Dehydration of alcohols is suitable for preparing ethers having primary alkyl groups only. Comment 

Explanation

During dehydration of alcohol the alkyl group should be unhindered and the temperature be kept low. Otherwise the reaction favours the formation of alkene. The reaction follows `S_N1` pathway when the alcohol is secondary or tertiary.
However, the dehydration of secondary and tertiary alcohols to give corresponding ethers is unsuccessful as elimination competes over substitution and as a consequence, alkenes are easily formed.