Lesson 13: Aromatics and Benzene Reactions

Lesson 13:

Aromatic vs. Non-Aromatic vs. Anti-Aromatic Compounds

Aromatic Compounds

• Definition:

  • Aromatic compounds are cyclic, planar molecules with a ring of resonance bonds that follow Huckel's rule, which states that a molecule is aromatic if it has 4n+24n + 24n+2 π-electrons (where nnn is a non-negative integer).

• Characteristics:

  • High stability due to delocalized π-electrons.

  • Exhibit resonance, leading to equal bond lengths in the ring.

  • Examples: Benzene (C₆H₆) with 6 π-electrons (4n + 2, where n=1).

Non-Aromatic Compounds

• Definition:

  • Non-aromatic compounds do not meet the criteria for aromaticity. They can be cyclic, acyclic, or non-planar, and do not have a continuous overlap of p-orbitals.

• Characteristics:

  • Lack the extra stability seen in aromatic compounds.

  • Can have any number of π-electrons or may lack a conjugated π-system.

  • Examples: Cyclohexane (C₆H₁₂), which is not conjugated and does not have a planar structure.

Anti-Aromatic Compounds

• Definition:

  • Anti-aromatic compounds are cyclic, planar molecules with a ring of resonance bonds but have 4n4n4n π-electrons, which leads to instability.

• Characteristics:

  • Highly unstable and reactive due to the unfavorable electronic structure.

  • Examples: Cyclobutadiene (C₄H₄) with 4 π-electrons (4n, where n=1).

Benzene Reactions: Electrophilic Aromatic Substitution (EAS)

1. Nitration

• Reagents:

  • Concentrated nitric acid (HNO₃) and concentrated sulfuric acid (H₂SO₄).

• Mechanism:

  • Generation of the electrophile: HNO3+H2SO4→NO2++HSO4−+H2OHNO₃ + H₂SO₄ → NO₂^+ + HSO₄^- + H₂OHNO3​+H2​SO4​→NO2+​+HSO4−​+H2​O.

  • The nitronium ion (NO₂⁺) attacks the benzene ring, forming a sigma complex (arenium ion).

  • Loss of a proton (re-aromatization) restores the aromaticity, producing nitrobenzene.

2. Sulfonation

• Reagents:

  • Concentrated sulfuric acid (H₂SO₄) or fuming sulfuric acid (SO₃ in H₂SO₄).

• Mechanism:

  • Generation of the electrophile: SO3+H2SO4→HSO3++HSO4−SO₃ + H₂SO₄ → HSO₃^+ + HSO₄^-SO3​+H2​SO4​→HSO3+​+HSO4−​.

  • The sulfonic acid ion (HSO₃⁺) attacks the benzene ring, forming a sigma complex.

  • Loss of a proton restores the aromaticity, producing benzene sulfonic acid.

3. Halogenation (e.g., Chlorination)

• Reagents:

  • Chlorine (Cl₂) and a Lewis acid catalyst such as aluminum chloride (AlCl₃).

• Mechanism:

  • Generation of the electrophile: Cl2+AlCl3→Cl++AlCl4−Cl₂ + AlCl₃ → Cl^+ + AlCl₄^-Cl2​+AlCl3​→Cl++AlCl4−​.

  • The chlorine cation (Cl⁺) attacks the benzene ring, forming a sigma complex.

  • Loss of a proton restores the aromaticity, producing chlorobenzene.

4. Friedel-Crafts Alkylation

• Reagents:

  • An alkyl halide (e.g., CH₃Cl) and a Lewis acid catalyst such as aluminum chloride (AlCl₃).

• Mechanism:

  • Generation of the electrophile: RCl+AlCl3→R++AlCl4−RCl + AlCl₃ → R^+ + AlCl₄^-RCl+AlCl3​→R++AlCl4−​.

  • The alkyl cation (R⁺) attacks the benzene ring, forming a sigma complex.

  • Loss of a proton restores the aromaticity, producing an alkylbenzene.

5. Friedel-Crafts Acylation

• Reagents:

  • An acyl chloride (e.g., CH₃COCl) and a Lewis acid catalyst such as aluminum chloride (AlCl₃).

• Mechanism:

  • Generation of the electrophile: RCOCl+AlCl3→RCO++AlCl4−RCOCl + AlCl₃ → RCO^+ + AlCl₄^-RCOCl+AlCl3​→RCO++AlCl4−​.

  • The acylium ion (RCO⁺) attacks the benzene ring, forming a sigma complex.

  • Loss of a proton restores the aromaticity, producing an acylbenzene (ketone).