Organic reductions or
organic oxidations or
organic redox reactions are redox reactions that take place with
organic compoundAn organic compound is any member of a large class of chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of compounds such as carbonates, simple oxides of carbon and cyanides, as well as the allotropes of carbon, are considered inorganic...
s. In
organic chemistryOrganic chemistry is a discipline within chemistry which involves the scientific study of the structure, properties, composition, reactions, and preparation of chemical compounds that contain carbon...
oxidations and reductions are different from ordinary redox reactions because many reactions carry the name but do not actually involve electron transfer in the
electrochemicalElectrochemistry is a branch of chemistry that studies chemical reactions which take place in a solution at the interface of an electron conductor and an ionic conductor , and which involve electron transfer between the electrode and the electrolyte or species in solution.If a chemical reaction is...
sense of the word .
Following the rules for determining the
oxidation numberIn coordination chemistry, the oxidation number of a central atom in a coordination compound is the charge that it would have if all the ligands were removed along with the electron pairs that were shared with the central atom...
for an individual carbon atom leads to
- oxidation number -4 for alkane
Alkanes, also known as paraffins, are chemical compounds that consist only of the elements carbon and hydrogen , wherein these atoms are linked together exclusively by single bonds without any cyclic structure...
s,
- oxidation number -2 for alkene
In organic chemistry, an alkene, olefin, or olefine is an unsaturated chemical compound containing at least one carbon-to-carbon double bond...
s, alcoholIn chemistry, an alcohol is any organic compound in which a hydroxyl group is bound to a carbon atom of an alkyl or substituted alkyl group. An important group of acohols is formed by the simple acyclic alcohols, the general formula for which is C
nH
2n+1OH...
s, alkyl halides, amineAmines are organic compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are derivatives of ammonia, wherein one or more hydrogen atoms have been replaced by a substituent such as an alkyl or aryl group. Important amines include amino acids, biogenic amines,...
s,
- oxidation number 0 for alkyne
Alkynes are hydrocarbons that have a triple bond between two carbon atoms, with the formula CnH2n-2. Alkynes are traditionally known as acetylenes, although the name acetylene also refers specifically to C2H2, known formally as ethyne using IUPAC...
s, ketoneIn organic chemistry, a ketone is a type of compound that features a carbonyl group bonded to two other carbon atoms, i.e., R3CCO-CR3 where R can be a variety of atoms and groups of atoms...
s, aldehydeAn aldehyde is an organic compound containing a terminal carbonyl group. This functional group, which consists of a carbon atom bonded to a hydrogen atom and double-bonded to an oxygen atom , is called the aldehyde group...
s, geminalIn chemistry, the term geminal refers to the relationship between two functional groups that are attached to the same atom...
diolA diol or glycol is a chemical compound containing two hydroxyl groups Vicinal diols have hydroxyl groups attached to adjacent atoms. Examples of vicinal diol compounds are ethylene glycol and propylene glycol...
s,
- oxidation number +2 for carboxylic acid
Carboxylic acids are organic acids characterized by the presence of a carboxyl group, which has the formula -COH, usually written -COOH or -CO2H. Carboxylic acids are Brønsted-Lowry acids — they are proton donors. Salts and anions of carboxylic acids are called...
s, amideIn chemistry, an amide is usually an organic compound that contains the functional group consisting of an acyl group linked to a nitrogen atom . The term refers both to a class of compounds and a functional group within those compounds...
s, chloroformChloroform is the organic compound with formula CHCl
3. This colourless, sweet-smelling, dense liquid is a trihalomethane. It is also considered somewhat hazardous...
and
- oxidation number +4 for carbon dioxide
Carbon dioxide is a chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. It is a gas at standard temperature and pressure and exists in Earth's atmosphere in this state...
, tetrachloromethane.
MethaneMethane is a chemical compound with the chemical formula . It is the simplest alkane, and the principal component of natural gas. Methane's bond angles are 109.5 degrees. Burning methane in the presence of oxygen produces carbon dioxide and water. The relative abundance of methane and its clean...
is punched to
carbon dioxideCarbon dioxide is a chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. It is a gas at standard temperature and pressure and exists in Earth's atmosphere in this state...
because the oxidation number changes from -4 to +4. Classical reductions include
alkeneIn organic chemistry, an alkene, olefin, or olefine is an unsaturated chemical compound containing at least one carbon-to-carbon double bond...
reduction to
alkaneAlkanes, also known as paraffins, are chemical compounds that consist only of the elements carbon and hydrogen , wherein these atoms are linked together exclusively by single bonds without any cyclic structure...
s and classical oxidations include oxidation of
alcoholIn chemistry, an alcohol is any organic compound in which a hydroxyl group is bound to a carbon atom of an alkyl or substituted alkyl group. An important group of acohols is formed by the simple acyclic alcohols, the general formula for which is C
nH
2n+1OH...
s to
aldehydeAn aldehyde is an organic compound containing a terminal carbonyl group. This functional group, which consists of a carbon atom bonded to a hydrogen atom and double-bonded to an oxygen atom , is called the aldehyde group...
s with manganese dioxide. In oxidations electrons are removed and the electron density of a molecule is reduced. In reductions electron density increases when electrons are added to the molecule. This terminology is always centered around the organic compound. So a
ketoneIn organic chemistry, a ketone is a type of compound that features a carbonyl group bonded to two other carbon atoms, i.e., R3CCO-CR3 where R can be a variety of atoms and groups of atoms...
is always reduced by
Lithium aluminium hydrideLithium aluminium hydride , commonly abbreviated to LAH, is a reducing agent used in organic synthesis. It is more powerful than the related reagent sodium borohydride due to the weaker Al-H bond compared to the B-H bond...
but it is bad form to have lithium aluminium hydride oxidized by a ketone. Many oxidations involve removal of
protonHydrogen ion is recommended by IUPAC as a general term for all ions of hydrogen and its isotopes. Depending on the charge of the ion, two different classes can be distinguished:-Cation :...
s from the organic molecule and the reverse reduction adds protons to an organic molecule.
Many reactions classified as reductions also appear in other classes. For instance conversion of the ketone to an alcohol by Lithium aluminium hydride can be considered a reduction but the hydride is also a good
nucleophileIn chemistry, a nucleophile is a reagent that forms a chemical bond to its reaction partner by donating both bonding electrons. Because nucleophiles donate electrons, they are by definition Lewis bases . All molecules or ions with a free pair of electrons can act as nucleophiles...
in
nucleophilic substitutionIn organic and inorganic chemistry, nucleophilic substitution is a fundamental class of substitution reaction in which an "electron rich" nucleophile selectively bonds with or attacks the positive or partially positive charge of an atom attached to a group or atom called the leaving group; the...
. Many redox reactions in organic chemistry have
coupling reactionA coupling reaction in organic chemistry is a catch-all for a range of reactions in organometallic chemistry where two hydrocarbon fragments are coupled with the aid of a metal containing catalyst....
reaction mechanismIn chemistry, a reaction mechanism is the step by step sequence of elementary reactions by which overall chemical change occurs .Although only the net chemical change is directly observable for most chemical reactions, experiments can often be designed that suggest the possible sequence of steps in...
involving free radical intermediates. True organic redox chemistry can be found in electrochemical organic synthesis or
electrosynthesisElectrosynthesis in organic chemistry is the synthesis of chemical compounds in a electrochemical cell The main advantage of electrosynthesis over an ordinary redox reaction is avoidance of the potential wasteful other half-reaction and the ability to precisely tune the required potential...
. Examples of organic reactions that can take place in an
electrochemical cellAn electrochemical cell is a device used for generating an electromotive force and current from chemical reactions, or the reverse, inducing a chemical reaction by a current. The current is caused by the reactions releasing and accepting electrons at the different ends of a conductor. A common...
are the
Kolbe electrolysisKolbe electrolysis or Kolbe reaction is an organic reaction named after Adolph Wilhelm Hermann Kolbe. The Kolbe reaction is formally a decarboxylative dimerisation and proceeds by a radical reaction mechanism...
In
disproportionationDisproportionation or dismutation is used to describe two particular types of chemical reaction:* A chemical reaction of the type: 2A → A' + A" where A, A' and A" are different chemical species. Most but not all are redox reactions...
reactions the reactant is both oxidised and reduced in the same chemical reaction forming 2 separate compounds.
Asymmetric catalytic reductionAsymmetric catalytic reduction is the use of various chiral catalysts to reduce a prochiral organic compound to obtain a chiral product. This is one of the several techniques used in chiral synthesis....
s and
asymmetric catalytic oxidationAsymmetric catalytic oxidation is a technique of oxidizing various substrates to give an enantiopure product using a catalyst.-Reactions:*Jacobsen epoxidation of alkenes using manganese-salen complex and NaOCl...
s are important in asymmetric synthesis.
Organic reductions
Several reaction mechanisms exist for organic reductions:
- Direct electron transfer in one-electron reduction
A one-electron reduction in organic chemistry involves the transfer of an electron from a metal to an organic substrate. It serves to differentiate between true organic reductions and other reductions such as hydride transfer reactions that actually involve two-electron species.The first...
with the Birch reductionThe Birch reduction is the organic reduction of aromatic rings with sodium and an alcohol in liquid ammonia to form 1,4-cyclohexadienes. The reaction was reported by the Australian chemist Arthur John Birch in 1944...
as example
- Hydride
Hydride is the name given to the negative ion of hydrogen, H−. Practically, the term hydride has two distinct but overlapping meanings. In the chemical vernacular the term hydride refers to a hydrogen atom that formally reacts as a hydrogen anion under common conditions as well as...
transfer in reductions with for example lithium aluminium hydrideLithium aluminium hydride , commonly abbreviated to LAH, is a reducing agent used in organic synthesis. It is more powerful than the related reagent sodium borohydride due to the weaker Al-H bond compared to the B-H bond...
or a hydride shift as in the Meerwein-Ponndorf-Verley reductionThe Meerwein-Ponndorf-Verley Reduction in organic chemistry is the reduction of ketones to secondary alcohols with aluminumisopropylate catalysis in isopropanol solution ....
- Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. At standard temperature and pressure, hydrogen is a colorless, odorless, nonmetallic, tasteless, highly flammable diatomic gas with the molecular formula H
2...
reductions with a catalyst such as the Lindlar catalystA Lindlar catalyst is a heterogeneous catalyst that consists of palladium deposited on calcium carbonate and treated with various forms of lead. The lead additive serves to deactivate the palladium sites. A variety of "catalyst poisons" have been used including lead acetate and lead oxide. The...
or the Adkins catalystCopper chromite is a complex inorganic composition Cu2Cr2O5, but often containing barium oxide that is used to catalyse certain reactions in organic synthesis. It was first described in 1908...
or in specific reductions such as the Rosenmund reductionThe Rosenmund reduction is a chemical reaction that reduces an acid halide to an aldehyde using hydrogen gas over palladium-on-carbon poisoned with barium sulfate...
.
- Disproportionation
Disproportionation or dismutation is used to describe two particular types of chemical reaction:* A chemical reaction of the type: 2A → A' + A" where A, A' and A" are different chemical species. Most but not all are redox reactions...
reaction such as the Cannizzaro reactionThe Cannizzaro reaction, named after its discoverer Stanislao Cannizzaro, is a chemical reaction that involves the base-induced disproportionation of an aldehyde lacking a hydrogen atom in the alpha position...
Reductions that do not fit in any reduction reaction mechanism and in which just the change in oxidation state is reflected include the Wolff-Kishner reaction.
Organic oxidations
Several reaction mechanisms exist for organic oxidations:
- Single electron transfer
- Oxidations through ester intermediates with chromic acid
Chromic acid generally refers to a collection of compounds generated by the acidification of solutions containing chromate and dichromate anions or the dissolving of chromium trioxide in sulfuric acid. Often the species are assigned the formulas H2CrO4 and...
or manganese dioxide
- Hydrogen atom transfer as in Free radical halogenation
In organic chemistry, free radical halogenation is a type of halogenation. This chemical reaction is typical of alkanes and alkyl-substituted aromatics under application of heat or UV light. The reaction is used for the industrial synthesis of chloroform , dichloromethane and hexachlorobutadiene...
- Oxidation with oxygen
Oxygen Oxygen Oxygen (acid, literally "sharp", from the taste of acids) and -γενής (-genēs) (producer, literally begetter) is the element with atomic number 8 and represented by the symbol O...
(combustionCombustion or burning is a complex sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat or both heat and light in the form of either a glow or flames, appearance of light flickering.Direct combustion by atmospheric oxygen is a reaction...
)
- Oxidation involving ozone
Ozone or trioxygen is a simple triatomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic O
2. Ground-level ozone is an air pollutant with harmful effects on the respiratory systems of animals...
in ozonolysisOzonolysis is the cleavage of an alkene or alkyne with ozone to form organic compounds in which the multiple carbon-carbon bond has been replaced by a double bond to oxygen...
and peroxideA peroxide is a compound containing an oxygen-oxygen single bond. The simplest stable peroxide is hydrogen peroxide. Superoxides, dioxygenyls, ozones and ozonides compound are considered separately.- Organic chemistry :...
s
- Oxidations involving an elimination reaction
An elimination reaction is a type of organic reaction in which two substituents are removed from a molecule in either a one or two-step mechanism...
mechanism such as the Swern oxidationThe Swern oxidation, named after Daniel Swern, is a chemical reaction whereby a primary or secondary alcohol is oxidized to an aldehyde or ketone using oxalyl chloride, dimethyl sulfoxide and an organic base, such as triethylamine...
, the Kornblum oxidationThe Kornblum oxidation is a chemical reaction of a primary halide with dimethyl sulfoxide to form an aldehyde. Like all DMSO-based oxidations, the Kornblum oxidation creates an alkoxysulphonium ion, which, in the presence of a base, such as triethylamine , will eliminate to form the desired...
and with reagents such as IBX acid and Dess-Martin periodinaneDess-Martin periodinane is a chemical reagent used to oxidize primary alcohols to aldehydes and secondary alcohols to ketones. This periodinane has several advantages over chromium- and DMSO-based oxidants that include milder conditions, shorter reaction times, higher yields, and simplified workups...
.
- oxidation by nitroso
Nitroso refers to a functional group in organic chemistry which has the general formula RNO. Nitroso compounds can be prepared by the reduction of nitro compounds or by the oxidation of hydroxylamines...
radicalsIn chemistry, radicals are atoms, molecules, or ions with unpaired electrons on an otherwise open shell configuration. These unpaired electrons are usually highly reactive, so radicals are likely to take part in chemical reactions...
Fremy's saltFrémy's salt, discovered in 1845 by Edmond Frémy , is a chemical compound and a strong oxidizing agent. The formal name is disodium nitrosodisulfonic acid or Na2NO2, but the expression "Frémy's salt" refers equally well to potassium nitrosodisulfonate, also known as potassium...
or TEMPO2,2,6,6-Tetramethylpiperidine-1-oxyl or TEMPO is the chemical compound with the formula 32NO. This heterocycle is a red-orange, sublimable solid. As a stable radical, it has applications throughout chemistry and biochemistry. TEMPO was discovered by Lebedev and Kazarnowskii...