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?
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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