Which mechanism do ethers follow when cleaved by hydrogen halides?
The first step of this reaction is always a proton transfer:



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:



Tertiary ethers undergo cleavage by the SN1 mechanism:



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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