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Carbocation Rearrangements

Article ID: 126
Last updated: 12 Feb, 2012
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Carbocations are prone to rearrangement via 1,2-hyride or 1,2-alkyl shifts provided it generates a more stable carbocation. For example:

Notice that the "predicted" product is only formed in 3% yield, and that products with a different skeleton dominate. 
The reaction proceeds via protonation to give the better leaving group which departs to give the 2o carbocation shown.  A methyl group rapidly migrates taking its bonding electrons along, giving a new skeleton and a more stable 3o carbocation which can then lose H+ to give the more stable alkene as the major product. 

2o carbocation to 3o carbocation

This is an example of a 1,2-alkyl shift.  The numbers indicate that the alkyl group moves to an adjacent position.
Similar migrations of H atoms, 1,2-hydride shifts are also known.

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Organic Chemistry -> Introduction to Organic Compounds and Reactions -> Carbocations

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Department of Chemistry & Chemical Biology