C₆H₅NH₂ — Aniline

Aniline (C₆H₅NH₂) is the simplest aromatic amine, consisting of a benzene ring attached to an amino group (–NH₂). It was first discovered in the 19th century during the distillation of indigo dye and later became a cornerstone of the synthetic dye industry. Aniline is a colorless, oily liquid that darkens upon exposure to air and light due to oxidation.

It is an important industrial compound used in the manufacture of dyes, pharmaceuticals, and rubber-processing chemicals. Despite its widespread use, aniline is toxic and must be handled with care.

Aniline consists of a benzene ring bonded to an amino group (–NH₂). The nitrogen atom in the amino group is sp³ hybridized and contains a lone pair of electrons that interact with the π-electrons of the aromatic ring. This causes partial delocalization of electrons, stabilizing the molecule and affecting its reactivity.

This resonance interaction makes aniline less basic than aliphatic amines because the lone pair on nitrogen is partially delocalized into the benzene ring, reducing its availability for protonation.

Aniline can be prepared through several laboratory and industrial methods:

1. Reduction of Nitrobenzene

This is the most common industrial method. Nitrobenzene is reduced using iron filings and hydrochloric acid or catalytic hydrogenation to form aniline.

2. Reduction with Tin and Hydrochloric Acid

Alternatively, nitrobenzene can be reduced using tin (Sn) and hydrochloric acid:

3. Ammonolysis of Chlorobenzene

Chlorobenzene reacts with ammonia under high temperature and pressure in the presence of a copper catalyst to produce aniline.

4. From Phenol via Ammonia

Phenol reacts with ammonia at high temperature and pressure in the presence of an iron catalyst to yield aniline.

Physical Properties:

• Oily, colorless liquid that turns brown upon oxidation.
• Boiling Point: 184°C; Melting Point: -6°C.
• Soluble in organic solvents but only sparingly soluble in water.
• Has a characteristic unpleasant odor typical of amines.

Chemical Properties:

• 1. Basicity: Aniline is a weak base due to delocalization of the nitrogen lone pair.

\(C_6H_5NH_2 + H^+ \leftrightarrow C_6H_5NH_3^+\)

• 2. Acetylation: Aniline reacts with acetic anhydride to form acetanilide, reducing the activity of the amino group.

\(C_6H_5NH_2 + (CH_3CO)_2O \rightarrow C_6H_5NHCOCH_3 + CH_3COOH\)

• 3. Electrophilic Substitution: The –NH₂ group is an activating, ortho-para directing group.
• Bromination: Aniline reacts with bromine water to form 2,4,6-tribromoaniline.

\(C_6H_5NH_2 + 3Br_2 \rightarrow C_6H_2Br_3NH_2 + 3HBr\)

• Nitration: Direct nitration gives a mixture of products; hence, acetylated aniline (acetanilide) is nitrated instead to form p-nitroacetanilide, which upon hydrolysis gives p-nitroaniline.

\(C_6H_5NHCOCH_3 + HNO_3 \xrightarrow{H_2SO_4} C_6H_4(NO_2)NHCOCH_3\)

\(C_6H_4(NO_2)NHCOCH_3 + H_2O \rightarrow C_6H_4(NO_2)NH_2 + CH_3COOH\)

• 4. Oxidation: Aniline oxidizes in air to form aniline black or quinone-like products.
• 5. Reaction with Carbon Disulfide: Forms thiocarbamide, an intermediate for dyes.

• 1. Dye Industry: Aniline is the starting material for azo dyes and indigo dye. Its derivatives are used in the manufacture of aniline yellow, methyl orange, and other dyes.
• 2. Pharmaceuticals: Used in the synthesis of drugs like paracetamol (acetaminophen).
• 3. Rubber Industry: Aniline derivatives act as antioxidants and vulcanization accelerators in rubber processing.
• 4. Polyurethane Production: Aniline is used to produce methylene diphenyl diisocyanate (MDI), a key precursor for polyurethane foams.
• 5. Agricultural Chemicals: Used in herbicide and pesticide synthesis.
• 6. Research and Laboratory Use: Serves as a reagent for organic synthesis and as a standard aromatic amine compound in laboratories.

Aniline is toxic and can be absorbed through the skin, lungs, or gastrointestinal tract. It can cause methemoglobinemia, a condition that reduces oxygen-carrying capacity in the blood. Chronic exposure may damage the liver, kidneys, and central nervous system.

Safety precautions:

• Use protective gloves, goggles, and fume hoods when handling aniline.
• Store in airtight containers away from oxidizing agents and heat sources.
• Dispose of aniline waste following environmental and safety regulations.