Q.The preparation of ether by acid catalysed dehydration of secondary or tertiary alcohol is not a suitable method. Give reasons.

The dehydration of ethanol with conc. H2SO4 is temperature-controlled: at 413 K it gives diethyl ether (intermolecular dehydration); at the higher 443 K it gives ethene (intramolecular dehydration).

Mechanism at 413 K: conc. H2SO4 protonates one ethanol molecule; a second, unprotonated ethanol molecule then acts as a nucleophile and attacks this protonated species (SN2-like), displacing water and forming an ether linkage:

C2H5OH + C2H5OH --(conc. H2SO4, 413 K)--> C2H5-O-C2H5 + H2O

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Two molecules of ethyl iodide react with dry $Ag_2O$ to give diethyl ether, with $AgI$ as the by-product.

Dry silver oxide is a classical reagent for converting alkyl halides into ethers: $2C_2H_5I + Ag_2O \rightarrow (C_2H_5)_2O + 2AgI$. Mechanistically, the oxide oxygen of $Ag_2O$ displaces iodide from one ethyl iodide molecule (assisted by the strong affinity of $Ag^+$ for $I^-$, which drives the reaction), generating an intermediate silver ethoxide/alkoxide species that then reacts with a second molecule of ethyl iodide in a Williamson-type $S_N2$ step, giving the symmetrical ether $C_2H_5-O-C_2H_5$. Option (d), silver ethoxide, is only the transient intermediate, not the isolated pro …