Cleavage of Ethers by HBr and HI

Unlike alcohols, ethers are not commonly used as synthetic intermediates because they do not undergo many reactions. This unreactivity makes ethers attractive as solvents. Even so, ethers do undergo a limited number of characteristic reactions. 

Ethers are cleaved by heating wi th HB r or HI to give alkyl bromides or alkyl iodides. 

Ethers are unreactive toward most bases, but they can react under acidic conditions. A pro-tonated ether can undergo substitution or elimination with the expulsion of an alcohol. Ethers react with concentrated HBr and HI because these reagents are sufficiently acidic to protonate the ether, while bromide and iodide are good nucleophiles for the substitution. 

In effect, this reaction converts a dialkyl ether into two alkyl halides. The conditions are very strong, however, and the molecule must not contain any acid-sensitive functional groups. 

Iodide and bromide ions are good nucleophiles but weak bases, so they are more likely to substitute by the Sn2 mechanism than to eliminate by the E2 mechanism. Mechanism 14-1 shows how bromide ion cleaves the protonated ether by displacing an alcohol. In most cases, the alcohol reacts further with HBr to give the alkyl bromide (see Section 11-7). The reaction of cyclohexyl ethyl ether with HBr is an example of this displacement. The cyclohexanol produced by the cleavage reacts further with HBr, and the final products are ethyl bromide and bromocyclohexane. 

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Synthesis of Ethers by Alkoxymercuration-Demercuration 636 14-7 Industrial Synthesis Bimolecular Condensation of Alcohols 637 Summary Syntheses of Ethers (Review) 638 14-8 Cleavage of Ethers by HBr and HI 638... 

Although the carbonium ion representation of aqueous acid-catalyzed etherification and cleavage is quite useful for correlating much important information, the actual course of the reaction must be more complex than it has been indicated here. The very fact that there is so great a difference in the rate at which ethers are hydrolyzed by the halogen acids (IICl HBr HI = 1 6 < ) is a clear indication that the function of the acid is not simply to provide protons for the reaction. 

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