"Bimolecular dehydration" is a way of preparing diethyl ether from the dehydrating action of concentrated sulfuric acid on ethanol. First step is protonation of one alcohol molecule (ethanol) by conc. H₂SO₄ and in second step nucleophilic attack by second ethanol molecule leading to loss of water molecule. abstracts a hydrogen from one ethanol and a hydroxyl (the – OH group) of a neighbor. In third step loss of proton takes place and symmetrical ether is formed.
So this method works only with symmetrical ethers: di[alkyl] ethers. Methyl Ethyl ether is better prepared from Sodium ethoxide and methanol or Sodium methoxide and ethanol.