Formaldehyde undergoes Cannizzaro action because

A. It has alpha hydrogen atom

B. It does not have alpha hydrogen atom

C. It does not undergo self-oxidation and reduction on heating with concentrated alkali

D. It undergo self-oxidation and reduction on heating with concentrated alkali

Choose the correct answer from the option given below:

B and D only

The correct answer is option 1. B and D only.

The Cannizzaro reaction is a disproportionation reaction in which an aldehyde undergoes self-oxidation and reduction in the presence of a strong base, typically concentrated alkali, to yield a corresponding alcohol and carboxylic acid. Formaldehyde \((HCHO)\) is a special case because it lacks alpha hydrogen atoms, making it unable to undergo aldol condensation or cross-aldol condensation reactions.

Let us look at each of the given statements:

A. It has alpha hydrogen atom: This statement is incorrect. Formaldehyde does not have alpha hydrogen atoms.

B. Formaldehyde lacks alpha hydrogen atoms: This statement is correct.  Formaldehyde (\(HCHO\)) is the simplest aldehyde, and its structure is \(H-C=O\). In this molecule, there are no hydrogen atoms directly attached to the carbonyl carbon (the carbon atom double-bonded to oxygen). Alpha hydrogen atoms are hydrogen atoms directly attached to the carbon next to the carbonyl carbon. Since formaldehyde lacks this structure, it does not have alpha hydrogen atoms.

C. It does not undergo self-oxidation and reduction on heating with concentrated alkali: This statement is incorrect. Formaldehyde does undergo self-oxidation and reduction (Cannizzaro reaction) when heated with concentrated alkali.

D. Formaldehyde undergoes self-oxidation and reduction on heating with concentrated alkali: This statement is correct. The Cannizzaro reaction is a disproportionation reaction involving aldehydes or ketones in the presence of a strong base, such as concentrated alkali. In this reaction, one molecule of aldehyde or ketone is reduced to an alcohol, while another molecule of the same compound is oxidized to a carboxylic acid.

Formaldehyde (\(HCHO\)) is a special case because it cannot undergo Aldol condensation due to the absence of alpha hydrogen atoms. Therefore, when formaldehyde is heated with concentrated alkali, it undergoes self-oxidation and reduction to yield methanol (\(CH_3OH\)) and formic acid (\(HCOOH\)).

The reaction mechanism involves the formation of a formate ion (\(HCOO^-\)) and a hydride ion (\(H^-\)), which is then used for the reduction of another molecule of formaldehyde.

Therefore, option 1. B and D only, correctly explains the characteristics of formaldehyde in relation to the Cannizzaro reaction.

Let us look at the Cannizzaro reaction of formaldehyde:

One molecule of aldehyde is reduced to the corresponding alcohol, while a second one is oxidized to the carboxylic acid.

Let us look at the mechanism clearly.

STEP 1: The reaction initiated by the nucleophilic attack of a hydroxide ion on the carbonyl carbon of a formaldehyde molecule by giving a hydrate anion, which can be further deprotonated in a strongly alkaline medium to give a dianion. Here, the hydroxide behaves as a base.

STEP 2: Now a hydride anion is transferred either from the dianionic species to the carbonyl carbon of another formaldehyde molecule. The strong electron donating effect of \(O^−\) groups facilitates the hydride transfer and the reaction goes on. That is why, it is the rate determining step of the reaction.

STEP 3: Here, one molecule is oxidized to carboxylic acid and the other one got reduced to an alcohol. It indicates that hydrogen is transferred from the second formaldehyde molecule. The reaction occurs very slowly when electron-donating groups are present but occurs at faster rates when electron withdrawing groups are present.