Cleavage of Ethers by Hydrogen Halides

Cleavage of Ethers by Hydrogen Halides THE OVERALL REACTION  

Step 1 Proton transfer to the oxygen of the ether to give a dialkyloxonium ion. 

Step 2 Nucleophilic attack of the halide anion on carbon of the dialkyloxonium ion. This step gives one molecule of an alkyl halide and one molecule of an alcohol. 

Steps 3 and 4 These two steps do not involve an ether at all. They correspond to those in which an alcohol is converted to an alkyl halide (Sections 4.8-4.12). 

With ethers of the type ROR (Rt R ), the question of which carbon-oxygen bond is broken first is not one that we need examine at our level of study. Note also that ethers of tertiary alcohols would react with hydrogen halides by an S l mechanism. 

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FIGURE 16 4 The mechanism for the cleavage of ethers by hydrogen halides using the reac tion of diethyl ether with hydrogen bromide as an example... 

The cleavage of dialkyl ethers by hydrogen halides was discussed in Section 16 8 where It was noted that the same pair of alkyl 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 prod nets Either hydrogen bromide or hydrogen iodide is normally used... 

Section 24 12 The cleavage of alkyl aryl ethers by hydrogen halides yields a phenol and an alkyl halide... 

Section 16 8 The only important reaction of ethers is their cleavage by hydrogen halides... 

If a carbonium ion were involved, we should expect the halide to be derived from the more stable, and therefore the more highly branched, alkyl group. It is also pertinent that anisole is not cleaved by dry hydrogen chloride or hydrogen bromide in carbon tetrachloride solution unless a small amount of pyridine, dimethylaniline, or aniline is added.18 Furthermore, the presence of these bases increases the rate of cleavage of ethers in acetic acid solution. All these experiments imply that more than two molecules participate in the formation of the transition complex leading to etherification or cleavage (see p. 83). 

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