Q.Williamson synthesis for preparing dimethyl ether is a/an

Diethyl ether is prepared industrially/in the lab by heating excess ethanol with concentrated sulphuric acid at about 413 K, which dehydrates two molecules of ethanol intermolecularly to give the ether.

Method: Acid-catalysed dehydration of ethanol (Williamson's continuous process).

When excess ethanol is heated with concentrated sulphuric acid at about 413 K (140 degC), two molecules of ethanol undergo intermolecular dehydration (loss of one water molecule between them) to give diethyl ether:

$C_2H_5-OH + H-O-C_2H_5 \xrightarrow[413\,K]{conc.\,H_2SO_4} C_2H_5-O-C_2H_5 + H_2O$

Mechanism (briefly):

1. Concentrated $H_2SO_4$ protonates the $-OH$ oxygen of one ethanol molecule, converting it into a good leaving group ($C_2H_5-\overset{+}{O}H_2$, a protonated alcohol/oxonium ion).
2. A second, un-protonated ethanol molecule then acts as a nucleophile, attacking the electrophilic carbon of the protonated alcohol (or its oxygen attacking via an $S_N2$-type displacement of water).
3. Loss of water and then loss of a proton (deprotonation of the resulting protonated ether) gives the neutral ether, $C_2H_5-O-C_2H_5$, regenerating the $H_2SO_4$ catalyst. …