Reactions of Ether
Involving the Cleavage C-O
The C-O cleavage in ethers takes place under drastic conditions. Some important reactions that involve carbon-oxygen cleavage in ethers are given below :
Reactions of Ether with Halogen Acids
Ethers react with HX (X=Br,I) at 373 K to form alcohols and alkyl halides.
Complete the following reaction :
If HX (X=Br,I) is in excess, only alkyl halide is formed.
What happens when HX is in excess
We have already studied that alcohols react with halogens to form alkyl halides.
When ethers react with HX, alcohols and alkyl halides are formed. The alcohol formed, reacts further with HX (which is in excess) to give alkyl halide. Hence, we get alkyl halide as the final product.
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.
Identify ?A ?B and ?C
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.
Alkyl aryl ethers
If one of the R groups is C6H5, the products formed are always a phenol and an alkyl halide.
Why one of the products is always phenol
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.