Class 12 Chemistry Aldehydes Ketones Acids Chemical properties of aldehydes and ketones
  • Oxidation
  • Aldehyde and ketones on oxidation give carboxylic acid .But this happens only,in presence of Oxidising agents like HNO3 , KMnO4 , k2Cr2O7 , Na2Cr2O7Collin’s Fehling reagent and many more.

When we use KMnO4 or k2Cr2O7 ,the medium gets acidic. There is a trend, that when we carry oxidation of aldehyde or ketone it forms acid like –

    Class_12_Aldehydes_&_Ketones_Oxidation_Reaction                                                   

           

We get acid, irrespective of the alkyl group used. Likewise, ketones also give acids but we get mixtures.

     Class_12_Aldehydes_&_Ketones_Oxidation_Reaction_1                     The product formed is in accordance with Popoff’s rule.

According to it : “ketones get oxidised in such a manner that the keto group remains with smaller alkyl group.

 Example:

  Class_12_Aldehydes_&_Ketones_Oxidation_Reaction_2                   Aldehyde

 They are easily oxidised even with mild oxidising agents like Collin’s, Fehling etc. They can be oxidised but in case of ketone we need to use strong oxidising agents .The use of weak and strong oxidising agents like Colin’s, Fehling’s can be used to distinguish between aldehydes and ketones.

 Mild Oxidising agent: Used for oxidising aldehyde to carboxylic acids.

        Mild Oxidising agents used are Tollen’s reagent.

                       

For aldehydes

  • Tollen’s reagent :It is a solution of Ammonical silver nitrate and it has formula [ Ag (NH3)2 ]+ OH- ] .Now ,Let us take example of any aldehyde like :

 Class_12_Aldehydes_&_Ketones_Oxidation_Reaction_Tollen's_Agent

                       

Therefore, it is called as “Silver mirror test”. This test  is known as silver mirror test and  is not given by ketones.

  • Fehling solution test
  • It is prepared by missing equal amount of Fehling’s Solution A and Fehling’s solution B.

Has to be changed

      (Aq. CuSO4) (Sodium potassium Carbonate " Rochelle Salt")     

  • We take test tube and add both solutions A & B equally.
  • The resulting solution is blue due to CuSO4.
  • Now when we add aldehyde this blue solution, it turns red. The medium is alkaline ketone due to Fehling solution B.
  • The following reaction occurs.

           

RCHO    +       Cu2+ +5OH- -->  RCOO-       +       Cu2O + 3H2O

(Aldehyde) Copper ion   carboxylate ion  (Cuprous oxide) (Reddish Brown ppt)

          

  • Identification : Colour changes from blue to red .This test is not given by ketones.
  •  Benedict’s solution test- It is alkaline solution of cupric ion complexed with citrate ions. When we react with aldehyde the following reaction occur:

          

RCHO    +       Cu2+ +5OH- -->  RCOO-       +       Cu2O + 3H2O

(Aldehyde) Copper ion   carboxylate ion  (Cuprous oxide) (Reddish Brown ppt)

for Ketones

  • Iodoform test: By performing this test, we get yellow precipitate due to formation of iodoform.
  • Limitation of this test: This is given only methyl ketones i.e. one of the R in ketone should be CH3 and also by ethanol as it contains the methyl group.

In this, aldehyde reacts with I2 in presence of NaOH. In this 3 hydrogen are substituted by Iodine as shown below:

Class_12_Aldehydes_&_Ketones_IodoForm_Test

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