Elimination (E1) of alkyl halides to form alkenes

by James

Description: Alkyl halides that can readily form carbocations will undergo elimination to form alkenes.
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Notes: X can be a halide (Cl, Br, or I) or a sulfonate (such as OTs or OMs).

The more substituted alkene is generally formed (Zaitsev product).

These reactions are generally accompanied by the substitution products (SN1).

Sometimes silver salts are used to facilitate formation of carbocations from alkyl halides.

Examples: 

Notes: Example 3 is an example of using a silver salt to facilitate the removal of halide. Silver binds to the halide and forms insoluble AgBr along with a carbocation, which then undergoes the E1 reaction.

Mechanism: This reaction proceeds through an E1 mechanism (elimination, unimolecular)

Loss of a good leaving group such as Br(–) (Step 1, arrow A) leaves behind a carbocation. Deprotonation on the carbon adjacent to the carbocation (Step 2, arrows B and C) leads to the formation of the alkene.

Notes: Note here that the more substituted alkene is formed preferentially.

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