Anilines

1.0Aniline and Its Reactions

1. Bromination:

• Reaction: Aniline reacts with bromine water to form 2,4,6-tribromoaniline.
• Mechanism: Highly activated aromatic ring due to the -NH₂ group facilitates substitution at ortho and para positions.

2. Nitration:

• Reaction: Direct nitration of aniline with concentrated nitric acid​ and H₂SO₄ leads to oxidation and undesired products.
• Protection: Acetylation of aniline to acetanilide is carried out before nitration to control the reaction and obtain para products selectively.

3. Sulfonation:

• Reaction: Treatment with concentrated H₂SO₄​ forms sulfanilic acid (aniline-2-sulfonic acid).
• Zwitterion formation occurs when the amino group protonates, and sulfonic acid (-SO₃H) deprotonates.

Protection Techniques for aniline group:

• • Protection of the -NH₂ group (e.g., acetylation) is often necessary to direct reactions selectively and avoid overreaction.
  • Acetylation is a fundamental technique in organic chemistry for protecting and modifying amino functionalities. 

Zwitterion Formation in Sulfonation

• In sulfonation, the -NH₂ group gains a proton (acting as a base), and the sulfonic acid group (-SO₃H) loses a proton, forming a zwitterionic species.
• Important for understanding the solubility and reactivity of sulfanilic acid.

2.0Directing Nature of Aniline

Ortho/Para/meta Directing:

• • In aniline the -NH₂ group is an electron-donating group, and reaction with HNO₃ and H₂SO₄ the reaction produces a mixture because of the dual effects of the activating nature of the −NH₂​ group and its protonation in the acidic medium, leading to these resulting products-
  • Para-product (51%): The para position is more sterically favorable than the ortho position, leading to the para product as the major product.
  • Meta-product (47%): The protonation of −NH_2​ in the acidic medium slightly favors meta substitution.
  • Ortho-product (2%): The ortho position is less favourable due to steric hindrance between the −NO₂​ group and the −NH₂ group.

3.0Electrophilic Substitution and Friedel-Crafts Reactions

Aniline is highly reactive in electrophilic substitution reactions because of the amino group (-NH₂). This high reactivity makes it suitable for reactions like bromination, nitration, and sulfonation. However, it creates difficulties in Friedel-Crafts reactions due to potential side reactions and overreactivity. To manage this, the amino group can be temporarily protected through acetylation, allowing for controlled and selective synthesis of the desired products.

Why Friedel-Crafts Does Not Work:

• Aniline reacts with the Lewis acid catalyst (e.g., AlCl₃), forming a complex that deactivates the ring for electrophilic substitution.
• Leads to unwanted side reactions or no reaction at all. For example-

Reaction Mechanisms Involving Aniline

Coupling Reactions:

• Aniline reacts with diazonium salts to form azo compounds in the presence of mild alkaline solutions.
• Nucleophilic attack of aniline on the diazonium ion at para position.

Halogenation:

• In bromination, 2,4,6-tribromoaniline forms due to strong activation.
• Bromination occurs readily without a catalyst due to the high activation provided by the -NH₂ group.
• Controlled bromination requires deactivation via protection.

4.0Important Points About Preparation of Aniline Derivatives

• Acetylation: Protects the -NH₂ group and controls reactivity.
• Controlled Reagents: Use of dilute halogen solutions or nitration mixtures to minimize side reactions.

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