Substitution (SN1) with hydride shift

by James

Description: 1,2-hydride shifts can occur during substitution reactions if an unstable carbocation is formed next to a tertiary carbon.
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Notes: Besides H–Cl, other strong acids with nucleophilic conjugate bases could also be used (e.g. HBr, HI). It’s called a “hydride shift” because hydrogen is migrating and taking the pair of electrons with it [i.e. H:(-)  ]

Examples:

Notes: Notice how in each case a hydrogen is shifting from a tertiary carbon to a secondary carbon. In the third example, silver ion (AgNO3) removes bromine to give the carbocation, which is then attacked by water.

Mechanism: In this mechanism, the oxygen is protonated by H-Cl (Step 1, arrows A and B) to give an oxonium ion, where the C-O bond breaks (Step 2, arrow C) to give a secondary carbocation. Next, migration of the hydrogen occurs, giving a (more stable) tertiary carbocation (Step 3, arrow D). Finally, the chloride ion attacks the carbocation to give the tertiary alkyl chloride (Step 4, arrow E).

Notes: Again, note that it is called a “hydride” shift because H is moving and taking the pair of electrons with it.

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