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 query requires the evaluation of a claim and its supporting justification concerning the reaction of haloalkanes with potassium cyanide (KCN) and silver cyanide (AgCN).

Concept Used:

The fundamental concept is the reactivity of ambident nucleophiles. An ambident nucleophile possesses two nucleophilic centers and can engage in an attack via either. The cyanide ion, \( [C \equiv N]^- \), serves as a prime illustration. It can target an electrophilic carbon center through either the carbon atom (resulting in a cyanide) or the nitrogen atom (forming an isocyanide).

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

The reaction's outcome is contingent upon the character of the attacking agent (ionic or covalent) and the prevailing reaction conditions.

1. Ionic Reagents (e.g., KCN): These compounds fully dissociate in the solvent, yielding free nucleophiles. The site of attack is then determined by factors such as bond strength.
2. Covalent Reagents (e.g., AgCN): Within these compounds, the nucleophilic center is not liberated. The attack proceeds through a site that offers greater accessibility for electron pair donation.

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 largely ionic. In a polar solvent, it dissociates into \(K^+\) and free \(CN^-\) ions. The \(CN^-\) ion then attacks the haloalkane (\(R-X\)). Although both C and N are nucleophilic, the attack predominantly occurs via the carbon atom. This preference is due to the greater strength and stability of the resultant C-C bond in the alkyl cyanide (\(R-C \equiv N\)) compared to the C-N bond in an isocyanide. \[ R-X + KCN \longrightarrow R-CN \text{ (Alkyl Cyanide - Major Product)} + KX \]
• Reaction with AgCN: Silver cyanide (AgCN) is predominantly covalent. The Ag-C bond exhibits substantial covalent character, preventing complete dissociation into free \(CN^-\) ions. In this scenario, the lone pair of electrons on the nitrogen atom is available for donation, enabling it to function as the nucleophilic center. The nitrogen atom then attacks the carbon of the haloalkane. \[ R-X + AgCN \longrightarrow R-NC \text{ (Alkyl Isocyanide - Major Product)} + AgX \]

Consequently, the assertion in statement (A) is factually accurate.

Step 2: Analyze the Reason (R).

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

• KCN: As previously elaborated, KCN is an ionic compound due to the significant electronegativity disparity between potassium (an alkali metal) and the cyanide group.
• AgCN: AgCN is not highly ionic; it is primarily covalent. According to Fajan's rules, a smaller, more highly charged cation exhibits greater polarizing power, leading to increased covalent character. Although \(K^+\) and \(Ag^+\) share similar charges, \(Ag^+\), with its pseudo-noble gas configuration, is more polarizing than \(K^+\). This results in significant covalent character within the Ag-C bond.

Given that the reason asserts both compounds are highly ionic, and AgCN is in fact covalent, the statement in Reason (R) is erroneous.

Step 3: Conclude the relationship between Assertion and Reason.

It has been established that Assertion (A) is a true statement, whereas Reason (R) is a false statement.

Final Result:

Following the analysis, Assertion (A) is true, but the provided Reason (R) is false.

Therefore, the most fitting response is: (A) is true but (R) is false.