Cleavage of Aryl Ethers by Hydrogen Halides

The cleavage of dialkyl ethers by hydrogen halides was discussed in Section 16.8, where it was noted that the same pair of aUcyl halides results, irrespective of the order in which the carbon-oxygen bonds of the ether are broken. 

Cleavage of alkyl aryl ethers by hydrogen halides always proceeds so that the alkyl-oxygen bond is broken and yields an alkyl halide and a phenol as the final products. 

Since phenols are not converted to aryl halides by reaction with hydrogen halides, reaction proceeds no further than shown in the preceding general equation. For example. 

Guaiacol is obtained by chemical treatment of lignum vitae, the wood from a species of tree that grows in warm climates. It Is sometimes used as an expectorant to help relieve bronchial congestion. 

The first step in the reaction of an alkyl aryl ether with a hydrogen halide is protonation of oxygen to form an alkylaryloxonium ion  

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Claisen rearrangement of allyl aryl ethers, 957-958, 964 cleavage by hydrogen halides, 628—630,... 

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