Question

Benzaldehyde undergoes _____ reaction due to ____ of $\alpha -$hydrogen atom.
A. Cannizzaro; presence
B. Cannizzaro; absence
C. Henry’s; absence
D. Henry’s; presence

Answer 1

Hint: We have to know that Cannizzaro reaction is an organic redox reaction that deals with aldehydes and ketones. Henry reaction deals with nitroalkane and an aldehyde (or) ketone to form beta-nitro alcohols. Both these reactions happen in presence of base.

Complete step by step answer:
We must remember that a carboxylic acid and a primary alcohol is formed by the base-induced disproportion of molecules of non-enolizable aldehyde reaction known as Cannizzaro reaction. Few of the ketone group also undergo Cannizzaro reaction. It transfers one of their groups of carbon before the hydride which could be found on an aldehyde.
We have to remember that Henry's reaction is the reaction of nitroalkane and an aldehyde or ketone in the presence of a base to give beta-nitro alcohols. This reaction is also called a nitroaldol reaction. Therefore, the option C and D are incorrect.
Two molecules of Benzaldehyde react with potassium hydroxide to form benzyl alcohol and potassium benzoate. We can write the chemical reaction as,
$2C_6{H}_{5}CHO+KOH\to C_6{H}_{5}C{H}_{2}OH+C_6{H}_{5}COOK$
Two molecules of Benzaldehyde also react with sodium hydroxide to form benzyl alcohol and sodium benzoate. We can write the chemical reaction as,
$2C_6{H}_{5}CHO+NaOH\to C_6{H}_{5}C{H}_{2}OH+C_6{H}_{5}COONa$
We have to remember that the Cannizzaro reaction is undergone by benzaldehyde as it does not contain an alpha hydrogen atom. Therefore, the option (B) is correct.
We must know that the compounds that contain alpha-hydrogen atoms do not undergo Cannizzaro reaction. Therefore, the option (A) is incorrect.

So, the correct answer is Option B.

Note: We have to know that Cannizzaro reaction is a redox reaction that involves the transfer of hydride from one molecule of substrate to the substrate of another molecule. Among these two molecules, acid is formed from oxidation of one acid, another aldehyde molecule is reduced to form molecule of alcohol.