Question

Which of the following reagents is used to prepare alkyl isocyanides from alkyl halides $(RX)$?

• A. $KCN$ (alc.)
• B. $AgCN$ (alc.)
• C. $NaCN$ (aq.)
• D. $HCN$

Hint:

Important ambident nucleophile results:

• \(KCN/NaCN \rightarrow\) Alkyl cyanide (Nitrile, \(R-CN\))
• \(AgCN \rightarrow\) Alkyl isocyanide (\(R-NC\))
• \(KNO_2 \rightarrow\) Alkyl nitrite (\(R-ONO\))
• \(AgNO_2 \rightarrow\) Nitroalkane (\(R-NO_2\))

Answer 1

The Correct Option is B

Concept:
The cyanide ion \(CN^-\) is an ambident nucleophile, meaning it has two possible sites of attack:

• Carbon end \((C)\)
• Nitrogen end \((N)\)

The type of reagent used determines which atom participates in the nucleophilic attack, thereby deciding the final product.
Step 1: {Reaction with \(KCN\).} \(KCN\) is an ionic compound and dissociates completely in solution to give free \(CN^-\) ions. The nucleophilic attack occurs through the carbon atom, forming a \(C-C\) bond and producing alkyl cyanides (nitriles). \[ R-X + KCN \rightarrow R-CN \]
Step 2: {Reaction with \(AgCN\).} \(AgCN\) is largely covalent in nature. In this compound, the carbon atom is strongly bonded to silver, leaving the nitrogen atom available for nucleophilic attack. As a result, the reaction forms alkyl isocyanides. \[ R-X + AgCN \rightarrow R-NC \]
Step 3: {Conclusion.} Since the formation of alkyl isocyanides occurs when nucleophilic attack takes place through nitrogen, the reagent required is: \[ AgCN \text{ (alc.)} \]