Problem 32 - A Complete Resource Book in Chemistry for JEE Main

Question

In the reaction of \(\mathrm{p}\)-chlorotoluene with \(\mathrm{KNH}_{2}\) in liquid \(\mathrm{NH}_{3}\), the major product is (a) o-toluidine (b) \(\mathrm{m}\)-toluidine (c) p-toluidine (d) p-chloroaniline

Step-by-Step Solution

Answer

(a) o-toluidine

Step 1: Understand the Reaction Conditions

The reaction involves p-chlorotoluene and potassium amide (KNH2) in liquid ammonia (NH3). This is characteristic of a nucleophilic aromatic substitution (NAS) through a benzyne intermediate, particularly common in harsh conditions like liquid NH3.

Step 2: Identify the Mechanism

In NAS with KNH2 in liquid NH3, p-chlorotoluene forms a benzyne intermediate when the chlorine is eliminated. The benzyne is an unstable intermediate that can react with NH2- ions.

Step 3: Predict the Benzyne Attack

The benzyne intermediate can be attacked by NH2- at either the ortho or meta position relative to the methyl group, as para is blocked by substitution.

Step 4: Determine Major Products

The main attack of NH2- will be at the position adjacent to the methyl group, leading primarily to the formation of o-toluidine.

Step 5: Conclusion

The compound formed from the reaction of p-chlorotoluene with KNH2 in liquid ammonia is predominately o-toluidine, making it the major product.

Key Concepts

Benzyne Intermediate

The reaction between \(\text{p-chlorotoluene}\) and \(\text{KNH}_2\) in liquid ammonia primarily involves the formation of a benzyne intermediate. Benzyne is a fascinating and somewhat mysterious intermediate in organic chemistry. It is a highly reactive, unstable compound. The structure of benzyne consists of a benzene ring in which two adjacent carbon atoms are connected by a triple bond instead of the usual double bond. This formation happens because the aromatic ring loses a substituent like chlorine under strong conditions, paving the way for the creation of a triple-bonded structure.

• Benzyne is crucial for understanding certain aromatic substitution reactions, as it enables the reaction to progress despite chlorine being a relatively poor leaving group.
• Unlike typical nucleophilic substitution reactions where intermediates with negative charges are formed, benzyne intermediates typically handle extreme conditions.

The presence and detection of the benzyne intermediate open up a path for nucleophiles, like \(\text{NH}_2^-\), to attack the aromatic ring and determine the final structure of the product.

P-Chlorotoluene

P-chlorotoluene acts as the starting substrate in this reaction. This compound consists of a benzene ring with a chlorine atom and a methyl group attached. The prefix 'p' stands for para, indicating that the substituents—the chlorine and the methyl group—are located opposite each other on the benzene ring.

• This para substitution pattern significantly impacts the reaction pathway and the positions available for additional substitutions.
• The chlorine atom in p-chlorotoluene is crucial for the generation of the benzyne intermediate. When influenced by a strong base like \(\text{KNH}_2\), it gets expelled, facilitating the nucleophilic aromatic substitution mechanism.

The interaction style of p-chlorotoluene makes it possible to establish either an ortho or a meta substitution product in the reaction setting.

Ortho Substitution

After the formation of the reactive benzyne intermediate, the nucleophile, here the \(\text{NH}_2^-\) ion, has two primary potential attack points on the aromatic ring. The positions to target are namely ortho and meta relative to the methyl group.One of the key outcomes in this specific reaction scenario is the prevalence of ortho substitution. This means \(\text{NH}_2^-\) primarily attacks the carbon adjacent to the methyl group, leading to the formation of ortho-toluidine. Factors contributing to this tendency include:

• Proximity: The ortho position is closer in terms of chemical stability after the formation of the benzyne.
• Substitution Restriction: The para position is already occupied by the methyl group, making ortho and meta positions the only viable choices.

Such a preference makes ortho-toluidine the major product in these reaction conditions.

Liquid Ammonia Reaction

Liquid ammonia plays an essential role as a solvent in this process. It provides a medium for the reaction to occur at low temperatures, which is unlike typical reactions that require heating.

• KNH2 in liquid ammonia forms a strong base and a potent nucleophile, the \(\text{NH}_2^-\) ion, which is essential for reacting with the benzyne intermediate.
• This reaction setup—using liquid ammonia—allows alterations to aromatic compounds under extreme conditions without causing unwanted side reactions that could occur in other solvents.

The reaction of p-chlorotoluene in liquid ammonia shows how varying the reaction medium can control the nature and outcome of particular chemical reactions, emphasizing the utility of liquid ammonia in achieving certain transformation efficiency.