Autoxidation

Autoxidation is any oxidation that occurs in open air or in presence of oxygen (and sometimes UV radiation) and forms peroxides and hydroperoxides. A classic example of autoxidation is that of simple ethers like diethyl ether, whose peroxides can be dangerously explosive. It can be considered to be a slow, flameless combustion of materials by reaction with oxygen. Autoxidation is important because it is a useful reaction for converting compounds to oxygenated derivatives, and also because it occurs in situations where it is not desired (as in the destructive cracking of the rubber in automobile tires or in rancidification).

Although virtually all types of organic materials can undergo air oxidation, certain types are particularly prone to autoxidation, including unsaturated compounds that have allylic or benzylic hydrogen atoms; these materials are converted to hydroperoxides by autoxidation.

Mechanism

Autoxidation is a free radical chain process. Such reactions can be divided into three stages: chain initiation, propagation, and termination. In the initiation process, some event causes free radicals to be formed. For example, free radicals can be produced purposefully by the decomposition of a radical initiator, such as benzoyl peroxide. In some cases, initiation occurs by a process that is not well understood but is thought to be the spontaneous reaction of oxygen with a material with a readily abstractable hydrogen. Destructive autoxidation processes also are initiated by pollutants such as those in smog.

Once free radicals are formed, they react in a chain to convert the material to a hydroperoxide. The chain is ended by termination reactions in which free radicals collide and combine their odd electrons to form a new bond.

Chain initiation

$\mathrm{ROOH + RH \ \xrightarrow {energy} \ RO{\cdot} + {\cdot}OH + RH \ \longrightarrow {} \ RO{\cdot} + H_2O + R{\cdot} \quad}$

$\mathrm{RO{\cdot} + RH \ \xrightarrow {H-abstraction} \ R{\cdot} + ROH \quad}$

Chain propagation

$\mathrm{R{^{\cdot}} + O_2 \ \xrightarrow {fast} \ ROO{^{\cdot}}}$

$\mathrm{ROO{^{\cdot}} + RH \ \xrightarrow {H-abstraction} \ ROOH + {^{\cdot}}R}$

Chain termination

$\mathrm{2 ROO{^{\cdot}} \ \xrightarrow {} \ 2 RO{^{\cdot}} + O_2 \ \longrightarrow {} \ ROH + QO + O_2}$

Source of alcohol and ketone

$\mathrm{ROOH + ROO{^{\cdot}} \ \longrightarrow {} \ ROOH + Q{^{\cdot}}OOH \ \longrightarrow {} \ ROOH + QO + ^{\cdot}OH }$

$\mathrm{ROOH + QO + ^{\cdot}OH + RH \ \longrightarrow {} \ ROOH + QO + H_2O + R^{\cdot} \ \longrightarrow {} \ RO^{\cdot} + ROH + QO + H_2O }$

Reaction rate

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.