Name: autoxidation in oils and ethers
Uploaded: 2023-04-30T11:21:36.000Z
Duration: 1 min 20 s
Channel: JoVE

Oxidation of organic compounds in the presence of air or atmospheric oxygen is termed autoxidation.
It is a free radical chain reaction consisting of initiation, propagation, and termination steps.
In the initiation step, an initiator abstracts hydrogen from the organic compound to form a carbon-centered radical.
The radical then enters the first propagation step, where it couples with the molecular oxygen to form a peroxy radical.
In the second propagation step, the peroxy radical abstracts hydrogen from another organic compound molecule forming the product peroxide and regenerating the carbon-centered radical.
The regenerated radical further propagates the chain reaction.
Finally, in the termination step, two carbon-centered radicals couple together to form a non-radical product.
Generally, the autoxidation process is initiated by light. In the absence of light, the process is very slow. For this reason, most organic chemicals are stored in dark bottles.
In particular, compounds with allylic and benzylic hydrogens are more susceptible to autoxidation as the free radicals formed in the initiation step are resonance stabilized.

radical autoxidation

Radical Autoxidation

The oxidation of an organic compound in the presence of air or oxygen is called autoxidation. For example, cumene reacts with oxygen to form hydroperoxide. Autoxidation involves initiation, propagation, and termination steps. Many organic compounds are susceptible to autoxidation&#8212;especially ethers in the presence of oxygen, which form hydroperoxides. Even though this reaction is slow, old ether bottles contain small amounts of peroxide, which leads to laboratory explosions during ether distillation, making them dangerous to use. Naturally occurring fats and oil, like vegetable oil, can also undergo autoxidation. These oils are generally a mixture of triglycerides, which consist of three long hydrocarbon chains with double bonds.
<img alt="Triglyceride structure, chemical formula diagram, lipid study, organic chemistry research." src="/files/ftp_upload/12893/PT_12893_Figure%201.svg" style="text-align: center; height: 215px;" data-altupdatedat="1761830439000" data-alt="Figure1" />
Typically, the allylic position of triglycerides gets oxidized to form hydroperoxides that are responsible for the foul smell of foods containing unsaturated oils. Consequently, foods with unsaturated oils have a short lifetime unless radical inhibitors are used to inhibit the radical formation. These radical inhibitors, such as BHT and BHA, are used as food preservatives. Both BHT and BHA react with radicals to form resonance stabilized radicals. Additionally, the tert-butyl groups of these compounds sterically hinder the radical center and decrease the reactivity of the radicals. Thus, these radical inhibitors are called antioxidants, as they scavenge and destroy radicals.
<img alt="Chemical structure diagrams of antioxidants BHT and BHA, showing molecular formulas." src="/files/ftp_upload/12893/PT_12893_Figure%202.svg" style="text-align: center; height: 175px;" data-altupdatedat="1761830439000" data-alt="Figure2" />
One can also find several natural antioxidants that help prevent the oxidation of cell membranes and biologically important compounds. For example, vitamin E and vitamin C are natural antioxidants. These compounds react with the reactive radicals to form less reactive and stabilized radicals.
<img alt="Chemical structures of vitamins; Vitamin E and Vitamin C, structural formula diagram." src="/files/ftp_upload/12893/PT_12893_Figure%203.svg" style="text-align: center; height: 135px;" data-altupdatedat="1761830439000" data-alt="Figure3" />

The oxidation of an organic compound in the presence of air or oxygen is called autoxidation. For example, cumene reacts with oxygen to form ...

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JoVE Core

Organic Chemistry

Unit 1

Radical Chemistry

KEY TERMS AND CONCEPTS

autoxidation in oils and ethers

Autoxidation is the oxidation of an organic compound in the presence of air or oxygen. In this process, compounds such as cumene can react with oxygen to form hydroperoxide. The reaction follows three main stages: initiation, propagation, and termination.

Many organic compounds can undergo autoxidation. Ethers are especially vulnerable when oxygen is present, and they can form hydroperoxides. Even though this reaction is slow, old ether bottles may contain small amounts of peroxide. That is why ether can become dangerous during distillation and may lead to laboratory explosions.

Fats and oils can also autoxidize. Vegetable oil is one common example. These oils are usually mixtures of triglycerides, which are molecules made of three long hydrocarbon chains with double bonds. In many cases, the allylic position of these triglycerides is oxidized to form hydroperoxides. These products help cause the foul smell that develops in foods with unsaturated oils.

Foods that contain unsaturated oils often have a short shelf life unless radical inhibitors are added. Common inhibitors such as BHT and BHA are used as food preservatives. They react with radicals to form resonance-stabilized radicals, and their tert-butyl groups also help block the reactive center. Because they remove radicals, these compounds act as antioxidants.

Natural antioxidants work in a similar way. Vitamin E and vitamin C can help prevent the oxidation of cell membranes and other biologically important compounds. They react with reactive radicals and form less reactive, stabilized radicals.

Autoxidation in Oils and Ethers

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Radical Geometry: Planarity and Inversion

Radical geometry depends on the electronic structure around the radical center. An unpaired electron, like a lone pair, affects the shape of the molecule. For a trivalent radical, the structure falls between the geometries of carbocations and carbanions. A carbocation with sp2 hybridization is trigonal planar. A carbanion with sp3 hybridization is trigonal pyramidal. The difference comes from the number of nonbonding electrons. The carbocation has none, while the carbanion has two. A carbon...

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EPR Hyperfine Splitting in Organic Radicals

Electron paramagnetic resonance (EPR) spectroscopy can show how an unpaired electron behaves in an organic radical. A simple radical may give a single peak because the electron moves between two spin energy states. In many radicals, the unpaired electron also interacts with nearby spin-active nuclei. This interaction is called hyperfine coupling. It causes hyperfine splitting, which divides the EPR signal into multiplets, or groups of peaks. The number of peaks follows the pattern 2nI + 1,...

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How Radicals Are Generated

Radicals are generated when bonds break in specific ways or when one radical leads to another. In homolytic bond cleavage, a bond splits evenly and forms radicals. In heterolytic bond cleavage, the bond breaks to form ions instead. A fishhook arrow shows the movement of a single electron during homolytic cleavage. Radicals can come from spin-paired molecules by homolysis or by electron transfer. Homolysis forms two radicals. Electron transfer adds an electron to the molecule, and this process...

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Homolysis and Radical Bonds

Homolysis is a bond-breaking process that splits a shared electron pair evenly. When a bond is broken with enough energy, the two electrons can also move to one atom in heterolysis, which forms ions. Homolysis gives one electron to each atom and creates radicals, which are highly reactive particles with an unpaired electron. HCl shows the difference between these bond cleavages. In solution, HCl can break into H+ and Cl− ions, and chlorine takes both bonding electrons, leaving a naked proton.

radical formation: abstraction

Radical Abstraction in Chain Reactions

Radical abstraction helps drive radical chain reactions. In this process, a relatively unstable radical removes an atom from a compound and forms a new radical that is more stable. The atom taken in this step can be hydrogen or a halogen. A radical initiator can first form radicals by homolysis, which is the breaking of a spin-paired molecule into two radicals. That is different from abstraction. In abstraction, a radical reacts with a spin-paired molecule and produces one new radical plus one...

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Radical Addition and Spin Conservation

Radical addition can form a new radical when an unpaired electron adds to a spin-paired molecule. This is often seen with unsaturated species. In these reactions, the radical adds across a pi bond and breaks that bond to make a new radical. A bromine radical adding to an alkene is a clear example. The product is a carbon-centered radical. Radical reactions also follow spin conservation, much like chemical reactions follow charge conservation. If the starting material has an unpaired electron,...

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Radicals from Elimination Reactions

Radicals can form through elimination reactions. This route is the opposite of the addition route. It is driven by the instability of the radical. Dibenzoyl peroxide is one example of this process. It first undergoes homolysis, which is an even split of a bond, to give a pair of unstable radicals. Because the radical is unstable, it then undergoes elimination on its own. During elimination, a C-C bond breaks. The cleavage occurs between the alpha and beta positions relative to the radical...

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Radical Cyclization in One Molecule

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Autoxidation is the oxidation of an organic compound in the presence of air or oxygen. In this process, compounds such as cumene can react with oxygen to form hydroperoxide. The reaction follows three main stages: initiation, propagation, and termination. Many organic compounds can undergo autoxidation. Ethers are especially vulnerable when oxygen is present, and they can form hydroperoxides. Even though this reaction is slow, old ether bottles may contain small amounts of peroxide. That is...

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Selective MnO2 Oxidation of Alcohols

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How Alkanes Form Alkyl Halides by Halogenation

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Alkane Halogenation: Stability and Reaction Rates

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Radical Halogenation: Chiral Products

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Allylic Chlorination by Radical Chain Reaction

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radical substitution: allylic bromination

Allylic Bromination with NBS

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Alkyl Halide Reduction with Tributyltin Hydride

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Peroxide Effect in HBr Addition to Alkenes

Hydrogen bromide can add to alkenes through a peroxide-driven radical pathway. In this reaction, hydroperoxides or peroxides change the usual addition pattern and lead to alkyl bromides with anti-Markovnikov regiochemistry, which means bromine ends up on the less substituted carbon. This regioselectivity can be explained by radical stability. When the bromine radical adds to the alkene at the less substituted carbon, the reaction forms a more stable tertiary radical intermediate. The same...

radical anti-markovnikov addition to alkenes: mechanism

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Hydrogen Bromide Addition to Alkenes by Radicals

Hydrogen bromide can add to alkenes through a radical chain reaction when hydroperoxides or peroxides are present. This pathway gives anti-Markovnikov addition, which means the bromine ends up on the less substituted side of the double bond. The reaction begins with chain initiation. First, mild heating breaks the weak peroxide bond by homolytic cleavage, which means each atom keeps one electron and two alkoxy radicals form. Next, an alkoxy radical abstracts a hydrogen atom and forms a bromine...

radical anti-markovnikov addition to alkenes: thermodynamics

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HBr Selectivity in Alkene Addition Thermodynamics

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vicinal diols via reductive coupling of aldehydes or ketones: pinacol coupling overview

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Pinacol Coupling and Solvent Effects

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Titanium-Coupled Alkenes from Carbonyls

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&alpha;-hydroxy ketones via reductive coupling of esters: acyloin condensation overview

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Acyloin Condensation: Ester Coupling Pathway

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key terms and concepts

Learn how autoxidation affects ethers, vegetable oils, and foods, and how antioxidants like BHT, BHA, vitamin E, and vitamin C slow radical reactions.

Autoxidation in Oils and Ethers | JoVE K12

organic chemistry

JoVE Core Organic Chemistry explains basic concepts of stereochemistry, reactions, and mechanisms using concise and easy-to-understand animated video lessons. Additionally, the scientist-in-action videos demonstrate the application of related concepts in classical and original research experiments performed in today's laboratories worldwide.

Autoxidation in Oils and Ethers

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