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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 where a substituent moves from one atom to another atom in a chemical compound. 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 isomers. 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. == Reaction mechanism == A 1,2-rearrangement is often initialised by the formation of a reactive intermediate such as: *a carbocation by heterolysis in a nucleophilic rearrangement or anionotropic rearrangement *a carbanion in a electrophilic rearrangement or cationotropic rearrangement *a free radical by homolysis *a nitrene. 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. A cyclic carbocationic transition state 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 carbanionic 1,2-shift is involved in the benzilic acid rearrangement. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「1,2-rearrangement」の詳細全文を読む スポンサード リンク
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