Class 12 Chemistry Aldehydes Ketones Acids Chemical properties of aldehydes and ketones

Chemical properties of aldehydes and ketones

  • Structure of Carbonyl group

 Class_12_Aldehydes_&_Ketones_Carbonyl_Group

 

  • If we look at structure ,multiple bonds are seen. Therefore ,they will show addition reactions .
  • Oxygen being electronegative,attract electrons .
  • As a result, partial negative charge is acquired on oxygen and partial postive on carbon .
  • Therefore ,nucleophilic attack occur on carbon .So, in them common reactions that is seen is nucleophilic reactions.
  • The electronegativity is same but due to approach of attacking agent temporary transfer of electrons occur .

Types of reactions shown

  1. Nucleophilic addition reactions
  2. Nucleophilic addition reactions followed by loss of water molecule
  3. Oxidation reactions
  4. Reduction reactions
  5. Miscellaneous reactions
  • Nucleophilic addition reaction

     In this given structure

 Class_12_Aldehydes_&_Ketones_Carbonyl_Group

The nucleophile attack on the carbon atom and the reaction occur in a way as shown below:

Class_12_Aldehydes_&_Ketones_Attack_Of_Nucleophilic_Addition_Reaction

This reaction is acid catalysed reaction .Due to which hydroxyl group is formed from aldehyde.

Relative strength of Aldehydes and ketones

  • On the basis of Inductive effect

 If the magnitude of positive charge is more on carbon atom ,then more readily it shows nucleophilic addition reaction .So, out of all members HCHO shows this reaction more readily as it has no alkyl group( electron releasing group) present.

So, if we compare formaldehyde, ethanol and propanone, the order of their strength towards nucleophilic addition is:

       Class_12_Aldehydes_&_Ketones_Inductive_Effect

  • It can also be explained on the basis of Steric effect

More the bulky groups attached, less space will be available for nucleophile to attack. Therefore, reactivity will be less. So, this can also be the reason to explain the trend.

      Class_12_Aldehydes_&_Ketones_Steric_Effect           

  • If we compare Benzaldehyde with any other aldehyde, its strength towards nucleophilic addition is less. The reason is due to delocalisation of electron that takes place. Therefore, the charge is not so intensified.

Let’s show some nucleophilic addition reactions:

 

(a)Addition of HCN (Cyanohydrins are formed)

If only HCN is used, the reaction is slow. So, the base is used that help in removing hydrogen ion from HCN and sets the nucleophile free. As a result, reaction becomes fast. The base reacts as shown below:

             Class_12_Aldehydes_&_Ketones_Addition_Of_HCN

 If we carry out hydrolysis of the above compound, 2-methyl 2-hydroxy propionitrile, then the lactic acid is produced due to hydrolysis of CN group as shown below:

 Class_12_Aldehydes_&_Ketones_Hydrolysis_1

 If we heat lactic acid, in presence of sulphuric acid then the water molecule gets removed and acrylic acid is formed as shown:

 Class_12_Aldehydes_&_Ketones_Heating_Of_Lactic_Acid

  •   Addition of Sodium bisulphite (this reaction is used in separation of aldehyde and ketone)

The reaction involved is given below:

  Class_12_Aldehydes_&_Ketones_Addition_Of_Sodium_BiSulphite                   

 

For aldehyde, the reaction goes in forward reaction and for ketone backside is favoured (as two methyl groups increase bulkiness and form unstable products).

Note: diethyl ketone, acetophenone and Benzophenone will not undergo these reactions because of bulkiness.

  •  Addition of Grignard reagent

When Grignard reagent is added following reactions occur:

Class_12_Aldehydes_&_Ketones_Addition_Of_Grignard_Reaction 

 

Please note: In this formaldehyde form primary alcohol, any other aldehyde form secondary alcohol and ketone forms tertiary alcohol.

 

(d)Addition of alcohols: In this we get two products that are hemi-acetals and acetals from aldehyde. From ketone we get, ketals.

The reaction occurs as shown:

  Class_12_Aldehydes_&_Ketones_Addition_Of_Alcohols               

 Please note in this we use dry HCl

  • So as, to protonate the carbonyl oxygen. Therefore, increasing the electrophilicity of carbonyl carbon to enhance the attack of alcohol.
  • It removes the water produced during the reaction and takes the reaction in forward direction.

 Class_12_Aldehydes_&_Ketones_Use_Of_Dry_HCl

           

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