1,2-rearrangement

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A 1,2-rearrangement or 1,2-shift or Whitmore 1,2-shift ^{[#endnote_Whitmore]} 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 C¹ to C².

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 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 S_N1 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.

An example of a less common radical 1,2-shift can be found in the gas phase pyrolysis of certain polycyclic aromatic compounds ^{[#endnote_Brooks]}. The energy required in an aryl radical for the 1,2-shift can be high (up to 60 Kcal/mole) but much less than that required for a proton abstraction to an aryne (82 Kcal/mole). In alkene radicals proton abstraction to an alkyne is preferred.

1,2 Rearrangements

The 1,2-rearrangement mechanism is found in the

1,3-Rearrangements

1,3-rearrangements take place over 3 carbon atoms. Examples:

the Fries rearrangement
a 1,3-alkyl shift of verbenone to Chrysanthenone

References

↑ The common basis of molecular rearrangements Frank C. Whitmore; J. Am. Chem. Soc.; 1932; 54(8); 3274-3283. [Abstract]
↑ 1,2-Shifts of Hydrogen Atoms in Aryl Radicals Michele A. Brooks and Lawrence T. Scott J. Am. Chem. Soc.; 1999; 121(23) pp 5444 - 5449; (Article) DOI: 10.1021/ja984472d [Abstract]

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