Question

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

Answer 1

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.