c10 k

1. C10 K Aromatic Chemistry Lecture 1 Dr. Gallimore

2. Lecture Outline Aromatic vs aliphatic compounds Benzene Structure/ stability Electronic configuration The concept of aromaticity Huckel’s rule Aromatic compounds other than benzene Nomenclature of benzene derivatives

3. Aromatic/ Aliphatic compounds Aromatic compounds have a low hydrogen-carbon ratio e.g. benzene, benzaldehyde Aliphatic compounds have a higher ratio e.g. alkanes, alkenes, alkynes

4. Examples of Aromatic compounds Benzaldehyde Naphthalene Coal Benzo[a]pyrene

5. Buckminsterfullerene

6. BENZENEStability of the ring The difference in energy between benzene and the hypothetical cyclohexatriene is called the resonance energy. The resonance energy is the energy lost (stability gained) by complete delocalization of electrons in the pi system. It is a measure of the added stability of the aromatic system compared to the localized system

7. Reactivity of benzene More energy is required for a reaction in which the aromatic character of the ring is lost Does NOT undergo addition of HX, X2 Does NOT oxidize with KMnO4

8. Benzene is a resonance hybrid of the contributing structures Each carbon atom is at the corner of a regular hexagon All C-C bond lengths are identical (1.39 Angstroms)

9. Bonding in benzene

10. The Concept of AromaticityRequirements for aromaticity Molecule must be cyclic Molecule must be planar Each atom of the ring(s) must have a p-orbital perpendicular to the plane of the ring Huckel’s rule must be obeyed Planar compounds must have (4n + 2) pi electrons, where n is an integer Ring with 2, 6, 10 or 14 pi electrons may be aromatic Ring with 8, 12 or 16 pi electrons will not be aromatic

11. For aromaticity, all pi electrons must be paired and all bonding orbitals filled Maximum and complete overlap is required for stabilization With unpaired pi electrons, overlap is not maximized The pi electrons in an aromatic compound are delocalized over the entire ring leading to stabilization

13. Bonding in Cyclooctatetraene

14. Bonding in Cyclopentadiene

15. The Cyclopentadienyl anion

16. The Cyclopentadienyl cation

17. Antiaromatic compounds The pi-electron energy of a compound is determined by experimentation and calculations The pi electrons in an aromatic compound are delocalized over the entire ring leading to stabilization Ring has lower pi electron energy than the open chain In antiaromatic compounds, the ring has a greater pi-electron energy than the open chain

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19. Nomenclature Substituents are added as prefixes to benzene e.g. bromobenzene Derivatives of benzene Common names are often used in the naming of some benzene derivatives

21. The benzene ring is numbered when more than two substituents are present The ring is numbered to give the lowest possible number to the substituents When more than two different substituents are present, they are listed alphabetically.

22. If substituted benzenes (e.g. aniline or toluene) are used as the parent, that substituent is understood to be at position 1 on the ring.

23. The designation phenyl is used when benzene is being treated as a substituent. The phenylmethyl group is designated benzyl.

24. Next Lecture Electrophilic aromatic substitution (EAS) Mechanism of EAS Specific examples of EAS Aromatic nitration Aromatic sulfonation Aromatic halogenation Friedel-Crafts alkylation and acylation