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How are aldehydes distinguished from ketones using Tollen's and Fehling's reagents ? Give complete chemical reactions. |
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Answer» SOLUTION :Aldehydes are more easily oxidised than ketones. They show the FOLLOWING two reactions which are not shown by ketones : (i) They reduce Fehling.s solution (alkaline solution of COPPER sulphate containing some sodium potassium tartrate) to form red precipitate of cuprous oxide : `CuSO_(4)+2NaOH to Cu(OH)_(2)+Na_(2)SO_(4)` `Cu(OH)_(2) underset("tartrate")overset("Sod.")to CuO + H_(2)O` `CH_(3)CHO+2CuO to CH_(3)COOH+underset("Red ppt.")(Cu_(2)O)` (ii) They reduce ammoniacal silver nitrate solution (Tollen.s reagent) to form silver which is obtained as a silver MIRROR. `AgNO_(3)+3NH_(4)OH to [Ag(NH_(3))_(2)]^(+)OH^(-)+NH_(4)NO_(3)+H_(2)O` `CH_(3)CHO+[Ag(NH_(3))_(2)]^(+)OH^(-) to CH_(3)COOH+underset(underset("mirror")("Silver"))(2Ag)+4NH_(3)+H_(2)O` The above two reactions are given by aldehydes but not by ketones. Therefore, these can be used to distinguish aldehydes and ketones. |
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