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Another feature of E1 reactions (and also of SN1 reactions) is the tendency of the initially formed carbocation to rearrange, especially if a more stable carbocation is formed thereby. For example, the very slow SN1 solvolysis of neopentyl iodide in methanoic acid leads predominantly to 2-methyl-2-butene:
In this reaction, ionization results in migration of a methyl group with its bonding pair of electrons from the β to the α carbon, thereby transforming an unstable primary carbocation to a relatively stable tertiary carbocation. Elimination of a proton completes the reaction.
Rearrangements involving shifts of hydrogen (as H:⊖) occur with comparable ease if a more stable carbocation can be formed thereby:
Rearrangements of carbocations are among the fastest organic reactions known and must be reckoned with as a possibility whenever carbocation intermediates are involved.
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