Auto-oxidation

Autoxidation refers to oxidation by air oxygen . The autoxidation proceeds very slowly and without any noticeable development of heat or flames, in contrast to combustion . Oxides are formed from oxidation-sensitive substances. Such oxides are e.g. E.g .: alcohols , aldehydes , ketones and carboxylic acids . From substances with conjugated double bonds such as B. linoleic acid , hydrocarbons or odor-active substances from the group of terpenes are initially formed hydroperoxides . These oxidized products can slowly react on to their next oxidation stage . The reaction is significantly accelerated by light, especially ultraviolet light, and traces of metals. Autoxidation is one reason why materials age in air. In the case of metals, the process is usually referred to as corrosion .

Occur

Typical examples are the fading of colors, the aging of plastics , the rancidity of cooking oil and the hardening of oxidatively drying paints . The classic textbook example is the formation of explosive peroxides from ethers (e.g. diethyl ether , tetrahydrofuran ) when standing in air for a long time.

mechanism

The auto-oxidation of hydrocarbons, e.g. B. Cyclohexane , is a radical chain reaction in which a variety of different partial reactions take place. First, an initiator radical reacts with oxygen to form a peroxy radical. This peroxy radical abstracts a hydrogen atom from an alkyl chain, resulting in a hydroperoxide and an alkyl radical. The alkyl radical in turn reacts with oxygen to form a peroxy radical. This process initially forms hydroperoxides (ROOH), which can break down into an alkoxy radical and a hydroxyl radical (OH ^• ) by breaking the O – O bond . These radicals can abstract further H atoms from the substrate and thereby form alcohol (ROH) or water (H ₂ O) and alkyl radicals. The latter in turn react with oxygen to form peroxy radicals. Because this process increases the quasi-stationary concentration of the chain carriers (peroxyl radicals ROO ^• ), auto-oxidations run faster at a high ROOH concentration. Oxygen itself is a diradical, but does not react with hydrocarbons at room temperature, since the formation of an alkyl radical and a hydroperoxyl radical from oxygen and alkane is endothermic and has a very high activation barrier.

Chain start

${\displaystyle \mathrm {ROOH+RH\ {\xrightarrow {Energie}}\ RO{\cdot }+{\cdot }OH+RH\ \longrightarrow {}\ RO{\cdot }+H_{2}O+R{\cdot }\quad } }$

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

Chain propagation

${\displaystyle \mathrm {R{^{\cdot }}+O_{2}\ {\xrightarrow {schnell}}\ ROO{^{\cdot }}} }$

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

Chain termination

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

Alcohol and ketone formation

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

${\displaystyle \mathrm {ROOH+QO+^{\cdot }OH+RH\ \longrightarrow {}\ ROOH+QO+H_{2}O+R^{\cdot }\ \longrightarrow {}\ RO^{\cdot }+ROH+QO+H_{2}O} }$

With simple substrates (e.g. cyclohexane), the ketone (QO) is not formed by primary reactions but, as shown in the diagram above, by secondary reactions with ROOH. The radical Q ^• OOH that appears in the meantime is not stable: It eliminates a hydroxyl radical and becomes the ketone QO. The alcohol (ROH) can be formed in a subsequent cage reaction (reactive particles close together). Because auto-oxidation is a chain reaction, the rate of chain termination is insufficient to explain the high alcohol and ketone levels.

Reaction speed

In the quasi-steady state, the radical concentration is constant, that is to say the speed of the chain start is identical to that of the chain termination.

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

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

Demarcation

In some sources, every oxidation with atmospheric oxygen is called autoxidation, including combustion and enzymatic processes such as the modernization of wood.

Individual evidence

1. ^ DA Pratt, KA Tallman, NA Porter: Free radical oxidation of polyunsaturated lipids: New mechanistic insights and the development of peroxyl radical clocks. In: Acc Chem Res. 44 (6), Jun 21, 2011, pp. 458-467. PMID 21486044 .
2. ↑ L. Hagvall, M. Sköld, J. Bråred-Christensson, A. Börje, AT Karlberg: Lavender oil lacks natural protection against autoxidation, forming strong contact allergens on air exposure. In: Contact Dermatitis . 59 (3), Sep 2008, pp. 143-150. PMID 18759894 .
3. ^ IV Berezin, ET Denisov: The Oxidation of Cyclohexane. Pergamon Press, New York 1996.
4. ^ I. Hermans, TL Nguyen, PA Jacobs, J. Peeters: ChemPhysChem. 6, 2005, pp. 637-645.