Elimination (E1) with hydride shift

by James

Description: When unstable carbocations are formed under conditions typical of E1 reactions, hydride shifts can occur.
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Notes: This is most common with alcohols in the presence of the strong, non-nucleophilic acid H2SO4, although it can also occur with other species that have good leaving groups such as alkyl halides.

Examples:

Notes: In the first and second examples, note how the alkene ends up in a position that is impossible to explain except through the intermediacy of a hydride shift. In example 3, silver salts (such as AgNO3) can be used to form carbocations from alkyl halides.

Mechanism:  In the first step, protonation of the oxygen (Step 1, arrows A and B) leads to the loss of water ((Step 2, arrow C). Since this is a secondary carbocation adjacent to a tertiary carbon, a hydride shift will lead to formation of the more stable tertiary carbocation (Step 3, arrow D). Finally, deprotonation at carbon (Step 4, arrows E and F) leads to formation of the more substituted alkene according to Zaitsev’s rule.

Notes: Note that water (H2O) could also be used as a base in the eliminatino step here instead of HSO4(-).

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