Unit 4: Polynuclear Hydrocarbons

February 25, 2026

Semester 3
BP301T

Introduction to Polynuclear Hydrocarbons

Polynuclear (polycyclic) aromatic hydrocarbons (PAHs) consist of two or more fused benzene rings. They are the structural backbone of steroids, bile acids, and many drugs. Phenanthrene’s ring system is the basis of morphine and codeine. Understanding the synthesis and reactions of these compounds is essential for Medicinal Chemistry.
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Syllabus & Topics

  • 1Introduction to Polynuclear/Polycyclic Aromatic Hydrocarbons (PAHs).
  • 2Naphthalene (C10H8): Structure (two fused benzene rings), orbital picture.
  • 3Synthesis of Naphthalene (Haworth synthesis from benzene and succinic anhydride → Diels-Alder approach).
  • 4Reactions of Naphthalene: Electrophilic substitution preferentially at C-1 position (α-position).
  • 5Nitration, sulphonation, halogenation, and reduction of naphthalene.
  • 6Medicinal uses: α-Naphthol and β-Naphthol (antiseptics), Naphthylamine (dye intermediate).
  • 7Anthracene (C14H10): Structure (3 linearly fused benzene rings), synthesis (from coal tar).
  • 8Reactions of Anthracene: EAS at 9,10-positions; Diels-Alder reaction (as diene at 9,10).
  • 9Medicinal uses of Anthracene derivatives: Anthraquinone laxatives (Senna, Cascara, Aloe).
  • 10Phenanthrene (C14H10): Structure (3 angularly fused rings, unlike linear anthracene), isomer of anthracene.
  • 11Medicinal significance of Phenanthrene ring system: Basis of steroids, bile acids, sex hormones, morphine.
  • 12Diphenylmethane: Structure and uses (fragrance industry, drug synthesis intermediate).
  • 13Triphenylmethane: Structure, very stable carbocation (triphenylmethyl cation), uses in dyes (Malachite Green, Crystal Violet).

Learning Objectives

Draw the structures of Naphthalene, Anthracene, and Phenanthrene correctly.
Explain why EAS in naphthalene occurs preferentially at the α(C-1) position.
State the medicinal significance of the Phenanthrene ring system.
Give one drug derived from each: Naphthalene, Anthracene, and Phenanthrene series.
Describe the key reaction of Anthracene at positions 9 and 10.

Frequently Asked Questions (FAQs)

Q1. What is the difference between Anthracene and Phenanthrene?

Both Anthracene and Phenanthrene are structural isomers with the molecular formula C₁₄H₁₀, each containing three fused benzene rings.

  • Anthracene → Linear structure (rings fused in a straight line: 1–2–3).

  • Phenanthrene → Angular (bent) structure (third ring fused at an angle).

Phenanthrene is considered more biologically significant due to its presence in important biomolecules.

Q2. Why is the Phenanthrene ring system medically important?

The phenanthrene ring system, when fused with a cyclopentane ring, forms the Sterane nucleus (cyclopentanoperhydrophenanthrene).

This is the core structural framework of:

  • Cholesterol

  • Bile acids

  • Sex hormones (estrogen, testosterone)

  • Corticosteroids

  • Opioid alkaloids (morphine, codeine)

Thus, it is fundamental to steroid chemistry and pharmacology.

Q3. Why are Anthraquinone derivatives used as laxatives?

Anthraquinone derivatives such as:

  • Sennosides (from Senna)

  • Cascarosides (from Cascara sagrada)

  • Barbaloin (from Aloe)

are hydrolyzed in the colon by intestinal bacteria to release active anthraquinones.

These compounds:

  • Stimulate colonic peristalsis

  • Inhibit water absorption

Result → Laxative effect.

Q4. What makes Triphenylmethyl cation (Trityl cation) so stable?

The Triphenylmethyl cation (+C(C₆H₅)₃) is highly stable because its positive charge is delocalized over three benzene rings through resonance and hyperconjugation. The large number of resonance structures distributes the charge, significantly increasing stability. This stability makes triphenylmethyl chloride (trityl chloride) useful as a protecting group in organic synthesis.

Q5. Why is Naphthalene sulphonation at C-1 favored at low temperature?

At low temperature, sulphonation at the C-1 position (α-substitution) is kinetically favored (forms faster, despite slightly lower stability). At high temperature (>160°C), sulphonation at the C-2 position (β-substitution) is thermodynamically favored because it forms the more stable product. Since sulphonation is reversible, higher temperatures allow equilibration to the more stable β-product.