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MHT-CET : Chemistry Entrance Exam

MHT - CET : Chemistry - Aldehydes and Ketones Page 3

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  1. Cannizzaro's Reaction
    This reaction is specific of aldehydes having no a-hydrogen. It is a self oxidation reduction
    oxidation reaction in which one molecule of aldehyde is oxidised to carboxylic acid,
    while other molecule is reduced to an alcohol. It is a nucleophilic addition reaction.

HCHO + HCHO + NaOH

HCOONa + CH3 - OH


  1. NOTE: Acetaldehydes and ketones containing
    a-hydrogen atoms do not show
               Cannizzaro's reaction.
  2. Clemmenson's Reduction

 

(i)

  1.  

 

(ii)

  1.  

 

(iii)

  1.  
  2. Reducing Properties of Aldehydes (Distinguishing Test for Aldehydes)
    1. Reduction of Fehling's Solution:

      When an aldehyde is heated with blue coloured Fehling's solution, red precipitate of cuprous oxide (Cu2O) is formed.

 

 

CuSO4 + 2NaOH

Cu(OH)2 + Na2SO4

    1.  

 

 

R - CHO + [2Cu2+ + 4OH-]   R - COOH + 

Cu2O

+ 22++ 4---

 

 

 

(red ppt.)

 

    1.  
    2. Reduction of Tollen's Reagent:

            Tollen's reagent is ammoniacal silver nitrate solution. When an aldehyde is
            heated with it, a black precipitate of colloidal silver or silver mirror is obtained.

 

 

AgNO3 + 3NH4OH [Ag(NH3)2]+ OH- + NH4NO3 + 2H2O

    1.                                                     Tollen's reagent

 

 

R - CHO + 2[Ag(NH3)2]+OH- R - COOH + 4NH3+ H2O + 2Ag

    1.  
    2. Schiff's Reagent Test:

      Schiff's reagent is pink in colour. Schiff's reagent (solution of magenta or p-rosaniline hydrochloride, i.e., Fuschin) is made colourless by passing SO2 gas.
      The solution obtained by reducing the pink dye Fuchsin by passing sulphur
      dioxide into its solution is called Schiff's reagent.

      When aldehyde is mixed with Schiff's reagent, Magenta red or pink colouration is obtained.

      Note: Ketones do not give these tests.

 

 

 

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