Cleavage of ethers using acid (SN1 reaction)

by James

Description: When treated with acid, ethers of tertiary, benzylic, or allylic carbons can fragment to give a carbocation, which is then trapped by a nucleophile in an SN1 reaction.
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Notes: This pathways will be favored when a stabilized carbocation can form (tertiary, benzylic, or allylic).

E1 products can also accompany this reaction

Examples: 

Notes: Note that in each case the bond between the tertiary carbon and the oxygen is broken.

Mechanism: The first step is protonation of the ether oxygen with acid (Step 1, arrows A and B). CH3OH is a much better leaving group (weaker base) than CH3O- . So the role of the acid here is to accelerate the loss of the leaving group (Step 2, arrow C) leaving behind a carbocation. The tertiary carbocation is then attacked by the solvent (water) leading to the protonated alcohol (Step 3, arrow D) which is then deprotonated by base (Step 4, arrows E and F) to give the neutral alcohol.

Notes: It’s probably more reasonable to use water as the base for step E, but it was shown here to demonstrate that acid is regenerated here (it’s a catalyst). Many other acids besides H2SO4 could be used.

The carbocation formed after Step 2 can alternatively be deprotonated by base to give an alkene (arrows G and H).

Notes: the proportion of E1 products (relative to SN1) will increase with heat.

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