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C O Bond Cleavage

Carbon oxygen bond in ethers can be cleaved by the use of reagents like halogen acids, sulphuric acid and phosphorus pentachloride etc.

Cleavage with halogen acid: ethers can be cleaved by the use of hydroiodic acid or hydrobromic acid to give alkyl halide and alcohol.

In case excess of HI is used, the alcohol formed reacts further with HI to form alkyl iodide. The overall reaction can be written as

Site of cleavage

In case of unsymmetrical ethers, the site of cleavage depends on the nature of alkyl groups.

(i) If one group is methyl and other group is primary or secondary alkyl group, then halide is formed from the lower alkyl group.

Explanation: Cleavage of such ethers occurs by mechanism and formation of product is controlled by steric factors. In this mechanism alkyl halide is formed by the attack of halide ion on the protonated ether. Now, bulkier the alkyl group, greater will be the steric hinderance for the approaching halide ion. Thus, halide ion preferably attacks the smaller alkyl group. Consequently, alkyl halide is formed from smaller alkyl group.

(ii) If one group is methyl and other group is tertiary alkyl group, the main product is methyl alcohol and tertiary alkyl halide.

Explanation: the cleavage in such cases occurs by mechanism and formation of product is controlled by the stability of intermediate carbocation form the protonated ether. The tertiary carbocation being more stable forms more readily and subsequently produces tertiary halide.

(iii) In case of phenolic ethers, the cleavage occurs with the formation of phenol and alkyl halide. It is probably due to the fact that phenyl oxygen bond has a partial double bond character due to resonance. Therefore, its cleavage is relatively difficult as compared to alkyl oxygen bond. It can also be explained on the basis of steric effects. Since a larger benzene ring offers hinderance, therefore, halide ion attacks the smaller alkyl group.

Diaryl ethers on the contrary are quite difficult to be cleaved even at high temperature.

The order of reactivity of various halogen acids towards this reaction is:

HI > HBr > HCl ……. Order of reactivity