Autoxidation

Autoxidation called oxidation by atmospheric oxygen. The autoxidation is very slow and no noticeable heat or flame appearance, in contrast to combustion. In this case be made ​​of oxidation- sensitive substances containing conjugated double bonds such as Linoleic acid, hydrocarbons or odor-active substances from the group of terpenes first formed hydroperoxides which can react slowly to form alcohols, aldehydes, ketones and carboxylic acids. The reaction is significantly accelerated by light, especially ultraviolet light, and traces of metals. The autoxidation is the reason for the aging of materials in the air. In metals, the process is usually referred to as corrosion.

Occurrence

Typical examples are the fading of colors, the aging of plastics, rancidity of edible fat and curing oxidatively drying paints. The classic textbook example is the formation of explosive peroxides from ethers ( eg diethyl ether, tetrahydrofuran) on prolonged standing in air.

Mechanism

The autoxidation of hydrocarbons, such as Cyclohexane, a radical chain reaction in which a number of different reaction steps expires. First, a free-radical initiator will react with oxygen to form a peroxy radical. This peroxy radical abstracts a hydrogen atom of an alkyl chain, which leads to a hydroperoxide and an alkyl radical. The alkyl radical, in turn, reacts with oxygen to form a peroxy radical. Through this process, hydroperoxides ( ROOH ) are first formed, which can break under the O- O bond to each alkoxy radical and a hydroxyl radical (OH • ) decay. These radicals can abstract more hydrogen atoms from the substrate and thereby alcohol (ROH ) or water ( H2O) to form alkyl radicals. The latter in turn react with oxygen to form peroxy radicals. Because through this process increases the steady-state concentration of the chain carrier ( peroxyl radicals ROO • ), autoxidations run faster at high ROOH concentration. Although oxygen itself is a diradical, but reacts at room temperature with no hydrocarbons, since the formation of an alkyl radical and a Hydroperoxylradikals of oxygen and alkane is endothermic and has a very high activation barrier.

Chain initiation

Chain propagation

Chain termination

Alcohol and ketone formation

The ketone ( QO ) is in simple substrates (eg cyclohexane) not by primary reactions, but as shown in the above scheme by secondary reactions with Next ROOH formed. The intercurrent radical Q • OOH is not stable: it eliminated a hydroxyl radical and the ketone QO. The alcohol (ROH) can be formed in a subsequent reaction cage ( reactive particles close together ). Because it is chain reactions in autoxidation, the rate of chain termination is not sufficient to explain the high alcohol and ketone levels.

Reaction rate

In steady-state ( "steady state" ) state, the radical concentration is constant, that is, the start speed of the chain is identical to that of chain termination.

Demarcation

In some sources, each is called oxidation with atmospheric oxygen as autoxidation, including combustion and enzymatic processes such as Modern wood.