Aniline - Structure, Properties, Preparation, Reactions, Uses

Anilines are organic aromatic compounds that are derived from benzene with an amino group (NH₂) joined to it. Because of its reactivity and structure, it plays a crucial role in the making of various industrial chemicals, which increases the use of aniline in pharmaceuticals, dyes, and other places in modern chemistry.

What are Anilines?

Anilines are also known as phenylamine, benzamine, or aminobenzene. It is an organic compound that consists of an amino group (-NH₂) joined to a phenyl group (-C₆H₅). Its general chemical formula is C₆H₅NH₂.

Aniline is an aromatic compound. It is the most basic aromatic amine. It has 6 carbon (C) atoms, 7 hydrogen (H) atoms, and 1 nitrogen (N) atom. It is basic in nature because of the electron-withdrawing nature of the benzene ring, which decreases the availability of a lone pair of electrons for protonation. It is a very important compound in the production of various chemicals, including dyes, pharmaceuticals, organic compounds, and rubber chemicals.

Nomenclature of Anilines

• IUPAC Name of aniline is Phenylamine

The nomenclature of anilines, follows standard IUPAC conventions:

• Parent Chain: Benzene Ring.
• Numbering: Numbering carbons starting from the amino group(-NH₂).
• Prefixes: Ortho (o-) or 2, meta (m-) or 3, or para (p-) or 4 for other substituents.
• Suffix: "-benzenamine" to indicate the compound is an benzenamine derivative.

Examples:

• Aniline: Simplest form.
• Ortho-toluidine: methyl group (-CH₃) at Ortho position .
• Meta-chloroaniline: chlorine (-Cl) at Meta position.
• Para-nitroaniline: nitro (-NO₂) at Para position.

Aniline	Common name	Preferred IUPAC name	Systematic IUPAC name
C6H5NH2	Aniline, phenylamine	Aniline	Benzenamine
CH3-C6H4-NH2 (o-isomer)	o-toluidine	2-Methylaniline	2-methylbenzenamine
Cl-C6H4-NH2 (m-isomer)	m-Chloroaniline	4-Chloroaniline	3-Chlorobenzenamine
O2N-C6H4-NH2 (p-isomer)	p-nitroaniline	4-Nitroaniline	4-Nitrobenzenamine

Anilines Functional Group

The functional group on anilines is amino group (-NH2), which is joined to a benzene ring. In aniline, nitrogen has lone pair electron which make aniline a aromatics compound and activate this compound to do electrophilic aromatic substitution.

Structure of Anilines

The chemical structure of aniline (C₆H₅NH₂) consists of a benzene ring (C₆H₅) in which one H atom is replaced with an amino group(-NH₂)

• 6 C atoms are arranged in a benzene ring.
• 1 N atom joined to benzene ring.
• 2 H atoms bonded to the N atom.
• A lone pair of electrons on N atom.
• Each C atom is bonded with one H atom.
• C−N bond length is 1.41 Å

👁 Aniline-Structure

Resonance of Aniline

The resonance structure of aniline is shown below:

👁 Resonance-Structure-of-Aniline

The above resonance can be understood by dividing the whole resonance process in two parts:

• In first part, the lone pair of electrons on the nitrogen atom forms a π bond with the adjacent carbon atom, resulting in a double bond between nitrogen and carbon
• In second part, the lone pair of electrons on the nitrogen atom forms a π bond with the adjacent carbon atom on the opposite side, resulting in a double bond between nitrogen and carbon.

Anilines Properties

The properties of aniline are tabulated below:

Properties	Aniline
Appearance	colorless to pale yellow
Odor	fishy, amino like smell
State	liquid
Solubility	Slightly soluble in water, More soluble in organic solvent
Density	1.0297 g/mL
Boiling Point	184°C
Melting Point	−6.30 °C
Acidity/Basicity	weak base
pKa value	4.63
pH value	9.63

The above properties of aniline can be classified into physical properties and chemical properties of benzene:

Physical Properties of Aniline

The physical properties of aniline is mentioned below:

• Appearance: Aniline is generally colorless to pale yellow.
• Odor: Aniline smells like fishy, also amine-like odors.
• State: Aniline it liquid at room temperature.
• Solubility: Slightly soluble in water, More soluble in organic solvent like alcohol, benzene, chloroform, and acetone.
• Density: 1.0297 g/mL which is denser than water.
• Boiling Point: Boils at 184°C
• Melting Point: Melting point −6.30 °C

Chemical Properties of Aniline

The chemical properties of aniline is discussed below:

Basicity of Aniline: Aniline is a weak base in nature. Its pKa value is around 4.6. It's basicity is lower than aliphatic amines due to the electron-withdrawing nature of benzene ring that decreases the availability of lone pair of electron on N atom for protonation.

Nucleophilic: The lone pair on the N atom of aniline make aniline to be attack by electrophiles.

Oxidation-sensitive: Undergoes oxidation in strong acidic conditions and form unwanted side products.

Reactivity of Aniline: The reactivity of aniline is discussed below:

• Aromatic electrophilic substitution: The benzene ring easily undergoes electrophilic substitution reactions because of the activating effect of the amino group. This allows various functional groups like halogens, nitro groups, and alkyl groups to attach to benzene ring at different positions.
• Diazotization: Aniline do diazotization reactions when it reacts with nitrous acid(HNO2) to form a diazonium salt. This reaction is widely used in making dyes and other aromatic compound.

Preparation of Aniline

Aniline is a versatile aromatic organic compound which can be synthesis by various methods. Some of the methods of preparation of aniline are mentioned below:

Preparation of Aniline by Reduction of Nitrobenzene

Process: In this method nitrobenzene (C₆H₅NO₂) is converted into aniline(C₆H₅NH₂) with the help of reducing agent by removing nitro group (-NO₂) and replacing it with amino group (NH₂).

Common reagents: Iron filings in acidic medium, catalytic hydrogenation with metal catalysts like platinum or palladium.

Reaction:

C₆H₅NO₂ + 6HCl + 3Fe → C₆H₅NH₂ + 3FeCl₂ + 2H₂O

Advantages: Simple and economical, widely used in industry.

Disadvantages: Requires strong reducing agents and it can also generate byproducts.

Preparation of Aniline by Ammonolysis of Haloarenes

Process: In this method halogen atom (Cl, Br, I) form haloarene (C₆H₅X) are replaced with an amino group (-NH₂) using ammonia (NH₃).

Common reagents: Ammonia in the presence of copper catalysts.

Reaction:

C₆H₅Br + NH₃ → C₆H₅NH₂ + HBr

Advantages: Can be used for various starting materials, suitable for producing specific substituted anilines.

Disadvantages: Requires high temperatures and pressures, it can also generate undesired byproducts.

Anilines Reaction

We know that aniline is a fundamental organic aromatic compound. It can undergo various reactions to form several other compounds. Some of the common reactions of aniline is mentioned below:

Coupling Reactions: Aniline reacts with a diazonium salt in a diazo coupling reaction, and form azo dye (like aminoazobenzene) with water and hydrogen chloride as byproducts.

C₆H₅N₂⁺Cl^- + C₆H₅NH₂ → C₆H₅N=NC₆H₄NH₂ + H2O + HCl

Electrophilic Aromatic Substitution: Aniline undergoes electrophilic aromatic substitution with bromine in the presence of ferric bromide catalyst to form bromoaniline (3-bromobenzenamine ) derivatives.

2C₆H₅NH₂ + Br₂ + 2FeBr₃ → 2C₆H₄(NH₂)Br + 2HBr + 2FeBr₂

Diazotization: Aniline reacts with sodium nitrite and hydrochloric acid to form anilinium chloride salt, water and sodium chloride.

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

Acylation: Aniline reacts with acetyl chloride to form N-phenylacetamide and hydrochloric acid.

C₆H₅NH₂ + CH₃COCl → C₆H₅NHCOCH₃ + HCl

👁 Acylation-of-Aniline

Reduction: Aniline reacts with hydrogen gas in the presence of a catalyst to form N-methylaniline and water produced as a byproduct.

C₆H₅NH₂ + 3H₂ → C₆H₅NHCH₃ + 2 H₂O

Nitration: Aniline reacts with nitric acid in the presence of sulfuric acid as a catalyst to substitute a H atom on the benzene ring with a nitro group, forming nitroaniline with water and sulfur dioxide as byproducts.

C₆H₅NH₂ + HNO₃ + 2H₂SO4 → C₆H₄(NH₂)(NO₂) + 2SO₂ + 2H₂O

Halogenation: Aniline reacts with bromine to substitute a H atom with a Br atom on the benzene ring to form bromoaniline and hydrobromic acid.

C₆H₅NH₂+ Br₂ → C₆H₄BrNH₂ + HBr

Sulfonation: Aniline reacts with sulfur trioxide (SO3) in the presence of sulfuric acid (H2SO4) to form p-aminobenzenesulfonic acid (C6H4SO3NH2) and water (H2O).

C₆H₅NH₂ + SO₃/H2SO4 → C₆H₄SO₃NH₂ + H₂O

👁 Sulphonation-of-Aniline

Aniline Uses

Aniline is a basic aromatic organic compound. It has got several used in industrial applications. Some of the uses of aniline is mentioned below:

• Pharmaceutical Industry: Used for the synthesis of various drugs and pharmaceutical intermediates.
• Dye Industry: Aniline is vital components in the production of dyes and pigments.
• Polymer Industry: Aniline is used as monomers in the production of polymers like polyurethanes and epoxy resins.
• Agrochemicals: Aniline help in the manufacture of pesticides and herbicides.
• Rubber Processing: Aniline are important in vulcanization of rubber.

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