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

1. LiAlH4 : Reduction with LiAlH4 - Aldehydes form primary alcohols. Ketones form secondary alcohols.
2. Fehlings solution: Aldehydes when heated with Fehling's solution give red precipitate of cuprous oxide (Cu2O). Ketones do not give precipitates.
3. Alcohol in the prsence of HCl: Action of Alcohol in the prsence of HCl: Aldehydes form acetals easily. Ketones do not form acetals easily.
4. Schiff's reagent; In reaction aldehydes give red precipitate. Ketones do not give precipitate.
5. Tollen's reagent: In reaction aldehydes form silver mirror. Ketones do not form silver mirror.

Distinction between some compounds

1. 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

2. Acetaldehdye and Benzaldehyde
Acetaldehyde gives yellow precipitate of iodoform (CHI3) with an alkaline solution of iodine (I2 + NaOH)
Benzaldehyde does not give the precipitate



3. Acetaldehyde and acetone
Tollen's reagent: In reaction aldehydes form silver mirror
Acetone does not give this test

4. Benzaldehyde and acetophenone
Tollen's reagent:benzaldehyde give silver mirror.
acetophenone does not react

5. 2-Pentanone and 3-pentanone
Iodoform test
2-pentanone gives iodoform test.
3-pentannone does not give precipitate

6. Acetophenone and benzophenone
Iodoform test:
Acetophenone gives yellow precipitate of iodoform.
Benzophenone does not give this test.