Read the Passage carefully and answer the questions.

The carbonyl group of aldehydes and ketones is highly polar. The positively charged carbon is readily attacked by electron-rich nucleophile and the negatively charged oxygen atom is attacked by an electro-deficient electrophile. Aldehydes and ketones undergo nucleophilic addition reactions in the presence of acid or a base. The mechanism involves attack of a nucleophile on the electron deficient carbonyl carbon to form a tetrahedral intermediate. A new bond is formed between the carbon and oxygen. Protonation of the anion furnished by the reaction medium yields a neutral addition product. The addition product is same whether the reaction is catalyzed by acid or a base. For example in the presence of base aldehydes and ketones react with hydrogen cyanide to form cyanohydrin.

The source of nucleophile in the base catalyzed cyanohydrin formation is

HCN

The correct answer is Option (1) → HCN **

While salts like $KCN$ or $NaCN$ provide cyanide ions ($CN^-$), the process of cyanohydrin formation specifically involves the reaction of an aldehyde or ketone with Hydrogen Cyanide ($HCN$).

The passage states: "In the presence of base, aldehydes and ketones react with hydrogen cyanide to form cyanohydrin."

The Role of the Base Catalyst

Pure $HCN$ is a very weak acid and does not ionize significantly on its own. Therefore, it is a poor source of the $CN^-$ nucleophile. The addition of a base (like $OH^-$) catalyzes the reaction by deprotonating the $HCN$ to generate a higher concentration of the strong nucleophile, cyanide ion ($CN^-$).

The chemical equation for the generation of the nucleophile is:

$HCN + OH^- \rightleftharpoons CN^- + H_2O$

Mechanism Summary

1. Nucleophilic Attack: The generated $CN^-$ nucleophile attacks the electrophilic carbonyl carbon to form a tetrahedral intermediate.
2. Protonation: The intermediate oxygen anion ($O^-$) is then protonated (usually by a water molecule or another $HCN$ molecule) to yield the final cyanohydrin.

Why other options are incorrect:

• KCN / NaCN (Options 2 & 3): While these salts are often added to provide a high concentration of $CN^-$ ions in practical laboratory settings, the specific reaction described in the passage and standard curriculum refers to the base-catalyzed reaction of HCN.
• $CH_3CN$ (Option 4): This is methyl cyanide (acetonitrile), which is a solvent and does not act as a source of the $CN^-$ nucleophile for this specific addition reaction.