question_answer

Dehydration of alcohol to form an alkene is always carried out with concentrated \[{{H}_{2}}S{{O}_{4}}\] and not with concentrated\[HCl\] or \[HN{{O}_{3}}\]. Explain.

Answer:

                Under the acidic conditions, alcohol is initially protonated and then loses \[{{H}_{2}}O\] to form a carbocation. If \[HCl\] is used, then \[C{{l}^{-}}\] ion being a strong nucleophile will result in nucleophilic substitution to form alkyl chloride. However,\[HSO_{4}^{-}\] ion released by \[{{H}_{2}}S{{O}_{4}}\] is a very weak nucleophile and cannot participate in the nucleophilic substitution. It will rather act as a base and eliminate a proton to form alkene as the product as follows : Concentrated \[HN{{O}_{3}}\] is a powerful oxidising agent. It will cause oxidation of alcohol to aldehyde and then to acid. Thus, out of the mineral acids listed, dehydration is carried by concentrated \[{{H}_{2}}S{{O}_{4}}\]. Even phosphoric acid can be used.