Formation and Cleavage of Ethers in Acidic Media

Formation and Cleavage of Ethers in Acidic Media. When alcohols are treated with acids, dehydration (p. 105) or etherification often occurs. Diethyl ether, for example, is prepared commercially by passing ethanol into a mixture of sulfuric acid and alcohol maintained at 140°. By modifying this method somewhat, satisfactory yields of certain mixed ethers have been obtained.4 In other instances etherification proceeds extremely readily. Triphenylcarbinol is etherified by boiling with methanol5 (unless it is carefully freed of traces of acid),6 and benzhydrol is reported to form dibenzhydryl ether by refluxing it with water.7 Again, traces of acid are probably responsible for reaction. 

Reactions of this kind appear to be carbonium ion processes (p. 36)  

Originally it was believed that etherification with sulfuric acid proceeded through the intermediate formation of alkyl sulfuric acids (R0S03H), since these products sometimes could be isolated from the reaction mixture. It now seems clear, however, that alkyl sulfuric acids are formed only as by-products in the reaction and are not actually intermediates in the etherification process. Thus, ethyl ter -butyl ether was prepared in yields of 95 per cent by heating a mixture of tert-butyl and ethyl alcohols containing 15 per cent sulfuric acid at 70°, yet titration of the cooled reaction mixture showed no evidence of the formation of an alkyl sulfuric acid.4 Furthermore, the yield of diethyl ether was 95 per cent when ethanol was passed through concentrated sulfuric acid heated to 140°, but when the reaction was carried out with ethyl sulfuric acid at the same temperature, much sulfur dioxide was evolved and the yield dropped to 70 per cent.4 8 We can, therefore, conclude that the 

Acetals are ethers of hemiacetals (p. 158). Often they are formed by heating aldehydes in alcoholic solution containing a trace of acid. The mechanism of the reaction is probably the following  

Experimentally it is known that it is easier to prepare an acetal than a simple ether. This is in accord with the mechanism presented for etherification since the carbonium ion involved in this sequence is stabilized by resonance, 

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