Account for the following: (a) Aniline does not undergo Friedel Crafts alkylation (b) Although - NH2 group is an ortho and para-directing group, nitration of aniline gives alongwith ortho & para- derivatives meta-derivative also.

To understand why aniline does not undergo Friedel-Crafts alkylation and why nitration of aniline produces a meta-derivative alongside ortho and para derivatives, we need to delve into the reactivity of aniline and the nature of the reactions involved.

Friedel-Crafts Alkylation and Aniline

Friedel-Crafts alkylation is a reaction that typically involves the introduction of an alkyl group into an aromatic ring using an alkyl halide and a Lewis acid catalyst, such as aluminum chloride. However, aniline, which contains an amino group (-NH2), does not participate in this reaction. The reason lies in the properties of the amino group.

The Role of the Amino Group

The amino group is a strong electron-donating group due to its lone pair of electrons. This electron donation enhances the nucleophilicity of the aromatic ring, making it more reactive towards electrophiles. However, during Friedel-Crafts alkylation, the Lewis acid forms a complex with the alkyl halide, generating a highly reactive carbocation. This carbocation can react with the aromatic ring, but the presence of the amino group complicates the reaction.

• The amino group can coordinate with the Lewis acid, effectively tying it up and preventing it from generating the necessary electrophile.
• Additionally, the carbocation formed can lead to rearrangements that are not favorable when aniline is involved, often resulting in side reactions or decomposition.

As a result, the Friedel-Crafts alkylation does not proceed with aniline, making it a unique case among aromatic compounds.

Nitration of Aniline: A Complex Outcome

When we consider the nitration of aniline, we see a different scenario. The amino group is indeed an ortho- and para-directing group, which means it typically directs incoming electrophiles to the ortho and para positions on the aromatic ring. However, the presence of the amino group also introduces some complications that can lead to the formation of meta derivatives.

Mechanism of Nitration

Nitration involves the introduction of a nitro group (-NO2) into the aromatic ring, usually using a mixture of nitric acid and sulfuric acid. The electrophile in this case is the nitronium ion (NO2+), which is generated in situ from the acids. The amino group, while directing the electrophile, also influences the stability of the intermediates formed during the reaction.

• In the presence of the amino group, the ortho and para positions are favored for substitution due to resonance stabilization of the carbocation intermediates.
• However, the amino group can also activate the ring to such an extent that it becomes more susceptible to further reactions, including potential rearrangements or competing reactions that can lead to the formation of meta products.

Moreover, steric hindrance can play a role. The ortho position is often less favorable due to the proximity of the bulky amino group, leading to a higher likelihood of meta substitution under certain conditions. This results in a mixture of ortho, para, and meta products during the nitration of aniline.

Summary of Key Points

In summary, aniline's inability to undergo Friedel-Crafts alkylation is primarily due to the strong electron-donating nature of the amino group, which interferes with the formation of the necessary electrophile. On the other hand, while nitration of aniline predominantly yields ortho and para derivatives, the presence of the amino group can also lead to the formation of meta derivatives due to steric effects and the reactivity of the intermediates involved. Understanding these nuances highlights the complexity of aromatic chemistry and the influence of substituents on reaction pathways.