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Dehydration of alcohols to form alkenes is always carried out with conc. H_(2)SO_(4) and not witth conc. HCl or HNO_(3), explain why? |
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Answer» Solution :DEHYDRATION of alcohols to form alkenes occurs through carbocation INTERMEDIATES. If HCl is used, then chloride `(CL^(-))` ion beinng a good nucleophile brings about substitution reaction to give an alkyl chloride. `underset("Alcohol")(R-CH_(2)CH_(2)-OH) underset(-H_(2)O)overset(H^(+))to underset(R-CH_(2)CH_(2)-Cl" Alkyl chloride (substitution product)")underset(darr+Cl^(-))underset("Carbocation")(R-CH_(2)-overset(+)(C)H_(2))underset(-H^(+))to underset("Alkene (elimination product)")(R-CH=CH_(2))` If `H_(2)SO_(4)` is used, then the bisulphate ion `(HSO_(4))^(-)` being non-nucleophilic cannot bring about the substitution reaction. instead the carbocation loses a proton to form an alkene. conc. `HNO_(3)` is not used because it being a strong oxidising AGENT, preferentially brings abou the oxidation of alcohols first to aldehydes or ketones and then to the carboxylic acids. thus, dehydration of alcohols to form alkenes is always carried out with such strong acids (e.g. `H_(2)SO_(4),H_(3)PO_(4),KHSO_(4)`) which have non-nucleophilic anions. |
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