Aniline is an aromatic amine with the molecular formula C₆H₅NH₂. It consists of a benzene ring directly bonded to an amino group. As one of the simplest aromatic amines, it plays a vital role in organic chemistry and serves as an essential starting material for the synthesis of dyes, pharmaceuticals, rubber-processing chemicals, and polyurethane precursors. ^[1-4]

Structure ^[1,2]

The benzene ring provides aromatic stability. On the other hand, the amino group contributes a basic character and significantly enhances the molecule’s reactivity. The lone pair of electrons on the nitrogen atom interacts with the π-electron cloud of the benzene ring through resonance. This delocalization increases electron density, particularly at the ortho and para positions, thereby making aniline much more reactive toward electrophilic substitution than benzene. Structurally, aniline is classified as a primary amine because its nitrogen atom bonds to only one aromatic group.

Physical Properties ^[2]

Preparation ^[4]

1. Reduction of Nitrobenzene

The most common industrial method of preparing aniline is the reduction of nitrobenzene with hydrogen gas (H₂) in the presence of a metal catalyst (Ni, Cu, Pd, Pt).

C₆H₅NO₂ + 3 H₂ → C₆H₅NH₂ + 2 H₂O (in the presence of Pd)

2. From Phenol

Aniline can also be prepared industrially from phenol, a process known as catalytic ammonolysis. When phenol is heated with ammonia at 300 °C in the presence of a suitable catalyst, the -OH group is replaced by an -NH₂ group. Although this route is effective, it generally requires high temperatures and pressures, which restrict its application mainly to industrial processes.

C₆H₅OH + NH₃ → C₆H₅NH₂ + H₂O (in the presence of anhydrous ZnCl₂)

Chemical Reactions ^[4]

1. Basic Nature

Aniline behaves as a weak base because its –NH₂ group can accept a proton. However, it is less basic than aliphatic amines because part of the nitrogen lone pair delocalizes into the benzene ring through resonance. It reacts with acids, producing the anilinium ion: 

C₆H₅NH₂ + HCl → C₆H₅NH₃⁺Cl^–

2. Electrophilic Substitution Reactions

The –NH₂ group strongly activates the benzene ring and directs incoming electrophiles to the ortho and para positions. Consequently, aniline undergoes electrophilic substitution much more readily than benzene.

i. Bromination

Aniline reacts rapidly with bromine water to form 2,4,6-tribromoaniline as a white precipitate:

C₆H₅NH₂ + 3 Br₂ → C₆H₂Br₃NH₂ ↓ + 3 HBr

ii. Nitration

Under controlled conditions, nitration produces mainly ortho- and para-nitroaniline:

C₆H₅NH₂ + HNO₃ → o- and p-NO₂C₆H₄NH₂ + H₂O.

Direct nitration of aniline is not very practical because, in the strongly acidic medium, the -NH₂ group becomes protonated and loses its activating effect. As a result, the yield is low, and many unwanted products are formed. Therefore, in actual practice, aniline is first converted to acetanilide to protect the -NH₂ group. Then, nitration of acetanilide produces the para product, which is later hydrolyzed to p-nitroaniline.

3. N-Alkylation

Although not a ring substitution, aniline can undergo alkylation at nitrogen when treated with alkyl halides. Here, the nitrogen lone pair participates in nucleophilic substitution, forming secondary and tertiary amines.

Primary Alkylation:

C₆H₅NH₂ + CH₃-Br → C₆H₅-NH-CH₃ + HBr

Secondary Alkylation:

C₆H₅-NH-CH₃ + CH₃-Br → C₆H₅-N(CH₃)₂ + HBr

Because the alkylated amines formed are often more reactive than aniline, the reaction commonly produces a mixture of mono- and dialkylated products.

4. Acylation

Aniline reacts with acid chlorides to form acetanilide:

C₆H₅NH₂ + CH₃COCl → C₆H₅NHCOCH₃ + HCl

5. Diazotization

At 0-5 °C, aniline reacts with nitrous acid to produce benzene diazonium chloride, an essential intermediate used to synthesize a wide range of aromatic compounds:

C₆H₅NH₂ + NaNO₂ + 2 HCl → C₆H₅N₂⁺Cl^– + NaCl + 2 H₂O

Applications ^[5]

• Dye Industry: Key starting material for the manufacture of azo and other synthetic dyes.
• Pharmaceuticals: Used as an intermediate in the synthesis of several medicinal compounds.
• Rubber Processing: Aniline derivatives act as antioxidants and accelerators in rubber manufacturing.
• Polyurethanes and Polymers: Essential in the production of methylene diphenyl diisocyanate (MDI), a major component of polyurethanes.
• Laboratory Reagent: Widely employed in organic synthesis for preparing numerous aromatic derivatives.

Aniline is a simple aromatic amine with exceptional chemical importance. Its resonance-stabilized structure, enhanced reactivity toward electrophilic substitution, and ability to undergo key reactions such as acylation and diazotization make it a highly versatile synthetic intermediate.