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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. [1] Often a substituent moves from one atom to another atom in the same molecule, hence these reactions are usually intramolecular.
A Wagner–Meerwein rearrangement is a class of carbocation 1,2-rearrangement reactions in which a hydrogen, alkyl or aryl group migrates from one carbon to a neighboring carbon. [ 1 ] [ 2 ] They can be described as cationic [1,2]- sigmatropic rearrangements, proceeding suprafacially and with stereochemical retention.
Wittig rearrangement: 1,2-Wittig rearrangement; 2,3-Wittig rearrangement; Wittig–Horner reaction; Wohl degradation; Wohl–Aue reaction; Wohler synthesis; Wohl–Ziegler reaction; Wolffenstein–Böters reaction; Wolff rearrangement; Wolff–Kishner reduction; Woodward cis-hydroxylation; Woodward–Hoffmann rule; Wulff–Dötz reaction; Wurtz ...
Among the simplest examples are the methenium CH + 3, methanium CH + 5, acylium ions RCO +, and vinyl C 2 H + 3 cations. [2] Until the early 1970s, carbocations were called carbonium ions. [3] 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.
Carbocation rearrangement reactions occur through three-center bond transition states. Because the three center bond structures have about the same energy as carbocations, there is generally virtually no activation energy for these rearrangements so they occur with extraordinarily high rates. Carbonium ions such as ethanium C 2 H +
The 1,2-rearrangement belongs to a broad class of chemical reactions called rearrangement reactions. A rearrangement involving a hydrogen atom is called a 1,2-hydride shift . If the substituent being rearranged is an alkyl group, it is named according to the alkyl group's anion : i.e. 1,2-methanide shift , 1,2-ethanide shift , etc.
The cationic rearrangement contraction proceeds through the loss of a leaving group and the migration of an endocyclic bond to the carbocation. Pinacol type rearrangements are often used for this type of contraction. [20] Like the expansion reaction this proceeds with an electron donating group aiding in the migration.
A good example of a substitution reaction is halogenation. When chlorine gas (Cl 2) is irradiated, some of the molecules are split into two chlorine radicals (Cl•), whose free electrons are strongly nucleophilic. One of them breaks a C–H covalent bond in CH 4 and grabs the hydrogen atom to form the electrically neutral HCl.