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
Answer & explanation
Correct answer: option 1
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
- Nucleophilic Attack: The generated $CN^-$ nucleophile attacks the electrophilic carbonyl carbon to form a tetrahedral intermediate.
- 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.