Dehydration of alcohols to alkene is carried out by treating with conc.`H_2SO_4` but not with conc. HCl or `HNO_3`. Give reasons.

Explanation

Dehydration of alcohol proceeds via the the formation of carbocation intermediate. If HCl is used as the dehydrating agent then chloride ion `(Cl^(-))`, being a good nucleophile, attacks at carbonium ion carbon `(C^(+))` thereby producing alkyl chloride as the substitution product together with the alkene as the elimination product.

If conc. `H_(2)SO_(4)` is used as the reagent then `HSO_(4)^(-)` ion derived from `H_(2)SO_(4)`, does not act as a nucleophile. Instead the carbocation loses a proton from the `beta`-carbon atom to give alkene `(R - CH = CH_(2))` as the elimination product.

If conc. `HNO_(2)` is used as the reagent then it being a strong oxidising agent, brings about oxidation of the alcohol first to an aldehyde or a ketone adn then to a carboxylic acid.