1, 2 - rearrangement - wikipedia 1, 2 - rearrangement 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 C to C. 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. Contents 1 Reaction mechanism 2 Radical 1, 2 - rearrangements 3 1, 2 rearrangements 4 1, 3 - Rearrangements 5 References 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 S 1 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. Radical 1, 2 - rearrangements The first radical 1, 2 - rearrangement reported by Heinrich Otto Wieland in 1911 was the conversion of bis ( triphenylmethyl ) peroxide 1 to the tetraphenylethane 2. The reaction proceeds through the triphenylmethoxyl radical 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 or a reactive intermediate 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 for the 1, 2 - shift can be high ( up to 60 kcal / mol or 250 kJ / mol ) but much less than that required for a proton abstraction to an aryne ( 82 kcal / mol or 340 kJ / mol ). In alkene radicals proton abstraction to an alkyne is preferred. 1, 2 rearrangements The following mechanisms involve a 1, 2 - rearrangement : 1, 2 - Wittig rearrangement Alpha - ketol rearrangement Beckmann rearrangement Benzilic acid rearrangement Brook rearrangement Criegee rearrangement Curtius rearrangement Dowd -- Beckwith ring expansion reaction Favorskii rearrangement Friedel -- Crafts reaction Fritsch -- Buttenberg -- Wiechell rearrangement Halogen dance rearrangement Hofmann rearrangement Lossen rearrangement Pinacol rearrangement Seyferth -- Gilbert homologation S 1 reaction ( generally ) Stevens rearrangement Wagner -- Meerwein rearrangement Westphalen -- Lettré rearrangement Wolff rearrangement 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 Jump up ^ Whitmore, Frank C. ( 1932 ). `` The common basis of molecular rearrangements ''. J. Am. Chem. Soc. 54 ( 8 ) : 3274 -- 3283. doi : 10.1021 / ja01347a037. Jump up ^ Über Triphenylmethyl - peroxyd. Ein Beitrag zur Chemie der freien Radikale Wieland, H. Chem. Ber. 1911, 44, 2550 -- 2556. doi : 10.1002 / cber. 19110440380 Jump up ^ Isomerization of Triphenylmethoxyl and 1, 1 - Diphenylethoxyl Radicals. Revised Assignment of the Electron - Spin Resonance Spectra of Purported Intermediates Formed during the Ceric Ammonium Nitrate Mediated Photooxidation of Aryl Carbinols K.U. Ingold, Manuel Smeu, and Gino A. DiLabio J. Org. Chem. ; 2006 ; 71 ( 26 ) pp 9906 -- 9908 ; ( Note ) doi : 10.1021 / jo061898z Jump up ^ Brooks, Michele A. ; Lawrence T. Scott ( 1999 ). `` 1, 2 - Shifts of Hydrogen Atoms in Aryl Radicals ''. J. Am. Chem. Soc. 121 ( 23 ) : 5444 -- 5449. doi : 10.1021 / ja984472d. Retrieved from `` https://en.wikipedia.org/w/index.php?title=1,2-rearrangement&oldid=742212898 '' Categories : Rearrangement reactions Talk Contents About Wikipedia Español فارسی Français Português 中文 Edit links This page was last edited on 2 October 2016, at 10 : 24. Text is available under the Creative Commons Attribution - ShareAlike License ; additional terms may apply. By using this site, you agree to the Terms of Use and Privacy Policy. Wikipedia ® is a registered trademark of the Wikimedia Foundation, Inc., a non-profit organization. About Wikipedia

Question: when does a 1 2 hydride shift occur
Answer: a substituent moves from one atom to another atom in a chemical compound