chem I

Chemistry of Aromatic Compounds

Introduction

  • All organic compounds are divided into two classes: aliphatic and aromatic.

  • Aliphatic: originally meant 'fatty'.

  • Aromatic: originally meant 'fragrant' or 'pleasant smell'.

  • Aromatic compounds include benzene and compounds that exhibit similar chemical behavior.

  • Distinctive properties of aromatic compounds:

    • Consist of conjugated planar ring systems with delocalized pi-electron clouds instead of individual alternating double and single bonds.

    • Not all fully conjugated rings are aromatic.

  • Huckel's Rule (for aromaticity):

    1. Planarity

    2. Complete delocalization of the π electrons in the ring

    3. Presence of (4n + 2) π electrons where n is an integer (n = 0, 1, 2...)

  • Compounds with (4n) π electrons are termed antiaromatic.

  • Aromatic compounds divided into:

    • Benzenoids: Compounds containing benzene rings (e.g., benzene, naphthalene).

    • Non-benzenoids: Compounds without benzene rings (e.g., pyrrole, furan).

Discovery of Benzene

  • Discovered by Michael Faraday in 1825 in illuminating gas.

  • In 1834, Eilhardt Mitscherlich prepared benzene by heating benzoic acid with lime.

  • In 1845, A.W. von Hofmann isolated benzene from coal tar.

Structure of Benzene

  • Benzene (C6H6) has been known since 1825, but a satisfactory structure was proposed around 1931.

Proposed Structures

A. Chain Structures

  • Rejected due to:

    1. Addition reactions typical of alkenes and alkynes do not occur with benzene.

    2. Benzene produces only one type of mono-substituted product, unlike open-chain structures.

    3. Open-chain compounds do not undergo reactions such as Friedel Craft reactions, nitration, or sulphonation.

    4. Reduction of benzene yields cyclohexane instead of open-chain products.

B. Kekule Structure of Benzene

  • Proposed by Joseph Loschmidt (1861) and August Kekule von Stradonitz (1866).

  • Features a cyclic arrangement of six carbons with alternating single and double bonds.

  • All carbon atoms bonded to hydrogen, forming a hexagonal structure.

Limitations of Kekule's Structure

  1. Predicts two isomeric disubstituted products, while only one form exists indicating resonance among the bonds.

  2. Suggests benzene's properties would resemble alkenes, but benzene shows high stability with easy substitution.

  3. Predicts bond lengths of 1.54 Å for single and 1.34 Å for double bonds, but all bonds in benzene are of equal length.

  4. Predicts three isomeric disubstitution products, while actual observations yield more.

  5. Aromatic stability of benzene is much higher than aliphatic compounds due to unique reactivity patterns.

Actual Structure of Benzene

  • In 1931, Linus Pauling suggested benzene as a resonance hybrid of two Kekule structures.

  • Benzene comprises six sp2 hybridized carbon atoms in a planar hexagonal ring, each bonded to one hydrogen atom.

  • All C-C bond lengths are equal at 140 pm, indicating electron delocalization.

  • Continuous overlap of p-orbitals leads to cyclic overlap involving all p-orbitals, creating circular electron density above and below the ring.

  • Depicted with a circle inside hexagonal carbon atoms to represent delocalization.

Polycyclic Aromatics

  • Features two or more fused aromatic rings, also called polycyclic aromatic hydrocarbons (PAHs).

Examples

1. Naphthalene

  • Simplest polycyclic aromatic compound with two fused benzene rings.

  • Contains 5 C=C bonds, 10 π electrons, and adheres to Huckel’s Rule (n=2).

  • Exhibits higher reactivity than benzene due to one aromatic ring and other acting like butadiene.

2. Anthracene

  • Colorless solid exhibiting blue fluorescence; present in coal tar.

  • Composed of three linear fused benzene rings with 14 π electrons (n=3).

3. Phenanthrene

  • Contains three aromatic rings and has 14 π electrons (n=3) but with different structural configuration than anthracene.

Resonance Energy of Fused Benzenoid Rings

  • Resonance energy increases with the number of canonical forms:

    • Benzene = 36 kcal/mol

    • Naphthalene = 61 kcal/mol

    • Anthracene = 84 kcal/mol

    • Phenanthrene = 92 kcal/mol

  • Example of naphthalene shows discrepancy between expected and observed resonance energy due to shared electron pairs between fused rings, affecting aromaticity and reactivity.