Acid-catalyzed dehydration of small 1º-alcohols constitutes a specialized method of preparing symmetrical ethers.
As shown in the following two equations, the success of this procedure depends on the temperature. At 110º to 130 ºC an SN2 reaction of the alcohol conjugate acid leads to an ether product. At higher temperatures (over 150 ºC) an E2 elimination takes place.
|2 CH3CH2-OH + H2SO4||130 ºC
||CH3CH2-O-CH2CH3 + H2O|
|CH3CH2-OH + H2SO4||150 ºC
||CH2=CH2 + H2O|
In this reaction alcohol has to be used in excess and the temperature has to be maintained around 413 K. If alcohol is not used in excess or the temperature is higher, the alcohol will preferably undergo dehydration to yield alkene.
If ethanol is dehydrated to ethene in presence of sulfuric acid at 433 K, but as 410 K, ethoxyethane is the main product. The dehydration of secondary and tertiary alcohols to get corresponding ethers is unsuccessful as alkenes are formed easily in these reactions.
This reaction cannot be employed to prepare unsymmetrical ethers. It is because a mixture of products is likely to be obtained.
- Binod Shrestha (University of Lorraine)