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Ethers can be cleaved under acidic conditions by treatment with hydrogen halides (HX, where X = Cl/Br/I). Some of these reactions proceed by SN1 mechanisms and some by SN2 mechanisms. How can I tell which mechanism is being followed?

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The first step of this reaction is always a proton transfer:

Protonation of ether by HX

Then, the substitution follows either the SN2 or SN1 pathway depending on whether R is methyl, primary, secondary, or tertiary.

Methyl and primary ethers undergo acidic cleavage by the SN2 mechanism:

SN2 process for methyl or primary ethers

Tertiary ethers undergo cleavage by the SN1 mechanism:

SN1 process for tertiary ethers

Secondary ethers probably undergo cleavage by both mechanisms competing with each other.

Note that unless R' is an aryl group, the alcohol R'OH that forms will also be converted into a halide by a similar mechanism.

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  • $\begingroup$ I suppose you were to say: If R is primary... or secondary... or tertiary... . I don't think you can extend this notation from alcohols to ethers, it is ambiguous which carbon you are referring to. $\endgroup$ – Martin - マーチン♦ Sep 12 '18 at 17:01

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