Either of two methods of producing ethers, both named after the British chemist Alexander Williamson (1824–1904). 1. The dehydration of alcohols using concentrated sulphuric acid. The overall reaction can be written
2ROH → H2O+ROR
The method is used for making ethoxyethane (C2H5OC2H5) from ethanol by heating at 140°C with excess of alcohol (excess acid at 170°C gives ethene). Although the steps in the reaction are all reversible, the ether is distilled off so the reaction can proceed to completion. This is Williamson's continuous process. In general, there are two possible mechanisms for this synthesis. In the first (favoured by primary alcohols), an alkylhydrogen sulphate is formed ROH+H2SO4⇌ROSO3H+H2O
This reacts with another alcohol molecule to give an oxonium ion ROH+ROSO3H → ROHR+
This loses a proton to give ROR.The second mechanism (favoured by tertiary alcohols) is formation of a carbonium ion
ROH+H+ → H2O+R+
This is attacked by the lone pair on the other alcohol molecule R++ROH → ROHR+
and the oxonium ion formed again gives the product by loss of a proton.The method can be used for making symmetric ethers (i.e. having both R groups the same). It can successfully be used for mixed ethers only when one alcohol is primary and the other tertiary (otherwise a mixture of the three possible products results). 2. A method of preparing ethers by reacting a haloalkane with an alkoxide. The reaction, discovered in 1850, is a nucleophilic substitution in which the negative alkoxide ion displaces a halide ion; for example:
RI+−OR′ → ROR′+I−
A mixture of the reagents is refluxed in ethanol. The method is particularly useful for preparing mixed ethers, although a possible side reaction under some conditions is an elimination to give an alcohol and an alkene.
