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?


1 Answer 1


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.

Read more at any of the sites in this Google search.

  • $\begingroup$ 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. $\endgroup$ Commented Sep 12, 2018 at 17:01
  • $\begingroup$ Hey, my teacher told that no matter what ether is it, it will always go through SN2 if the HX used is anhydrous. Is it true? Does anhydrous HX guarantee SN2? Thanks $\endgroup$ Commented Sep 12, 2022 at 2:24
  • $\begingroup$ @An_Elephant Did you get an answer somewhere? $\endgroup$
    – Silica19
    Commented Dec 11, 2022 at 5:02
  • $\begingroup$ @Silica19 Yes, the SN2 will be major. $\endgroup$ Commented Dec 11, 2022 at 5:46
  • 1
    $\begingroup$ @Silica19 According to my teacher the major will be SN2. Unfortunately, I do not know any online source. Sorry :( $\endgroup$ Commented Dec 12, 2022 at 7:42

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