturated fatty acids causes them to crosslink to form polymers.{{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}} This phenomenon has been known since antiquity and forms the basis of drying oils, which were traditionally used to make many varnishes and paints.{{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 }} Linseed oil, which is rich in polyunsaturated fats, is a prime example.

Conversely, autoxidation can also cause polymers such as plastics to deteriorate.{{cite book |last1=Grassie |first1=Norman |title=Polymer degradation & stabilisation |year=1988 |orig-year=1985 |publisher=Cambridge University Press |location=Cambridge [England] |isbn=9780521357975 |edition=1st pbk.}} 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.

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

• in the cumene process, isopropylbenzene undergoes autoxidation to give cumene hydroperoxide. This compound is then converted to phenol and acetone, both commodity chemicals. are made from benzene and propylene. Many variations of this reaction have been developed, e.g. use of diisopropylbenzene as a substrate.
• the autoxidation of cyclohexane yields cyclohexanol and cyclohexanone.I.V. Berezin, E.T. Denisov, The Oxidation of Cyclohexane, Pergamon Press, New York, 1996.
• p-xylene undergoes auoxidation to terephthalic acid.
• ethylbenzene is oxidized to ethylbenzene hydroperoxide, an epoxidizing agent in the propylene oxide/styrene process POSM

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.{{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 }}

• 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: {{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 }}

Category:Organic redox reactions