Very confused by this video I stumbled across describing the cleavage of ethers with strong acids:

enter image description here

From what I can gather, the cleavage of ethers is run in aqueous strong acids such as HI or HBr.

Given that strong acids in water level to hydronium ion, why might HI or HBr be preferable to say HCl (another strong acid) in the clevage of ethers?

The video from which the above picture is taken says that HI and HBr are "stronger acids" and thus are better able to cleave the ether.

This sounds like garbage to me because if we're running this reaction in aqueous solution, there probably isn't a predominance of undissociated HI or HBr (very strong acids) reacting with the ethers. Instead, we'd only have hydronium ion and the conjugate bases of the strong acids.

So would the real reason HCl wouldn't work in cleaving ethers have to do with the nucleophilicity of the chloride anion in water solution (which is lower than that of either iodide or bromide ion) due to chloride anion's greater negative charge density and thus heavier solvation in water?

  • 6
    $\begingroup$ I agree with you, the higher nucleophilicity of bromide and iodide ions is what makes HBr and HI better reagents than HCl. But I leave the answer for anyone who has some references. A secondary reason might be because commercial HBr and HI come in higher concentrations than HCl (aqueous). $\endgroup$
    – K_P
    Aug 3, 2014 at 23:29
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    $\begingroup$ I'd say your on the right track. Protonation of the ether is the first step and shouldn't depend on which acid you use, so long as it is sufficiently strong. The reaction then proceeds by SN1 or SN2, both of which are obviously dependent on a nucleophile. $\endgroup$
    – canadianer
    Aug 3, 2014 at 23:38

1 Answer 1


You are right that $\ce{HBr}$ and $\ce{HI}$ are preferred over $\ce{HCl}$ for this reaction because $\ce{Br-}$ and $\ce{I-}$ are better nucleophiles than $\ce{Cl-}$, for the reason you have provided. Actually, the hydrolysis proceeds significantly slower with hydrochloric acid, and reasonable rates are only achieved with hydrobromic and hydroiodic acid (source). And while all of the acids level to the hydronium ion in water, the $\ce{H3O+}$ concentration will be higher in aqueous solutions of $\ce{HBr}$ and $\ce{HI}$, as both are available in higher concentrations than hydrochloric acid (57 and 48 wt.-% versus 38 wt.-% for $\ce{HCl}$).


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