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A 1,2-rearrangement or 1,2-migration or 1,2-shift or Whitmore 1,2-shift is an
organic reaction Organic reactions are chemical reactions involving organic compounds. The basic organic chemistry reaction types are addition reactions, elimination reactions, substitution reactions, pericyclic reactions, rearrangement reactions, mechanistic organ ...
where a
substituent In organic chemistry, a substituent is one or a group of atoms that replaces (one or more) atoms, thereby becoming a moiety in the resultant (new) molecule. The suffix ''-yl'' is used when naming organic compounds that contain a single bond r ...
moves from one atom to another atom in a
chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...
. In a 1,2 shift the movement involves two adjacent atoms but moves over larger distances are possible. In the example below the substituent R moves from carbon atom C2 to C3. The rearrangement is intramolecular and the starting compound and reaction product are
structural isomer In chemistry, a structural isomer (or constitutional isomer in the IUPAC nomenclature) of a compound is a compound that contains the same number and type of atoms, but with a different connectivity (i.e. arrangement of bonds) between them. The ...
s. The 1,2-rearrangement belongs to a broad class of chemical reactions called
rearrangement reaction In organic chemistry, a rearrangement reaction is a broad class of organic reactions where the carbon skeleton of a molecule is rearranged to give a structural isomer of the original molecule. Often a substituent moves from one atom to another at ...
s. A rearrangement involving a
hydrogen Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...
atom is called a 1,2-hydride shift. If the substituent being rearranged is an
alkyl In organic chemistry, an alkyl group is an alkane missing one hydrogen. The term ''alkyl'' is intentionally unspecific to include many possible substitutions. An acyclic alkyl has the general formula of . A cycloalkyl group is derived from a cy ...
group, it is named according to the alkyl group's
anion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...
: i.e. 1,2-methanide shift, 1,2-ethanide shift, etc.


Reaction mechanism

A 1,2-rearrangement is often initialised by the formation of a
reactive intermediate In chemistry, a reactive intermediate or an intermediate is a short-lived, high-energy, highly reactive molecule. When generated in a chemical reaction, it will quickly convert into a more stable molecule. Only in exceptional cases can these comp ...
such as: *a
carbocation Carbocation is a general term for ions with a positively charged carbon atom. In the present-day definition given by the IUPAC, a carbocation is any even-electron cation with significant partial positive charge on a carbon atom. They are further ...
by heterolysis in a nucleophilic rearrangement or anionotropic rearrangement *a
carbanion In organic chemistry, a carbanion is an anion with a lone pair attached to a tervalent carbon atom. This gives the carbon atom a negative charge. Formally, a carbanion is the conjugate base of a carbon acid: : where B stands for the base (chemist ...
in an electrophilic rearrangement or cationotropic rearrangement *a
free radical A daughter category of ''Ageing'', this category deals only with the biological aspects of ageing. Ageing Biogerontology Biological processes Causes of death Cellular processes Gerontology Life extension Metabolic disorders Metabolism ...
by homolysis *a
nitrene In chemistry, a nitrene or imene () is the nitrogen analogue of a carbene. The nitrogen atom is uncharged and valence (chemistry)#monovalent, monovalent, so it has only 6 electrons in its valence level—two covalent bonded and four non-bonded e ...
. The driving force for the actual migration of a substituent in step two of the rearrangement is the formation of a more stable intermediate. For instance a tertiary carbocation is more stable than a secondary carbocation and therefore the SN1 reaction of neopentyl bromide with ethanol yields tert-pentyl ethyl ether. Carbocation rearrangements are more common than the carbanion or radical counterparts. This observation can be explained on the basis of
Hückel's rule In organic chemistry, Hückel's rule predicts that a planar ring molecule will have aromatic properties if it has 4''n'' + 2 π-electrons, where ''n'' is a non-negative integer. The quantum mechanical basis for its formulation was f ...
. A cyclic carbocationic
transition state In chemistry, the transition state of a chemical reaction is a particular configuration along the reaction coordinate. It is defined as the state corresponding to the highest potential energy along this reaction coordinate. It is often marked w ...
is aromatic and stabilized because it holds 2 electrons. In an anionic transition state on the other hand 4 electrons are present thus antiaromatic and destabilized. A radical transition state is neither stabilized or destabilized. The most important carbocation 1,2-shift is the
Wagner–Meerwein rearrangement A Wagner–Meerwein rearrangement is a class of carbocation 1,2-rearrangement rearrangement reaction, reactions in which a hydrogen, alkyl or aryl group migrates from one carbon to a neighboring carbon. They can be described as cationic ,2sig ...
. A carbanionic 1,2-shift is involved in the
benzilic acid rearrangement The benzilic acid rearrangement is formally the 1,2-rearrangement of 1,2- diketones to form α- hydroxy–carboxylic acids using a base. This reaction receives its name from the reaction of benzil with potassium hydroxide to form benzilic acid. ...
.


Radical 1,2-rearrangements

The first radical 1,2-rearrangement reported by
Heinrich Otto Wieland Heinrich Otto Wieland (; 4 June 1877 – 5 August 1957) was a German chemist. He won the 1927 Nobel Prize in Chemistry for his research into the bile acids. Career In 1901 Wieland received his doctorate at the University of Munich while studying ...
in 1911 was the conversion of bis(triphenylmethyl)peroxide 1 to the tetraphenylethane 2. The reaction proceeds through the triphenylmethoxyl
radical Radical (from Latin: ', root) may refer to: Politics and ideology Politics *Classical radicalism, the Radical Movement that began in late 18th century Britain and spread to continental Europe and Latin America in the 19th century *Radical politics ...
A, a rearrangement to diphenylphenoxymethyl C and its dimerization. It is unclear to this day whether in this rearrangement the cyclohexadienyl radical intermediate B is a
transition state In chemistry, the transition state of a chemical reaction is a particular configuration along the reaction coordinate. It is defined as the state corresponding to the highest potential energy along this reaction coordinate. It is often marked w ...
or a
reactive intermediate In chemistry, a reactive intermediate or an intermediate is a short-lived, high-energy, highly reactive molecule. When generated in a chemical reaction, it will quickly convert into a more stable molecule. Only in exceptional cases can these comp ...
as it (or any other such species) has thus far eluded detection by ESR spectroscopy. An example of a less common radical 1,2-shift can be found in the gas phase pyrolysis of certain polycyclic aromatic compounds. The energy required in an
aryl radical An aryl radical in organic chemistry is a reactive intermediate and an arene compound incorporating one free radical carbon atom as part of the ring structure. As such it is the radical counterpart of the arenium ion. The parent compound is the ...
for the 1,2-shift can be high (up to 60
kcal KCAL may refer to: * KCAL (AM), a radio station (1410 AM) licensed to Redlands, California, United States * KCAL-FM, a radio station (96.7 FM) licensed to Redlands, California, United States *KCAL-TV KCAL-TV (channel 9) is an independent tel ...
/ mol or 250 kJ/mol) but much less than that required for a proton abstraction to an
aryne In organic chemistry, arynes and benzynes are a class of highly Reactivity (chemistry), reactive chemical Chemical species, species derived from an aromatic ring by removal of two substituents. Arynes are examples of didehydroarenes (1,2-didehydro ...
(82 kcal/mol or 340 kJ/mol). In
alkene In organic chemistry, an alkene, or olefin, is a hydrocarbon containing a carbon–carbon double bond. The double bond may be internal or at the terminal position. Terminal alkenes are also known as Alpha-olefin, α-olefins. The Internationa ...
radicals proton abstraction to an
alkyne \ce \ce Acetylene \ce \ce \ce Propyne \ce \ce \ce \ce 1-Butyne In organic chemistry, an alkyne is an unsaturated hydrocarbon containing at least one carbon—carbon triple bond. The simplest acyclic alkynes with only one triple bond and n ...
is preferred.


1,2-Rearrangements

The following mechanisms involve a 1,2-rearrangement: *
1,2-Wittig rearrangement A 1,2-Wittig rearrangement is a categorization of chemical reactions in organic chemistry, and consists of a 1,2-rearrangement of an ether with an alkyllithium compound. The reaction is named for Nobel Prize winning chemist Georg Wittig. The in ...
* Alpha-ketol rearrangement *
Beckmann rearrangement The Beckmann rearrangement, named after the German chemist Ernst Otto Beckmann (1853–1923), is a rearrangement reaction, rearrangement of an oxime functional group to substituted amides. The rearrangement has also been successfully performed on ...
*
Benzilic acid rearrangement The benzilic acid rearrangement is formally the 1,2-rearrangement of 1,2- diketones to form α- hydroxy–carboxylic acids using a base. This reaction receives its name from the reaction of benzil with potassium hydroxide to form benzilic acid. ...
*
Brook rearrangement In organic chemistry the Brook rearrangement refers to any ,''n''carbon to oxygen silyl migration. The Rearrangement reaction, rearrangement was first observed in the late 1950s by Canadian chemist Adrian Gibbs Brook (1924–2013), after which ...
*
Criegee rearrangement The Criegee rearrangement is a rearrangement reaction named after Rudolf Criegee. Description In this organic reaction, a tertiary alcohol is cleaved in an organic oxidation by a peroxyacid to a ketone. The acid used is often p-nitroperoxybenzoic ...
*
Curtius rearrangement The Curtius rearrangement (or Curtius reaction or Curtius degradation), first defined by Theodor Curtius in 1885, is the thermal decomposition of an acyl azide to an isocyanate with loss of nitrogen gas. The isocyanate then undergoes attack by a ...
* Dowd–Beckwith ring expansion reaction *
Favorskii rearrangement The Favorskii rearrangement is principally a rearrangement of cyclopropanones and α-halo ketones that leads to carboxylic acid derivatives. In the case of cyclic α-halo ketones, the Favorskii rearrangement constitutes a ring contraction. This ...
*
Friedel–Crafts reaction The Friedel–Crafts reactions are a set of organic reaction, reactions developed by Charles Friedel and James Crafts in 1877 to attach substituents to an Aromatic hydrocarbon, aromatic ring. Friedel–Crafts reactions are of two main types: alky ...
*
Fritsch–Buttenberg–Wiechell rearrangement The Fritsch–Buttenberg–Wiechell rearrangement, named for Paul Ernst Moritz Fritsch (1859–1913), Wilhelm Paul Buttenberg, and Heinrich G. Wiechell, is a chemical reaction A chemical reaction is a process that leads to the chemistry, chemic ...
*
Halogen dance rearrangement The halogen dance rearrangement, also known as halogen scrambling, halogen migration, or halogen isomerization, is the migration of halogen substituents to a different position on an aromatic or heteroaromatic ring, resulting in a net positional shi ...
*
Hofmann rearrangement The Hofmann rearrangement (Hofmann degradation) is the organic reaction of a primary amide to a primary amine with one less carbon atom. The reaction involves oxidation of the nitrogen followed by rearrangement of the carbonyl and nitrogen to gi ...
*
Lossen rearrangement The Lossen rearrangement is the conversion of a hydroxamic acid, hydroxamate ester to an isocyanate. Typically O-acyl, sulfonyl, or phosphoryl O-derivative are employed. The isocyanate can be used further to generate ureas in the presence of ami ...
*
Pinacol rearrangement The pinacol–pinacolone rearrangement is a method for converting a diol, 1,2-diol to a carbonyl compound in organic chemistry. The 1,2-rearrangement takes place under acidic conditions. The name of the rearrangement reaction comes from the rearra ...
*
Seyferth–Gilbert homologation The Seyferth–Gilbert homologation is a chemical reaction of an aryl ketone 1 (or aldehyde) with dimethyl (diazomethyl)phosphonate 2 and potassium tert-butoxide to give substituted alkynes 3. Dimethyl (diazomethyl)phosphonate 2 is often called th ...
* SN1 reaction (generally) *
Stevens rearrangement The Stevens rearrangement in organic chemistry is an organic reaction converting quaternary ammonium salts and sulfonium salts to the corresponding amines or sulfides in presence of a strong base in a 1,2-rearrangement. The reactants can be o ...
* Stieglitz rearrangement *
Wagner–Meerwein rearrangement A Wagner–Meerwein rearrangement is a class of carbocation 1,2-rearrangement rearrangement reaction, reactions in which a hydrogen, alkyl or aryl group migrates from one carbon to a neighboring carbon. They can be described as cationic ,2sig ...
*
Westphalen–Lettré rearrangement The Westphalen–Lettré rearrangement is a classic organic reaction in organic chemistry describing a rearrangement reaction of cholestane-3β,5α,6β-triol diacetate with acetic anhydride and sulfuric acid. In this reaction one equivalent of wat ...
*
Wolff rearrangement The Wolff rearrangement is a reaction in organic chemistry in which an α-diazocarbonyl compound is converted into a ketene by loss of dinitrogen with accompanying 1,2-rearrangement. The Wolff rearrangement yields a ketene as an intermediate prod ...


References

{{DEFAULTSORT:1,2-rearrangement Rearrangement reactions