Aromatic Hydrocarbons: Benzene, Nomenclature, Reactions & Uses

TAKEAWAYS:

• Aromatic hydrocarbons contain a benzene ring (C₆H₆) with delocalized π-electrons, making them stable and reactive via electrophilic substitution.
• Nomenclature uses prefixes (ortho-, meta-, para-) or numbers to name substituents on the benzene ring.
• Electrophilic substitution (e.g., nitration, halogenation) preserves the benzene ring, while addition reactions break aromaticity.
• Applications include synthetic dyes, detergents, and polymers like polystyrene.
• Comparisons: Aromatic vs aliphatic hydrocarbons in structure, reactivity, and uses.

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### **1. What Are Aromatic Hydrocarbons?**
Aromatic hydrocarbons are **organic compounds** containing a **benzene ring** (C₆H₆) or similar ring structures. The term "aromatic" comes from the pleasant smells of many early-discovered compounds like benzene (from coal tar).

```figure
{"type":"circle","center":"C","radius":"r","chord":true,"angleAtCentre":"120°","angleAtCircumference":"60°","tangent":true,"caption":"Benzene’s 120° bond angles and equal C–C bond lengths (1.39 Å)"}

Key Features of Benzene (C₆H₆)

• Structure: A flat, hexagonal ring with 6 carbon atoms and 6 hydrogen atoms.
• Bonding: Each carbon is bonded to two others via single bonds (σ-bonds) and one delocalized π-bond (shared by all 6 carbons).
• Stability: Benzene is extra stable due to delocalized electrons (resonance). It does not easily undergo addition reactions like alkenes.

Why Is Benzene Special?

• Resonance: Benzene can be represented by two equivalent structures (Kekulé structures), but the real structure is a hybrid (resonance hybrid).
• Delocalization: The π-electrons are not fixed between two carbons but spread over the entire ring, making it more stable than expected.

2. Nomenclature of Aromatic Hydrocarbons

Naming substituted benzenes follows IUPAC rules and common prefixes.

Types of Substituents

Naming Rules

1. Single substituent: Use the substituent name + "benzene".
   • Example: Toluene (C₆H₅-CH₃, methylbenzene).
2. Multiple substituents:
   • Use numbers (1, 2, 3...) or ortho/meta/para.
   • Alphabetical order for substituents.
   • Example: 1,3-Dimethylbenzene (m-Xylene) or 2,4,6-Trinitrotoluene (TNT).

Common Aromatic Compounds

3. Reactions of Aromatic Hydrocarbons

Aromatic compounds prefer substitution over addition to maintain the stable benzene ring.

A. Electrophilic Substitution Reactions

These reactions involve an electrophile (electron-loving species) attacking the benzene ring.

1. Nitration (Formation of Nitrobenzene)

• Reagents: Concentrated HNO₃ + H₂SO₄ (forms NO₂⁺ electrophile).
• Mechanism:
  1. Electrophile formation: H₂SO₄ protonates HNO₃ → NO₂⁺.
  2. Attack: NO₂⁺ attacks benzene, forming a sigma complex (carbocation).
  3. Restoration: Electrons delocalize, and H⁺ is lost, restoring aromaticity.

flowchart LR
    A["C₆H₆ + HNO₃/H₂SO₄"] --> B["NO₂⁺ formation"]
    B --> C["Electrophilic attack"]
    C --> D["Sigma complex"]
    D --> E["Loss of H⁺ → C₆H₅NO₂ + H₂O"]
    E --> F["Nitrobenzene"]

2. Halogenation (Formation of Chlorobenzene)

• Reagents: Cl₂ + FeCl₃ (forms Cl⁺ electrophile).
• Product: Chlorobenzene (used in dyes and pesticides).

3. Friedel-Crafts Alkylation/Acylation

• Alkylation: Adds an alkyl group (e.g., CH₃⁺ from CH₃Cl + AlCl₃).
• Acylation: Adds an acyl group (e.g., CH₃CO⁺ from CH₃COCl + AlCl₃).

B. Addition Reactions (Rare!)

Aromatic compounds do not easily undergo addition because it breaks the stable ring. However, under extreme conditions (high heat/pressure), addition can occur:

• Hydrogenation: Benzene + 3H₂ → Cyclohexane (requires Ni catalyst, high temperature).

4. Applications of Aromatic Hydrocarbons

5. Aromatic vs Aliphatic Hydrocarbons

6. Solved Examples

Example 1: Naming Substituted Benzenes

Question: Name the following compound:

   OH
    |
C₆H₄---CH₃

Solution:

• Two substituents: OH (hydroxyl) and CH₃ (methyl).
• Positions: 1 and 4 (para-).
• Name: 4-Methylphenol or p-Cresol.

Example 2: Reaction Mechanism

Question: Write the mechanism for the bromination of benzene using Br₂ + FeBr₃. Solution:

1. FeBr₃ polarizes Br₂ → Br⁺ (electrophile).
2. Br⁺ attacks benzene → sigma complex.
3. Electrons delocalize → bromobenzene + HBr.

7. NEB Board-Style Questions

Short Answer (5 marks)

Q1: Explain why benzene undergoes electrophilic substitution rather than addition reactions. Answer:

• Benzene has delocalized π-electrons, making it stable.
• Addition reactions break aromaticity, requiring high energy.
• Substitution preserves the ring, so it is favored.

Long Answer (10 marks)

Q2: Describe the nitration of benzene with a mechanism. What are the uses of nitrobenzene? Answer:

1. Reagents: HNO₃ + H₂SO₄.
2. Mechanism:
   • H₂SO₄ protonates HNO₃ → NO₂⁺.
   • NO₂⁺ attacks benzene → sigma complex.
   • Loss of H⁺ → nitrobenzene.
3. Uses:
   • Dye industry (aniline production).
   • Explosives (TNT).

Multiple Choice (1 mark)

Q3: Which of the following is not an aromatic compound? a) Benzene b) Toluene c) Cyclohexane d) Naphthalene Answer: c) Cyclohexane (saturated, no delocalized π-electrons).

Exam Tip

1. Memorize benzene’s structure (hexagon with a circle inside for delocalized electrons).
2. Practice naming substituted benzenes (use ortho/meta/para or numbers).
3. Understand electrophilic substitution (NO₂⁺, Br⁺, CH₃⁺ as common electrophiles).
4. Compare aromatic vs aliphatic reactivity in exam questions.
5. Draw mechanisms for nitration/halogenation—examiners love clear arrow-pushing!

Example of an aromatic polymer used in daily life. (Image: Walmart Corporate, CC BY 2.0, via Openverse)