Chapter Overview

This chapter explores the fascinating world of aromatic hydrocarbons, focusing on benzene and its derivatives. You’ll learn about the unique structure, stability, and reactivity of benzene, which sets it apart from other hydrocarbons.

Learning Objective 1

Understand the nomenclature of benzene derivatives including mono-, di-, and polysubstituted benzenes

Learning Objective 2

Explain the unique structure of benzene including resonance and hybridization

Learning Objective 3

Describe the reactivity of benzene and its preference for substitution over addition reactions

Learning Objective 4

Understand electrophilic aromatic substitution reactions including halogenation, nitration, and Friedel-Crafts reactions

Learning Objective 5

Explain directing effects of substituents in electrophilic aromatic substitution

Historical Context

Michael Faraday first isolated benzene in 1825 from the oily residue collected in London’s gas pipes. The name “benzene” comes from gum benzoin, a resin from the balsam tree of Java. Eilhardt Mitscherlich synthesized it in 1834 by heating benzoic acid with lime, establishing its empirical formula as CH.

Health Warning

Benzene is a known carcinogen that can cause leukemia. People exposed to car exhaust or working in factories with benzene need protection. There are strict regulations worldwide regarding permissible benzene concentration levels in workplaces.

Aromatic Compounds Classification

Nomenclature of Benzene

Benzene derivatives are named based on the number and position of substituents on the benzene ring. The IUPAC system retains some common names for simplicity.

Monosubstituted Benzenes

These have one substituent attached to the benzene ring. Some common examples retain their trivial names:

Toluene
C₆H₅-CH₃

Toluene
Methylbenzene

Phenol
C₆H₅-OH

Phenol
Hydroxybenzene

Aniline
C₆H₅-NH₂

Aniline
Aminobenzene

Disubstituted Benzenes

These have two substituents and can show three positional isomers. Positions are indicated using numbers or the prefixes ortho (1,2), meta (1,3), and para (1,4).

Position Prefix Example IUPAC Name
1,2 ortho- (o-) 1,2-dimethylbenzene o-Xylene
1,3 meta- (m-) 1,3-dimethylbenzene m-Xylene
1,4 para- (p-) 1,4-dimethylbenzene p-Xylene

Polysubstituted Benzenes

When three or more substituents are present, we specify their locations by numbers. If one substituent gives a special name (like in toluene, phenol, or aniline), the molecule is named as a derivative of that parent.

Naming Rule

When neither group gives a special name, substituents are listed in alphabetical order before the word ‘benzene’. Number the ring to give the smallest set of numbers.

Structure of Benzene

The structure of benzene puzzled chemists for decades due to its unusual stability despite apparent unsaturation.

Kekulé’s Structure

In 1865, August Kekulé proposed the cyclic structure of benzene after dreaming of a snake biting its tail. He suggested a hexagonal ring with alternating single and double bonds.

Kekulé’s Structure

Kekulé proposed a hexagonal ring with alternating single and double bonds

Modern Understanding

The actual structure of benzene is a resonance hybrid with delocalized π electrons:

Resonance Hybrid Structure

The circle represents delocalized π electrons in the resonance hybrid

Hybridization in Benzene

Each carbon atom in benzene is sp² hybridized, forming:

Sigma (σ) Bonds
Formed by sp²-sp² overlap between carbon atoms

Pi (π) Bonds
Formed by p-orbital overlap, creating delocalized electron cloud

Key Features of Benzene Structure
  • All carbon atoms are sp² hybridized
  • Bond angles are 120° (perfect hexagon)
  • Carbon-carbon bond length is 1.39 Å (between single and double bonds)
  • Delocalized π electron cloud above and below the ring
  • Extremely stable due to resonance energy (152 kJ/mol)
Experimental Evidence

X-ray diffraction shows all C-C bonds in benzene are equal in length (1.39 Å), intermediate between typical single (1.54 Å) and double (1.34 Å) bonds. This supports the concept of delocalization.

Resonance in Benzene

Resonance is a key concept that explains the exceptional stability and unique properties of benzene.

What is Resonance?

Resonance occurs when a molecule cannot be adequately represented by a single Lewis structure. Instead, the true structure is a hybrid of two or more contributing structures.

Resonance between two Kekulé structures

True Structure: Resonance Hybrid

Resonance Energy

The resonance energy of benzene is 152 kJ/mol, which is the difference between the expected and actual energy of hydrogenation.

Key Points About Resonance in Benzene
  • Benzene is a resonance hybrid of two equivalent Kekulé structures
  • The true structure has delocalized π electrons represented by a circle
  • All carbon-carbon bonds are identical (1.39 Å)
  • Resonance provides extra stability (152 kJ/mol)
  • The molecule is perfectly hexagonal with bond angles of 120°
Important Note

Resonance structures are not in equilibrium with each other. The benzene molecule does not oscillate between the two Kekulé forms. Instead, it exists as a single hybrid structure with delocalized electrons.

Reactivity of Benzene

Despite its apparent unsaturation, benzene does not behave like typical alkenes. It prefers substitution over addition reactions to preserve its aromatic stability.

Electrophilic Aromatic Substitution

Benzene undergoes substitution reactions where an electrophile replaces a hydrogen atom. The aromatic ring is temporarily disrupted but quickly restored.

General Mechanism

1 Generation of Electrophile: A Lewis acid catalyst helps generate the electrophile.
2 Formation of Arenium Ion: The electrophile attacks the π electron cloud, forming a carbocation intermediate.
3 Deprotonation: A base removes a proton, restoring the aromatic ring.

Addition Reactions

Benzene can undergo addition reactions under vigorous conditions, but these are less favorable as they destroy the aromatic stability.

C₆H₆ + 3H₂ → C₆H₁₂ (Cyclohexane)

Conditions: Ni/Pt/Pd catalyst, 150-200°C, 20-30 atm pressure

Oxidation Reactions

Alkyl side chains on benzene can be oxidized to carboxylic acids, while the ring itself is resistant to mild oxidizing agents.

C₆H₅-CH₃ + 2[O] → C₆H₅-COOH + H₂O

Reagents: Hot KMnO₄ (alkaline or acidic)

Electrophilic Aromatic Substitution

These are the most characteristic reactions of benzene, where an electrophile replaces a hydrogen atom while preserving the aromatic system.

Halogenation

Benzene reacts with halogens in the presence of Lewis acid catalysts to form halobenzenes.

C₆H₆ + Br₂ → C₆H₅Br + HBr

Catalyst: FeBr₃ or AlCl₃

Mechanism of Bromination

1 Generation of Electrophile: FeBr₃ + Br₂ → FeBr₄⁻ + Br⁺
2 Arenium Ion Formation: Br⁺ attacks benzene ring
3 Deprotonation: FeBr₄⁻ removes H⁺, regenerating catalyst

Nitration

Benzene reacts with a mixture of concentrated nitric and sulfuric acids to form nitrobenzene.

C₆H₆ + HNO₃ → C₆H₅NO₂ + H₂O

Reagents: Conc. HNO₃ + Conc. H₂SO₄ (nitrating mixture)

Mechanism of Nitration

1 Generation of Electrophile: HNO₃ + H₂SO₄ → NO₂⁺ + H₃O⁺ + HSO₄⁻
2 Arenium Ion Formation: NO₂⁺ attacks benzene ring
3 Deprotonation: HSO₄⁻ removes H⁺, regenerating H₂SO₄

Friedel-Crafts Reactions

These reactions introduce alkyl or acyl groups onto the benzene ring.

Reaction Type Reagents Product Catalyst
Alkylation R-Cl Alkylbenzene AlCl₃
Acylation R-CO-Cl Phenyl ketone AlCl₃

Directing Effects in Substituted Benzenes

Existing substituents on benzene rings influence the position where new substituents will attach during electrophilic substitution.

Ortho/Para Directing Groups

These groups activate the ring and direct incoming electrophiles to ortho and para positions.

Characteristics
  • Generally electron-donating groups
  • Activate the benzene ring toward electrophilic substitution
  • Direct to positions 2 and 4 relative to themselves
  • Examples: -OH, -NH₂, -OCH₃, -CH₃, -X (halogens)
Special Case: Halogens

Halogens are ortho/para directors but deactivate the ring. This is because their strong electron-withdrawing inductive effect dominates their weak electron-donating resonance effect for activation, but the resonance effect controls orientation.

Meta Directing Groups

These groups deactivate the ring and direct incoming electrophiles to meta positions.

Characteristics
  • Generally electron-withdrawing groups
  • Deactivate the benzene ring toward electrophilic substitution
  • Direct to position 3 relative to themselves
  • Examples: -NO₂, -COOH, -CHO, -CN, -SO₃H
Group Type Effect on Reactivity Directing Effect Examples
Ortho/Para Directors Activating (except halogens) 2- and 4- positions -OH, -NH₂, -CH₃, -OCH₃, -X
Meta Directors Deactivating 3- position -NO₂, -COOH, -CHO, -CN

Key Points

  • Benzene (C₆H₆) is the simplest aromatic hydrocarbon with a hexagonal planar structure
  • All carbon atoms in benzene are sp² hybridized with bond angles of 120°
  • Benzene has delocalized π electrons represented as a circle in the ring
  • The resonance energy of benzene is 152 kJ/mol, explaining its exceptional stability
  • Benzene undergoes electrophilic substitution rather than addition to preserve aromaticity
  • Common electrophilic substitution reactions include halogenation, nitration, and Friedel-Crafts reactions
  • Substituents on benzene direct incoming groups to ortho/para or meta positions
  • Ortho/para directors are generally activating (except halogens)
  • Meta directors are deactivating
  • Benzene was first isolated by Michael Faraday and its structure was proposed by Kekulé
  • Alkyl side chains on benzene can be oxidized to carboxylic acids while the ring remains intact
  • Benzene requires harsh conditions for addition reactions like hydrogenation

Multiple Choice Questions

1. The IUPAC name of the following compound is:

A 1,3-dichlorophenol
B 2,6-dichlorophenol
C 2-hydroxy-1,3-dichlorobenzene
D 1,5-dichlorophenol

Answer: C) 2-hydroxy-1,3-dichlorobenzene

Explanation: When naming polysubstituted benzenes, if one substituent gives a special name (like -OH for phenol), the molecule is named as a derivative of that parent with the special substituent at position 1. The other substituents are numbered to give the smallest set of numbers.

2. The type of intermolecular forces in benzene are:

A Dipole-dipole forces
B London dispersion forces
C Hydrogen bonding
D Ion-dipole forces

Answer: B) London dispersion forces

Explanation: Benzene is a symmetrical, nonpolar molecule. The primary intermolecular forces in nonpolar molecules are London dispersion forces, which result from temporary dipoles created by electron movement.

Quiz Results

0/2
Score
0%
Percentage
0
Correct
0
Incorrect

Short Answer Questions

1. Cyclohexene decolourizes bromine water whereas benzene cannot. Explain why.

Answer: Cyclohexene has localized π bonds that can easily undergo electrophilic addition with bromine, resulting in decolorization of bromine water. Benzene, however, has delocalized π electrons in an aromatic system that is exceptionally stable. The resonance energy makes benzene reluctant to undergo addition reactions that would destroy its aromaticity. Instead, benzene requires a catalyst (like FeBr₃) for electrophilic substitution with bromine, which preserves the aromatic ring.

2. In Friedel-Craft acylation of benzene, aluminium chloride (AlCl₃) acts as a Lewis acid. Justify this statement.

Answer: In Friedel-Crafts acylation, AlCl₃ acts as a Lewis acid by accepting a pair of electrons from the acyl chloride (R-CO-Cl). This interaction polarizes the C-Cl bond and facilitates the formation of the acylium ion (R-C≡O⁺), which is the electrophile that attacks the benzene ring. The AlCl₃ coordinates with the chlorine atom, making the carbonyl carbon more electrophilic and promoting the reaction.

Concept Assessment Exercises

Concept Assessment Exercise 8.1

Draw the structure of benzene and explain why it is considered aromatic.

A Benzene has a planar hexagonal structure with delocalized π electrons
B Benzene has alternating single and double bonds
C Benzene has a linear structure with triple bonds
D Benzene has a tetrahedral carbon framework

Explanation: Benzene has a planar hexagonal structure with all carbon atoms sp² hybridized. The six p-orbitals overlap to form a continuous π electron cloud above and below the ring. This delocalization gives benzene exceptional stability (resonance energy of 152 kJ/mol) and makes it aromatic according to Hückel’s rule (4n+2 π electrons, where n=1).

Concept Assessment Exercise 8.2

Draw the two Kekulé structures of benzene. How did Kekulé switch to closed structure of benzene molecule, although other chemists tried open structure for it?

A Kekulé envisioned a cyclic structure after dreaming of a snake biting its tail
B Kekulé conducted X-ray diffraction studies
C Kekulé followed the work of other chemists who proposed cyclic structures
D Kekulé used computer modeling to determine the structure

Explanation: Kekulé reported that he dreamed of atoms dancing and forming chains that turned into rings, like a snake biting its tail. This vision inspired him to propose the cyclic structure of benzene, which was revolutionary at the time when other chemists were trying to fit the molecular formula C₆H₆ into open-chain structures.

Define resonance in benzene. Why is benzene exceptionally stable compared to alkenes?

A Resonance is the delocalization of π electrons over several atoms, giving extra stability
B Resonance means benzene has alternating single and double bonds
C Resonance refers to the vibration of atoms in the benzene molecule
D Resonance indicates that benzene has three distinct double bonds

Explanation: Resonance in benzene refers to the delocalization of π electrons over all six carbon atoms in the ring, rather than being localized between specific pairs of atoms. This delocalization gives benzene extra stability (152 kJ/mol resonance energy) compared to hypothetical cyclohexatriene with localized double bonds. The continuous electron cloud above and below the ring makes benzene less reactive than typical alkenes.

Resonance Energy Calculation:
Expected hydrogenation energy for cyclohexatriene: 3 × -120 kJ/mol = -360 kJ/mol
Actual hydrogenation energy for benzene: -208 kJ/mol
Resonance energy = 360 – 208 = 152 kJ/mol

Concept Assessment Exercise 8.3

How does chloroethane react with benzene molecule? Name the electrophile and end product of this reaction.

Answer: Chloroethane reacts with benzene in a Friedel-Crafts alkylation reaction. The electrophile is the ethyl carbocation (CH₃CH₂⁺) generated with the help of AlCl₃ catalyst. The end product is ethylbenzene (C₆H₅CH₂CH₃).

C₆H₆ + CH₃CH₂Cl → C₆H₅CH₂CH₃ + HCl

Catalyst: AlCl₃

Mechanism

1 Generation of Electrophile: CH₃CH₂Cl + AlCl₃ → CH₃CH₂⁺ + AlCl₄⁻
2 Arenium Ion Formation: CH₃CH₂⁺ attacks benzene ring
3 Deprotonation: AlCl₄⁻ removes H⁺, regenerating AlCl₃ and forming ethylbenzene

Which two roles are being played by aluminium chloride (AlCl₃) in Friedel-Craft alkylation?

Answer: AlCl₃ plays two roles in Friedel-Crafts alkylation: (1) It acts as a Lewis acid to generate the carbocation electrophile from the alkyl halide, and (2) It helps regenerate the aromatic ring by accepting the proton from the arenium ion intermediate.

Detailed Explanation

  • Role 1: Electrophile Generation – AlCl₃ coordinates with the halogen atom of the alkyl halide, polarizing the C-X bond and facilitating the formation of the carbocation electrophile.
  • Role 2: Deprotonation – The AlCl₄⁻ anion formed acts as a base to remove a proton from the arenium ion intermediate, restoring aromaticity and regenerating the AlCl₃ catalyst.