Question

The Cannizzaro reaction is not given by;
 $ A) $ Formaldehyde
 $ B) $ Acetaldehyde
 $ C) $ Benzaldehyde

Answer 1

Hint: Cannizzaro reaction can be defined as base-induced disproportionation of an aldehyde lacking a hydrogen atom in the alpha position. This reaction was named after its discoverer Stanislao Cannizzaro.

Complete answer:
Cannizaro reaction is a characteristic reaction of aldehydes lacking $ \alpha $ - hydrogen. The reaction involves disproportionation (oxidation and reduction) of aldehyde in the presence of a strong base forming carboxylic acid and alcohol.
Now, coming back to our question.
We have three aldehydes and we have to tell which one will not give Cannizzaro reaction.
-Formaldehyde ( $ HCHO $ )
It doesn’t have $ \alpha $ - hydrogen and will not form an enolate . Thus, Formaldehyde will undergo Cannizzaro reaction very efficiently. It will be disproportionate to give acid and alcohol.
-Acetaldehyde ( $ C{H_3}CHO $ )
Acetaldehyde is having $ \alpha $ - hydrogen. In the presence of base it will form an enolate ion and will undergo Aldol Condensation reaction. Thus, Acetaldehyde will not show Cannizzaro reaction.
-Benzaldehyde ( $ {C_6}{H_5}CHO $ )
Benzaldehyde also lacks $ \alpha $ - hydrogen. Thus, Benzaldehyde will also undergo Cannizzaro reaction.
Hence, option $ B) $ Acetaldehyde is correct.

Additional Information:
The nucleophile ( $ O{H^ - } $ ) attacks the carbonyl carbon of aldehyde causing the disproportionation reaction of aldehyde. Crossed Cannizzaro reaction is used in order to avoid low yields. In Cross Cannizzaro two different aldehydes lacking $ \alpha $ - hydrogens are made to undergo the reaction.

Note:
In order to check which aldehyde will give Cannizzaro reaction, we just have to check the $ \alpha $ - hydrogens on aldehyde. The one having $ \alpha $ - hydrogen will undergo Aldol Condensation instead of Cannizzaro reaction. Examples of aldehydes without $ \alpha $ - hydrogen include Vanillin, Benzaldehyde, Formaldehyde etc.