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

Short answer

(a) o-toluidine

Step-by-step solution

Identify the Reaction Type

The reaction given involves liquid
e NH3
e and KNH2
e, which is typical for nucleophilic aromatic substitution (NAS) reactions, specifically the Elimination-Addition
e mechanism.

Analyze the Substrate

The substrate p-chlorotoluene
e is a benzene derivative with a chlorine atom at the para
e position relative to a methyl group. This creates a potential site for substitution by the strong nucleophile NH2-
e.

Understand Nucleophilic Attack

In NAS via elimination-addition
e, the NH2-
e first eliminates a H+
e creating a benzyne intermediate.
e This intermediate allows for nucleophilic attack. Given the structure of p-chlorotoluene,
e the major products emerge based on available positions for the nucleophile's attack on the benzyne.

Predict the Position of Nucleophilic Attack

The nucleophile NH2-
e can attack either position adjacent to the initial chloro group
e on the benzyne intermediate,
e resulting in two possible sites: ortho (o-) and para (p-
e) positions relative to the methyl group.

Determine the Major Product

Given that nucleophilic attack often favors the path with least steric hindrance and para
e
e relative to the methyl group,
e the predominant product of the reaction will most likely be o-toluidine.
e

Key concepts

Elimination-Addition Mechanism

The elimination-addition mechanism is a key pathway in nucleophilic aromatic substitution (NAS) reactions. Imagine it as a two-step "attack and transform" process. In the first step, elimination occurs when the strong nucleophile, such as \( ext{NH}_2^-\), removes a hydrogen (\( ext{H}^+ \)) from the aromatic ring. This action is not just a simple removal but rather a transformation that temporarily disrupts the usual stability of the aromatic ring. This leads to the formation of an unusual intermediate called benzyne.

• Begins with elimination of \( ext{H}^+\)
• Formation of benzyne intermediate
• Nucleophilic attack introduces substitution

In the second step, this benzyne intermediate, now exposed and reactive, becomes highly susceptible to nucleophilic attack. The nucleophile swoops in to form a new bond with the carbon ring, effectively substituting itself in place of a group like chlorine. This substitution restores stability to the aromatic structure after the turmoil of elimination.

Benzyne Intermediate

The benzyne intermediate is a pivotal concept in understanding the elimination-addition mechanism of nucleophilic aromatic substitution. Imagine the benzene ring, ordinarily flat and stable, twisting into a highly strained form with an additional bond. This structure, benzyne, is rare and incredibly reactive because the usual aromatic stability is temporarily compromised.

• Formed upon the removal of \( ext{H}^+\)
• Features an unusual triple bond within the aromatic ring
• Highly reactive due to ring strain

For example, when p-chlorotoluene undergoes elimination of \( ext{H}^+\), the resulting benzyne features a triple bond that makes the positions adjacent to the original chlorine and methyl groups sites for potential attack. This reactivity is harnessed to substitute groups on the ring, allowing for new molecules like o-toluidine to be formed.

Stereochemistry in Reactions

Stereochemistry in reactions, particularly in nucleophilic aromatic substitution via the elimination-addition mechanism, is crucial to predict which products are more likely formed. While benzynes have added complexity due to their reactive nature, understanding the spatial arrangement around the reaction sites helps in predicting the outcome. Let's break it down:

• Nucleophile attacks where least steric hindrance occurs
• Orientation affects which product dominates
• Stereochemistry determines molecular geometry and function

In this process, the spatial arrangement is controlled by the benzyne intermediate's available reactive sites. Specifically, in the reaction of \( ext{p-chlorotoluene}\)with \( ext{KNH}_2\)in liquid ammonia, the nucleophile can attack at positions relative to the methyl group. The path chosen minimizes strain and steric interference, leading to the formation of o-toluidine as the major product. Thus, understanding stereochemistry is not just about knowing where atoms are; it's about predicting how the reaction can proceed efficiently and favorably.