Question

ln the reaction \(\mathrm{p}\) -chlorotoluene with \(\mathrm{KNH}_{2}\) in liquid \(\mathrm{NH}_{3}\) the major product is (1) 0 -toludine (2) m-toludine (3) p-toludinc (4) p-chloroanilinc

Short answer

The major product is p-toluidine, which corresponds to option (3).

Step-by-step solution

Identify the Reactants and Reagent

The reactant in the problem is p-chlorotoluene, and the reagent used is \(\text{KNH}_2\) in liquid \(\text{NH}_3\).

Understand the Reaction Conditions

The reaction occurs in liquid ammonia, which is a common medium for nucleophilic substitution reactions involving \(\text{KNH}_2\).

Determine the Type of Reaction

This is likely a nucleophilic aromatic substitution reaction, where the amide ion (\(\text{NH}_2^-\)) acts as a nucleophile and replaces the chlorine atom in p-chlorotoluene.

Analyze the Product Formation

When \(\text{NH}_2^-\) substitutes the Cl atom in p-chlorotoluene, the major product formed is p-toluidine (para-toluidine).

Match the Product to the Given Options

Among the given options, the correct product, p-toluidine, corresponds to (3).

Key concepts

Reaction Conditions

The reaction conditions for nucleophilic aromatic substitution involving p-chlorotoluene and \(\text{KNH}_2\) typically require a polar aprotic solvent. Liquid ammonia (\(\text{NH}_3\)) is an excellent choice because it can stabilize the amide ion (\(\text{NH}_2^-\)).
Liquid ammonia provides a conducive environment by solvating the nucleophile, making it more reactive. It also has a relatively low boiling point, enabling easy handling.
Employing low temperature conditions can help control the reaction rate and prevent unwanted side reactions.

Nucleophiles

In nucleophilic aromatic substitution, the nucleophile plays a crucial role. Here, the nucleophile is the amide ion (\(\text{NH}_2^-\)).
The amide ion is highly nucleophilic due to its strong negative charge, making it effective in attacking the aromatic ring.
It preferentially substitutes the electron-withdrawing chlorine atom because chlorine creates a partial positive charge on the aromatic ring, making it more susceptible to nucleophilic attack.

Chemical Reagents

The primary chemical reagent in this reaction is potassium amide (\(\text{KNH}_2\)), which provides the necessary amide ion (\(\text{NH}_2^-\)).
Potassium amide is a strong base, effectively deprotonating liquid ammonia to form the essential nucleophile for the substitution.
Another reagent playing a secondary but important role is liquid \(NH_3\), which acts as the solvent. It dissolves the reactants and stabilizes the transition state, promoting the overall reaction.

Product Formation

Product formation in nucleophilic aromatic substitution hinges on the successful displacement of the chlorine atom by a nucleophile.
When \(\text{NH}_2^-\) substitutes for chlorine in p-chlorotoluene, the resulting major product is para-toluidine (p-toluidine).
Para-toluidine is formed because the para position (opposite to where the chlorine is attached) is more stable due to steric and electronic factors compared to ortho or meta positions. Therefore, the para-isomer is the predominant product, as observed in the problem's step-by-step solution.