Unit 1: Benzene and its Derivatives

February 23, 2026

Semester 3
BP301T

Introduction to Benzene and its Derivatives

The most fundamental concept in aromatic chemistry – why benzene is uniquely stable and how it reacts. Understanding Electrophilic Aromatic Substitution (EAS) and directing effects is the gateway to Medicinal Chemistry, since most drugs contain aromatic rings.
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Syllabus & Topics

  • 1Kekulé’s structure of benzene and its drawbacks.
  • 2Analytical, synthetic, and spectroscopic evidences for benzene structure.
  • 3Orbital picture of benzene: sp2 hybridization, delocalized π cloud.
  • 4Resonance in benzene: The two Kekulé structures as resonance hybrids.
  • 5Aromatic character: Criteria (cyclic, planar, fully conjugated, Huckel’s rule).
  • 6Huckel’s Rule: (4n+2) π electrons. n=0 (cyclopropyl cation), n=1 (benzene, 6e), n=2 (10e).
  • 7Electrophilic Aromatic Substitution (EAS) – General mechanism: electrophile attacks π cloud → σ-complex (Wheland intermediate/arenium ion) → loss of H⁺ → product.
  • 8Nitration of Benzene: Reagent (Conc. H2SO4 + Conc. HNO3), nitronium ion (NO₂⁺) as electrophile.
  • 9Sulphonation: Reagent (Fuming H2SO4/SO3), electrophile SO3.
  • 10Halogenation: Reagent (X2 + Lewis acid catalyst, e.g., FeCl3), electrophile Cl⁺ or Br⁺.
  • 11Friedel-Crafts Alkylation: Reagent (RX + AlCl3), carbocation as electrophile. Limitations: polyalkylation, carbocation rearrangement, does not work with deactivated rings.
  • 12Friedel-Crafts Acylation: Reagent (RCOCl + AlCl3), acylium ion as electrophile. Advantages over alkylation.
  • 13Substituent effects on reactivity and orientation of EAS: Activating groups (o/p directors), Deactivating groups (m directors, except halogens).
  • 14Structure and uses of DDT (insecticide), Saccharin (sweetener), BHC (Benzene Hexachloride – insecticide), Chloramine (antiseptic).

Learning Objectives

Draw and explain the resonance structures of benzene.
State Huckel’s rule and apply it to determine if a cyclic compound is aromatic.
Write the step-by-step EAS mechanism for nitration of benzene.
List 3 limitations of Friedel-Crafts alkylation and explain why acylation avoids them.
Classify a given substituent as an activating (o/p) or deactivating (m) director and predict the major product of EAS.

Frequently Asked Questions (FAQs)

Q1. What is Hückel’s Rule?

A cyclic molecule is aromatic if it satisfies the following conditions:

  1. It is cyclic

  2. It is planar

  3. It is fully conjugated (continuous overlap of p-orbitals with alternating single and double bonds)

  4. It contains (4n + 2) π electrons, where n = 0, 1, 2…

Examples:

  • Benzene → 6π electrons (n = 1)

  • Naphthalene → 10π electrons (n = 2)

Q2. What is the Wheland Intermediate in EAS?

The Wheland intermediate (also called σ-complex or arenium ion) is the positively charged cyclohexadienyl cation formed during Electrophilic Aromatic Substitution (EAS).

It is produced when an electrophile (E⁺) attacks one carbon of benzene, temporarily breaking the aromatic π system. The intermediate then loses H⁺ to restore aromaticity.

Q3. What are the limitations of Friedel–Crafts Alkylation?

  1. Polyalkylation – The alkylated product is more reactive than benzene and undergoes further substitution.

  2. Carbocation rearrangement – Primary carbocations rearrange to more stable secondary or tertiary carbocations.

  3. Does not work with strongly deactivated rings (e.g., nitrobenzene).

Friedel–Crafts Acylation avoids these problems.

Q4. How do activating groups direct to ortho/para positions?

Activating groups such as –OH, –NH₂, –OR, and alkyl groups donate electrons to the aromatic ring via resonance or inductive effects.

This increases electron density particularly at the ortho and para positions, making these positions more reactive toward electrophilic attack.

Q5. Uses of Saccharin?

Saccharin (o-sulphobenzamide) is an artificial sweetener approximately 300–500 times sweeter than sucrose.

  • It has zero calories

  • Suitable for diabetic patients

  • It is odorless but may leave a slightly bitter or metallic aftertaste