Why are secondary and tertiary alcohols not suitable for praparing ethers by dehydration with conc. `H_(2)SO_(4)` ?

Explanation

Both secondary and tertiary alcohols react wih conc. `H_(2)SO_(2)` according to `S_(N^(1))` mechanism in which carbocation intermediates are formed. These intermediates preferbly lose a `H^(+)` ion from `beta` carbon leading to alkenes as a result of elimination rather than taking part in subsitutio reactions forming ethers. For details, consult methods of preparation of ethers (section 22)