Practical Organic Chemistry - Aldehydes & Ketones

Aldehydes and Ketones Core Points


Tests to distinguish aldehydes and ketones

1. LiAlH4
   
2. Fehlings solution
   
3. Alcohol in the prsence of HCl
   
4. Schiff’s reagent
   
5. Tollen’s reagent
   
   Mnemonic to remember Learn FAST
   
   
6. LiAlH4 : Reduction with LiAlH4 - Aldehydes form primary alcohols. Ketones form secondary alcohols.
   
7. Fehlings solution: Aldehydes when heated with Fehling’s solution give red precipitate of cuprous oxide (Cu2O). Ketones do not give precipitates.
   
8. Alcohol in the prsence of HCl: Action of Alcohol in the prsence of HCl: Aldehydes form acetals easily. Ketones do not form acetals easily.
   
9. Schiff’s reagent; In reaction aldehydes give red precipitate. Ketones do not give precipitate.
   
10. Tollen’s reagent: In reaction aldehydes form silver mirror. Ketones do not form silver mirror.
    
    Distinction between some compounds
    
    
11. Formaldehyde (HCHO) and acetaldehyde (CH3CHO)
    
    Acetaldehyde gives yellow precipitate of iodoform (CHI3) with an alkaline solution of iodine (I2 + NaOH)
    
    Formaldehyde does not give this test
    
    
12. Acetaldehdye and Benzaldehyde
    Acetaldehyde gives yellow precipitate of iodoform (CHI3) with an alkaline solution of iodine (I2 + NaOH)
    Benzaldehyde does not give the precipitate
    
    
    
    
13. Acetaldehyde and acetone
    Tollen’s reagent: In reaction aldehydes form silver mirror
    Acetone does not give this test
    
    
14. Benzaldehyde and acetophenone
    Tollen’s reagent:benzaldehyde give silver mirror.
    acetophenone does not react
    
    
15. 2-Pentanone and 3-pentanone
    Iodoform test
    2-pentanone gives iodoform test.
    3-pentannone does not give precipitate
    
    
16. Acetophenone and benzophenone
    Iodoform test:
    Acetophenone gives yellow precipitate of iodoform.
    Benzophenone does not give this test.