11.5: Ethers to Alkyl Halides: Acidic Cleavage
Ethers are generally unreactive and unsuitable for direct nucleophilic substitution reactions since the alkoxy groups are strong bases and, therefore, poor leaving groups. However, ethers readily undergo acidic-cleavage reactions. Ethers can be converted to alkyl halides when heated with strong acids such as HBr and HI in a sequence of two substitution reactions.
In the first step, the ether is converted into an alkyl halide and alcohol.
In the second step, the alcohol reacts with the excess HX acid to form another equivalent of an alkyl halide.
Acidic cleavage of ethers can occur either by the SN1 mechanism or SN2 mechanism, depending on the substrate. Ethers with primary and secondary alkyl groups as substrates react with the SN2 mechanism in which the nucleophile attacks the protonated ether at the less hindered site. Ethers with a tertiary, benzylic, or allylic group undergo the SN1 mechanism because the substrates can produce more stable intermediate carbocations. Reactivity of the halogen acids with ethers increases relative to the nucleophilicity of the halide ions. HI and HBr are more reactive, HCl is less efficient, and HF is unsuitable.