Question

Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A) : Haloalkanes react with KCN to form alkyl cyanides as a main product while with AgCN form isocyanide as the main product.
Reason (R) : KCN and AgCN both are highly ionic compounds. In the light of the above statement
Choose the most appropriate answer from the options given below:

• A. (A) is correct but (R) is not correct
• B. Both (A) and (R) are correct but (R) is not the correct explanation of (A)
• C. (A) is not correct but (R) is correct
• D. Both (A) and (R) are correct and (R) is the correct explanation of (A)

Answer 1

The Correct Option is A

This question asks us to evaluate an assertion and a reason concerning the reaction of haloalkanes with potassium cyanide (KCN) and silver cyanide (AgCN).

Concept Used:

The core concept is the reactivity of ambident nucleophiles. An ambident nucleophile is a species that has two nucleophilic centers and can attack through either one. The cyanide ion, \( [C \equiv N]^- \), is a classic example. It can attack an electrophilic carbon center either through the carbon atom (forming a cyanide) or through the nitrogen atom (forming an isocyanide).

\[ :C \equiv N:^- \longleftrightarrow ^- :C = \ddot{N}: \]

The outcome of the reaction depends on the nature of the attacking reagent (ionic or covalent) and the reaction conditions.

1. Ionic Reagents (like KCN): These compounds completely dissociate in the solvent to provide free nucleophiles. The site of attack is then determined by factors like bond strength.
2. Covalent Reagents (like AgCN): In these compounds, the nucleophilic center is not free. The attack occurs through a site that is more available to donate its electron pair.

Step-by-Step Solution:

Step 1: Analyze the Assertion (A).

The assertion states: "Haloalkanes react with KCN to form alkyl cyanides as a main product while with AgCN form isocyanide as the main product."

• Reaction with KCN: Potassium cyanide (KCN) is predominantly an ionic compound. In a polar solvent, it dissociates to give \(K^+\) and free \(CN^-\) ions. The \(CN^-\) ion attacks the haloalkane (\(R-X\)). Although both C and N are nucleophilic sites, the attack preferentially occurs through the carbon atom. This is because the resulting C-C bond in the alkyl cyanide (\(R-C \equiv N\)) is stronger and more stable than the C-N bond that would be formed in an isocyanide. \[ R-X + KCN \longrightarrow R-CN \text{ (Alkyl Cyanide - Major Product)} + KX \]
• Reaction with AgCN: Silver cyanide (AgCN) is predominantly a covalent compound. The silver-carbon bond has significant covalent character, so it does not dissociate to give free \(CN^-\) ions. In this case, the lone pair of electrons on the nitrogen atom is available for donation, and it acts as the nucleophilic center. The nitrogen atom attacks the carbon of the haloalkane. \[ R-X + AgCN \longrightarrow R-NC \text{ (Alkyl Isocyanide - Major Product)} + AgX \]

Therefore, the statement made in Assertion (A) is factually correct.

Step 2: Analyze the Reason (R).

The reason states: "KCN and AgCN both are highly ionic compounds."

• KCN: As explained above, KCN is an ionic compound due to the large difference in electronegativity between potassium (an alkali metal) and the cyanide group.
• AgCN: AgCN is not highly ionic; it is predominantly covalent. According to Fajan's rules, the smaller and more highly charged a cation is, the greater its polarizing power, leading to more covalent character. While \(K^+\) and \(Ag^+\) have similar charges, \(Ag^+\) has a pseudo-noble gas configuration and is more polarizing than \(K^+\). This results in a significant covalent character in the Ag-C bond.

Since the reason claims that both compounds are highly ionic, and AgCN is covalent, the statement made in Reason (R) is incorrect.

Step 3: Conclude the relationship between Assertion and Reason.

We have determined that Assertion (A) is a true statement, but Reason (R) is a false statement.

Final Result:

Based on the analysis, Assertion (A) is true, but the Reason (R) provided for it is false.

Therefore, the most appropriate answer is: (A) is true but (R) is false.