The C-O cleavage in ethers takes place under drastic conditions. Some important reactions that involve carbon-oxygen cleavage in ethers are given below :
Ethers react with HX (X=Br,I) at 373 K to form alcohols and alkyl halides.
If HX (X=Br,I) is in excess, only alkyl halide is formed.
Ethers being basic (due to lone pairs on oxygen) undergo protonation to give oxonium salts (here protonated ether).
The protonated ether formed in step 1 is attacked by the halide ion to form alkyl halide and alcohol.
Since the protonated ether is attacked by a nucleophile (electron rich species), the reaction is nucleophilic.
In case of unsymmetrical ethers, the alkyl halide is always formed from the smaller alkyl group.
If an ether does not contain a tertiary alkyl group, the halide ion always attacks the smaller alkyl group by SN2 mechanism.
If an ether contains a tertiary part, the halide ion always attacks the tertiary alkyl group by SN1 mechanism.
If one of the R groups is C6H5, the products formed are always a phenol and an alkyl halide.
There is some double bond character between the benzene ring and the ethereal oxygen due to resonance and hence is difficult to break. This is also the reason why diphenyl ethers do not react with HX.