aromatic compounds

Importance of Aromatic Chemistry

  • Biological Activity of Aromatic Molecules

    • Examples:

      • Caffeine

      • MDMA

      • Chloroxylenol

      • Nicotine

      • Lidocaine

      • Ibuprofen

Why Is It Called "Aromatic"?

  • Term Origin: Introduced in 1855 by Hoffmann

    • Initial Association:

      • Early aromatic compounds associated with pleasant smells (e.g., thyme, vanilla, almonds, cinnamon)

    • Modern Definition:

      • Refers to benzene derivatives or compounds with one or more benzene-like rings

      • Not always pleasant fragrances

Benzene Structure and History

  • Discovered by Michael Faraday (1825)

  • Structure proposed by August KekulĂ© (1865)

  • Representation of electron density with resonance structures

  • Each structure is a canonical form contributing to overall electron delocalization

    resonance / canonical

Hybridization and Aromaticity

  • In benzene, (c6h6) each carbon is are sp2 hybridized

    • Each sp2 carbon has one perpendicular p orbital

  • Bonding:

    • 4 bonding electrons per carbon:

      • 1 in each sp2 orbital forms sigma (s) bonds with neighbouring carbons with hydrogen

      • 1 in p orbital forms a pi (p) bond

  • Delocalization:

    • 6 p orbital electrons can hybridise to form a set of contiguous Ď€ bonds

    • these 6 Ď€ bond electrons are delocalised above and below the ring

    • this Ď€ system means that all six sides of the aromatic ring are identical due to this delocalization

Interactions of Benzene with Functional Groups

  • Benzene has delocalized p electrons which interact with functional groups

  • Electron Density Variations:

    • Areas in benzene can be electron-rich or electron-poor, leading to cationic-pi interactions

  • Aromatic Stacking:

    • Important in biological systems, such as stabilizing DNA

  • face to face means they’re slightly offset, positive and negative parts.

Aromatic Nomenclature

lots of non-systematic names in common use

Functional Groups Derived from Aromatic Rings (Aryl group)

  • Aryl group denotations: R = Ar

  • Benzyl group and phenyl group defined as derivatives of benzene

Substitution Patterns in Aromatic Nomenclature

  • 1,2 relationship (ortho), 1,3 relationship (meta), 1,4 relationship (para)

Stability of Benzene

  • Reactivity Studies:

      • No reaction occurs when Br2 is added to benzene, demonstrating its stability compared to unsaturated compounds

    • Alkenes readily undergo addition reactions while benzene (arenes) remains unchanged

Reactivity Comparison: Alkyl Halides vs Aryl Halides

  • Alkyl halides undergo nucleophilic substitution easily

    • aryl halides do not

  • Aryl halides showcase different behavior:

    • No typical nucleophilic substitution or elimination under normal conditions

    • Specific conditions required for reactivity, no elimination of aryl halide.