Reactions of Ether
^{^{Involving the Cleavage C-O}}

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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 :

Reaction of ethers with halogen acids example

If HX (X=Br,I) is in excess, only alkyl halide is formed.

Reactions of ether with excess halogen acids only yield alkyl halide

What happens when HX is in excess

We have already studied that alcohols react with halogens to form alkyl halides.

Reaction of alcohol with halogen acids yields alkyl halide

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.

Reason why excess HX yield alkyl halide : alcohol formed further reacts to form alkyl halide

Mechanism

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.

Reaction of ether with hydrogen iodide or bromide mechanism step 2 : Formation of 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

Unsymmetrical ethers

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 S_N² mechanism.

Alkyl halide is formed from smaller alkyl group in case of primary and secondary ethers

If an ether contains a tertiary part, the halide ion always attacks the tertiary alkyl group by S_N¹ mechanism.

Alkyl halide is formed from larger alkyl group in case of tertiary ethers

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.