BENZENE notes for students

BENZENE AND ITS DERIVATIVES

1. Introduction

Benzene is the simplest aromatic hydrocarbon with the molecular formula C₆H₆. It is a colourless, flammable liquid with a pleasant smell and is an important raw material in the chemical industry.

Although benzene contains three double bonds, it is much more stable than ordinary unsaturated compounds. This unusual stability is due to its aromatic nature.

2. Structure of Benzene

(a) Kekulé Structure

Friedrich Kekulé proposed that benzene consists of six carbon atoms arranged in a ring with alternating single and double bonds.

However, this structure could not explain:

• The equal length of all C–C bonds in benzene

• The exceptional stability of benzene

• Why benzene undergoes substitution instead of addition reactions

(b) Resonance Structure

Benzene actually exists as a resonance hybrid of two Kekulé structures. The π-electrons are delocalized around the ring.

This delocalization:

• Makes all C–C bonds equal in length

• Gives benzene extra stability (resonance energy)

Benzene is often represented as a hexagon with a circle inside to show delocalized electrons.

         

3. Aromaticity

For a compound to be aromatic, it must:

1. Be cyclic

2. Be planar

3. Have continuous overlap of p-orbitals

4. Obey Hückel’s rule: contain (4n + 2) π-electrons

Benzene has 6 π-electrons (n = 1), so it is aromatic.

4. Physical Properties of Benzene

i. Molecular formula: C₆H₆

ii. Molecular mass: 78 g mol⁻¹

iii. Colourless liquid

iv. Sweet smell

v.  Melting point: 5.5 °C

 vi. Boiling point: 80.1 °C

 vii. Insoluble in water

viii. Soluble in organic solvents

  x.   Highly flammable

5. Chemical Properties of Benzene

Benzene mainly undergoes electrophilic substitution reactions to preserve its aromatic ring.

(a) Nitration

Reaction with concentrated nitric acid in the presence of concentrated sulfuric acid:

C₆H₆ + HNO₃ → C₆H₅NO₂ + H₂O
(Product: Nitrobenzene)

(b) Halogenation

Reaction with chlorine or bromine in the presence of FeCl₃ or FeBr₃:

C₆H₆ + Cl₂ → C₆H₅Cl + HCl
(Product: Chlorobenzene)

(c) Sulfonation

Reaction with fuming sulfuric acid:

C₆H₆ + H₂SO₄ → C₆H₅SO₃H + H₂O
(Product: Benzenesulfonic acid)

(d) Friedel–Crafts Alkylation

Reaction with an alkyl halide in the presence of AlCl₃:

C₆H₆ + CH₃Cl → C₆H₅CH₃ + HCl
(Product: Toluene)

(e) Friedel–Crafts Acylation

Reaction with an acyl chloride:

C₆H₆ + CH₃COCl → C₆H₅COCH₃ + HCl
(Product: Acetophenone)

6. Benzene Derivatives

Benzene derivatives are compounds formed when one or more hydrogen atoms in benzene are replaced by other atoms or groups (functional groups).

(a) Toluene (Methylbenzene)

Formula: C₆H₅CH₃   

Toluene is a derivative of benzene for when one of the hydrogens is substituted by an alkyls group

Toluene (Methylbenzene, C₇H₈) – Physical and Chemical Properties

Toluene is an aromatic hydrocarbon obtained mainly from petroleum and coal tar. It consists of a benzene ring attached to a methyl group (–CH₃), which slightly increases its reactivity compared to benzene.

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✅ Physical Properties of Toluene

1. Appearance

• Colourless, clear liquid

2. Odour

• Sweet, pleasant, benzene-like smell

3. Molecular formula

• C₇H₈

4. Molar mass

• 92 g mol⁻¹

5. Boiling point

• 110–111°C

6. Melting point

• −95°C

7. Density It has a density0.87 g cm⁻³ (lighter than water; floats on water)

8. Solubility: It is Insoluble in water but Soluble in organic solvents (ether, benzene, alcohol, chloroform)

9. Volatility: It is Volatile and evaporates easily

10. Flammability: It  is ighly flammable; burns with a smoky (sooty) flame

Uses:

• Solvent

• Manufacture of TNT

• Paint thinner

(b) Phenol (Hydroxybenzene)

Formula: C₆H₅OH

Uses:

• Antiseptics

• Plastics

• Dyes

(c) Aniline (Aminobenzene)

Formula: C₆H₅NH₂

Uses:

• Dyes

• Drugs

• Rubber chemicals

(d) Nitrobenzene

Formula: C₆H₅NO₂

Uses:

• Manufacture of aniline

• Dyes

• Perfumes

(e) Chlorobenzene

Formula: C₆H₅Cl

Uses:

• Solvent

• Manufacture of pesticides

• Dyes

7. Orientation in Substituted Benzene

Substituents on a benzene ring affect the position of further substitutions.

(a) Ortho–Para Directors

Electron-donating groups direct substitution to the ortho (1,2) and para (1,4) positions.

Examples:

• –OH

• –NH₂

• –CH₃

(b) Meta Directors

Electron-withdrawing groups direct substitution to the meta (1,3) position.

Examples:

• –NO₂

• –COOH

• –SO₃H

8. Uses of Benzene

• Manufacture of plastics

• Synthetic fibres

• Detergents

• Dyes

• Pharmaceuticals

⚠ Safety Note:
Benzene is toxic and carcinogenic. Prolonged exposure can cause serious health problems.

9. Summary

• Benzene is an aromatic hydrocarbon with formula C₆H₆.

• Its structure is best described by resonance.

• It undergoes electrophilic substitution reactions.

• Benzene derivatives are formed by replacing hydrogen atoms with functional groups.

• Substituents influence the position of further substitutions.