Autoxidation

Autoxidation (sometimes auto-oxidation) refers to oxidations brought about by oxygen at normal temperatures, without the intervention of flame or electric spark. The term is usually used to describe the degradation of organic compounds in air. Many common phenomena can be attributed to autoxidation, such as food going rancid, the 'drying' of varnishes and paints and the perishing of rubber. It is also an important concept in both industrial chemistry and biology. Autoxidation is therefore a fairly broad term and can encompass examples of photooxygenation and catalytic oxidation.

The common mechanism is a free radical chain reaction, where the addition of oxygen gives rise to hydroperoxides and their associated peroxy radicals (ROO•). 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 effected 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 with the autoxidation of cyclohexane to cyclohexanol and cyclohexanone which takes place at 160°C.

Mechanism

The free radical chain reaction can be divided into three stages: initiation, propagation, and termination. The initiation step is often ill-defined and many agents have been proposed as radical initiators. The autoxidation of unsaturated compound may be initiated by reactions with singlet oxygen or environmental pollutants such as ozone and NO₂. Saturated polymers, such as polyolefins would be expected to resist autoxidation, however in practise they contain hydroperoxides formed by thermal degradation during their high temperature moulding and casting. In biological systems the superoxide radical is 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 (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 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. Chain initiation

${\begin{array}{l}{\ce {{ROOH}+RH->[{\overset {}{\ce {energy}}}]{RO.}+{.OH}+RH->{RO.}+{H2O}+R.}}\\{\ce {{RO.}+RH->[{\ce {H-abstraction}}]{R.}+ROH}}\end{array}}$

Chain propagation

${\begin{array}{l}{\ce {{R.}+ O2 ->[{\ce {fast}}] ROO.}}\\{\ce {{ROO.}+ RH ->[{\ce {H-abstraction}}] {ROOH}+ .R}}\end{array}}$

Chain termination

${\ce {2 ROO. -> {2 RO.}+ O2 -> {ROH}+ {QO}+ O2}}$

Source of alcohol and ketone

${\begin{array}{l}{\ce {{ROOH}+ {ROO.}-> {ROOH}+ {Q.}OOH -> {ROOH}+ {QO}+ .OH}}\\{\ce {{ROOH}+ {QO}+ {.OH}+ RH -> {ROOH}+ {QO}+ {H2O}+ R. -> {RO.}+ {ROH}+ {QO}+ H2O}}\end{array}}$

In steady state, the concentration of chain-carrying radicals is constant, thus the rate of initiation equals the rate of termination.

$\mathrm {r_{init}=k_{init}\cdot [ROOH]=k_{term}\cdot [ROO^{\cdot }]^{2}}$

$\mathrm {r_{prop}=k_{prop}\cdot [RH]\cdot [ROO^{\cdot }]=k_{prop}\cdot [RH]\cdot {\sqrt[{\,}]{\frac {k_{init}}{k_{term}}}}\cdot {\sqrt[{\,}]{[ROOH]}}}$

Autoxidations in industry

Autoxidation is a process of enormous economic impact, since all foods, plastics, gasolines, oils, rubber, and other materials that must be exposed to air undergo continuous destructive reactions of this type. All plastics and rubber and most processed foods contain antioxidants to protect them against the attack of oxygen.

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.
• p-xylene is oxidized to terephthalic acid
• ethylbenzene is oxidized to ethylbenzene hydroperoxide, an epoxidizing agent in the propylene oxide/styrene process POSM.

Autoxidation in food

It is well known that fats become rancid, even when kept at low temperatures. This is especially true for polyunsaturated fats.

The complex mixture of compounds found in wine, including polyphenols, polysaccharides, and proteins, can undergo autoxidation during the aging process. Simple polyphenols can lead to the formation of B-type procyanidins in wines or in model solutions. This is correlated to the browning color change characteristic of this process.

This phenomenon is also observed in carrot puree.