Autoxidation

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.

The common mechanism is a

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.

Chain initiation

${\displaystyle {\ce {Polymer->P\bullet +\ P\bullet }}}$

Chain propagation

${\displaystyle {\ce {P\bullet +\ O2->POO\bullet }}}$

${\displaystyle {\ce {POO\bullet +\ PH->{POOH}+\ P\bullet }}}$

Chain branching

${\displaystyle {\ce {POOH->PO\bullet +\ OH\bullet }}}$

${\displaystyle {\ce {{PH}+OH\bullet ->P\bullet +\ H2O}}}$

${\displaystyle {\ce {PO\bullet ->Chain\ scission\ reactions}}}$

Termination

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

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

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

In oils and polymers

The autoxidation of unsaturated

Conversely, autoxidation can also cause polymers such as plastics to deteriorate.

In the chemical industry many chemicals are produced by autoxidation:

• in the cumene process phenol and acetone are made from benzene and propylene
• the autoxidation of cyclohexane yields cyclohexanol and cyclohexanone.^[20]
• p-xylene is oxidized to terephthalic acid
• ethylbenzene is oxidized to ethylbenzene hydroperoxide, an epoxidizing agent in the propylene oxide/styrene process POSM

See also

• 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: Frank, Charles E. (1950). "Hydrocarbon Autoxidation". Chemical Reviews. 46 (1): 155–169.