A number of symmetrical ethers containing the lower alkyl groups are prepared on a large scale, chiefly for use as solvents. The most important of these isethyl ether, the familiar anesthetic and the solvent we use in extractions and in the preparation of Grignard reagents; others include isopropyl ether and tf-butyl ether.
These ethers are prepared by reactions of the corresponding alcohols with
sulfuric acid. Since a molecule of water is lost for every pair of alcohol molecules, the reaction is a kind of dehydration. Dehydration to ethers rather than to alkenesis controlled by the choice of reaction conditions. For example, ethylene is prepared by heating ethyl alcohol with concentrated sulfuric acid to 180U; ethylether is prepared by heating a mixture of ethyl alcohol and concentrated sulfuric acid to 140, alcohol being continuously added to keep it in excess.
Dehydration is generally limited to the preparation of symmetrical ethers, because, as we might expect, a combination of two alcohols usually yields a mixture of three ethers.
Ether formation by dehydration is an example of nucleophilic substitution^ with the protonated alcohol as substrate and a second molecule of alcohol as nucleophile.
Problem 17.1 (a) Give all steps of a likely mechanism for the dehydration of an alcohol to an ether, (b) Is this the only possibility? Give all steps of an alternative mechanism. (Hint: See Sec. 14.16.) (c) Dehydration of w-butyl alcohol gives w-butyl ether. Which of your alternatives appears to be operating here?
Problem 17.2 In ether formation by dehydration, as in other cases of substitution, there is a competing elimination reaction. What is this reaction, and what products does it yield? For what alcohols would elimination be most important?
Problem 17.3- (a) Upon treatment with sulfuric acid, a mixture of ethyl and
H-propyl alcohols yields a mixture of three ethers. What are they? (b) On the otherhand, a mixture of /?r/-butyl alcohol and ethyl alcohol gives a good yield of a single, ether. What ether is this likely to be? How do you account for the good yield?
On standing in contact with air, most aliphatic ethers are converted slowly
into unstable peroxides. Although present in only low concentrations, these
peroxides are very dangerous, since they can cause violent explosions during the distillations that normally follow extractions with ether.
The presence of peroxides is indicated by formation of a red color when the
ether is shaken with an aqueous solution of ferrous ammonium sulfate and potassium thiocyanate; the peroxide oxidizes ferrous ion to ferric ion, which reacts with thiocyanate ion to give the characteristic blood-red color of the complex.
Peroxides can be removed from ethers in a number of ways, including washing with solutions of ferrous ion (which reduces peroxides), or distillation from concentrated H2SO4 (which oxidizes peroxides).
For use in the preparation of Grignard reagents, the ether (usually ethyl)
must be fuee of traces of water and alcohol. This so-called absolute ether can be prepared by distillation of ordinary ether from concentrated H2SO4.