Chapter 9: Aromatic Hydrocarbons – Chemistry 2nd Year Notes

This chapter covers Aromatic Hydrocarbons from the 2nd Year (FSc Part-II) Chemistry syllabus of the Punjab Curriculum and Textbook Board (PTB/PCTB). Aromatic hydrocarbons are ring compounds based on benzene (C6H6). Although benzene is unsaturated, it is unusually stable and prefers substitution reactions to addition. This special stability, called aromaticity, comes from the delocalisation of its six pi electrons around the ring.

Here you will learn the structure of benzene, why it is so stable, its electrophilic substitution reactions, and its uses in making dyes, drugs, plastics and detergents.

Learning Objectives

• Describe the structure of benzene using Kekule structures and resonance.
• Explain aromaticity and the unusual stability of benzene.
• Explain why benzene undergoes substitution rather than addition.
• Describe the electrophilic substitution reactions of benzene.
• Describe the addition reactions benzene can undergo under special conditions.
• State the important uses of benzene and its derivatives.

Key Concepts

Structure of Benzene

Benzene is a flat, six-membered ring of carbon atoms, each bonded to one hydrogen. Kekule represented it with alternating single and double bonds, but all six carbon to carbon bonds are identical, with a length between that of a single and a double bond. This is explained by resonance: the true structure is a resonance hybrid in which the six pi electrons are delocalised evenly around the ring.

Aromaticity and Stability

The delocalisation of the pi electrons lowers the energy of benzene and makes it much more stable than an imaginary molecule with three fixed double bonds. This extra stability, called resonance energy, is why benzene is described as aromatic. Because of it, benzene resists reactions that would destroy the delocalised ring; it undergoes substitution, which keeps the ring, rather than addition, which would break it.

Electrophilic Substitution Reactions

Benzene characteristic reactions are electrophilic substitutions, in which an electrophile replaces a ring hydrogen while the aromatic system is preserved. In halogenation, chlorine or bromine with an iron(III) halide catalyst gives chlorobenzene or bromobenzene. In nitration, concentrated nitric acid with concentrated sulphuric acid gives nitrobenzene. In sulphonation, fuming sulphuric acid gives benzenesulphonic acid. In Friedel-Crafts alkylation and acylation, anhydrous aluminium chloride is used to attach an alkyl group or an acyl group.

Addition Reactions and Uses

Under forcing conditions benzene can add: with hydrogen under high pressure and a nickel catalyst it forms cyclohexane, and with chlorine in bright sunlight it forms benzene hexachloride. Benzene and its derivatives are used to make dyes, medicines, phenol for plastics, styrene for polystyrene, detergents and explosives. Benzene itself is toxic and must be handled with care.

Important Definitions

Formulas & Rules

Reaction	Reagents / Product
Halogenation	Cl2 with FeCl3 gives chlorobenzene
Nitration	conc. HNO3 + conc. H2SO4 gives nitrobenzene
Sulphonation	fuming H2SO4 gives benzenesulphonic acid
Friedel-Crafts	RCl or RCOCl with anhydrous AlCl3
Addition	H2/Ni gives cyclohexane; Cl2 in sunlight gives BHC

Diagrams & Illustrations

Structure of benzene: the two Kekule structures and the resonance hybrid drawn as a hexagon with a circle inside, representing the delocalised pi electrons.

Electrophilic substitution reactions: a map around the benzene ring showing halogenation, nitration, sulphonation, and Friedel-Crafts alkylation and acylation.

Solved Examples & Numericals

Example 1: Substitution versus addition

Why does benzene undergo substitution rather than addition? Its six pi electrons are delocalised, giving extra stability; addition would destroy this stable ring, whereas substitution replaces a hydrogen and keeps the aromatic system.

Example 2: Nitration

Give the reagents and product for the nitration of benzene. Benzene is treated with a mixture of concentrated nitric and sulphuric acids at about 55 degrees Celsius to give nitrobenzene and water.

Example 3: Friedel-Crafts catalyst

What catalyst is required for Friedel-Crafts alkylation and why must it be anhydrous? Anhydrous aluminium chloride is required to generate the electrophile; water destroys the catalyst, stopping the reaction.

Short Questions & Answers

Long Questions & Answers

Q1: Describe the structure of benzene and explain its stability.

Benzene, C6H6, is a flat regular hexagon of six carbon atoms, each carrying one hydrogen. Kekule first suggested a ring with alternating single and double bonds, but this cannot be the whole truth because all six carbon to carbon bonds are exactly the same length, between a single and a double bond. The modern explanation is resonance: the molecule is a resonance hybrid of the two Kekule structures, in which each carbon is sp2 hybridised and contributes one electron to a set of pi orbitals that merge into a delocalised pi cloud above and below the ring. This delocalisation spreads the electrons over all six carbons and lowers the energy; the difference from a fixed-bond structure is the resonance energy. Because of this large resonance energy, benzene is unusually stable and resists reactions that break up the ring.

Q2: Discuss the electrophilic substitution reactions of benzene.

Benzene reacts mainly by electrophilic substitution, in which an electron-loving electrophile replaces one hydrogen atom of the ring while the aromatic system is preserved. In halogenation, benzene reacts with chlorine or bromine with an iron(III) halide catalyst to give a halobenzene. In nitration, a mixture of concentrated nitric and sulphuric acids at about 55 degrees introduces a nitro group to give nitrobenzene. In sulphonation, hot fuming sulphuric acid gives benzenesulphonic acid. In the Friedel-Crafts reactions, anhydrous aluminium chloride is the catalyst: alkylation with an alkyl halide gives an alkylbenzene, and acylation with an acyl chloride gives an aromatic ketone. In every case an electrophile replaces a ring hydrogen and the aromatic ring remains intact.

Q3: Compare the reactivity of benzene with that of an alkene.

Both benzene and alkenes are unsaturated and contain pi electrons, yet they behave differently. In an alkene the two pi electrons are localised between two carbons; this exposed double bond is easily attacked, so alkenes readily undergo addition and decolourise bromine water and potassium permanganate. In benzene the six pi electrons are delocalised over the whole ring, giving a large resonance energy and great stability, so benzene does not decolourise bromine water under ordinary conditions and undergoes electrophilic substitution instead of addition, keeping the ring intact. Only under forcing conditions can benzene be made to add. Thus the delocalisation of electrons makes benzene far less reactive than an ordinary alkene.

MCQs with Answers

Quick Revision Summary

• Benzene C6H6: planar ring, sp2 carbons, delocalised pi electrons (resonance hybrid).
• Aromaticity gives extra resonance stability, so the ring resists breaking.
• Characteristic reaction is electrophilic substitution, which keeps the ring.
• Reactions: halogenation (FeCl3), nitration (HNO3/H2SO4), sulphonation, Friedel-Crafts (AlCl3).
• Addition only under force: H2/Ni gives cyclohexane; Cl2/sunlight gives BHC.
• Derivatives make dyes, drugs, plastics and explosives.

Exam Tips

• Always explain benzene behaviour with delocalised pi electrons and resonance stability.
• Learn the five substitution reactions with reagents and catalysts.
• Remember catalysts: FeCl3 for halogenation, AlCl3 for Friedel-Crafts (must be anhydrous).
• State clearly why substitution is preferred over addition.
• Nitration mixture is concentrated HNO3 plus concentrated H2SO4.