Aromatic Compounds

1. Aromatic Compounds 15.7 Introduction to Aromatic Compounds

2. Benzene, C6H6, a Puzzling “Alkene” =CH2 15.7 Introduction to Aromatic Compounds

3. Benzene is inert to typical reactions of ordinary alkenes • Halogen addition • Hydroboration • Hydration • Ozonolysis

4. 15.7 Introduction to Aromatic Compounds

5. Structure of Benzene 15.7 Introduction to Aromatic Compounds

6. Comparison of Structures 15.7 Introduction to Aromatic Compounds

7. The Benzene p System • Each carbon in benzene is trigonal planar and sp2 hybridized • Hence, all of the p orbitals in benzene are parallel and readily overlap 15.7 Introduction to Aromatic Compounds

9. Stability of Benzene • Benzene: DH°f = 82.93 kJ/mol • 13.8 kJ/molper CH group • COT: DH°f = 298.0 kJ/mol • 37.3 kJ/mol per CH group • Therefore, benzene is 23.5 kJ/mol (37.3-13.8) more stable than COT per every CH group • Empirical resonance energy of benzene • (23.5 kJ/mol) x 6 CH groups = 141 kJ/mol • Estimate of energy by which benzene is stabilized by resonance

10. Criteria for Aromaticity • Four structural criteria must be satisfied for a compound to be aromatic • A molecule must be cylic • Each p orbital must be connected with p orbitals on adjacent atoms Not Aromatic Aromatic Benzene 1,3,5-hexatriene 15.7 Introduction to Aromatic Compounds

11. A molecule must be planar • All adjacent p orbitals must be aligned so that the π electron density can be delocalized Benzene Cyclooctatetraene

12. The molecule must be completely conjugated • There must be a p orbital at every atom No p orbitals • Completely conjugated ring • A p orbital at every atom in the ring • Aromatic • 1,3-cyclohexadiene • Not completely conjugated • Not aromatic

13. A molecule must satisfy Hückel’s Rule • Hückel’s Rule: in order to be aromatic, a molecule must contain 4n + 2 π electrons • n = 0, 1, 2 etc. • Molecules containing 4n π electrons are antiaromatic • Often quite unstable • 6 π electrons • 4(1) + 2 = 6 • Aromatic • 4 π electrons • 4(1) = 4 • Aromatic

14. Hückel’s Rule • Note that Hückel’s Rule refers to the number of π electrons, not the number of atoms in the ring

15. Summary • Aromatic: cyclic, planar, completely conjugated compound with 4n + 2 π electrons • Anti-aromatic: cyclic, planar, completely conjugated compound with 4n π electrons • Non-aromatic: a compound that lacks one or more of the following requirements: being cyclic, planar, or completely conjugated.

16. Aromatic Heterocycles • Allylic electrons • Electrons on atom next to a doubly bonded atom • Included in π electron count • Vinylic electrons • Electrons on doubly bonded atoms • Not included in π electron count

17. Aromatic Ions

20. Polycyclic Aromatic Compounds • The Hückel 4n+2 rule applies to single ring compounds however, many fused bicyclic and polycyclic compounds are also aromatic 15.7 Introduction to Aromatic Compounds

22. Problems • Classify the following molecules as aromatic, anti-aromatic, or non-aromatic

24. Spectroscopy of Aromatic Compounds • Infrared Spectroscopy • Aromatic ring • C–H stretching at 3030 cm1 • Two peaks at 1450-1600 and 1660-2000 cm1 • Ordinary Alkene • C-H absorbance at 3020-3100 cm-1 • C=C at 1640-1680 cm-1

27. Ultraviolet spectroscopy • Intense peak near 200 nm and then less intense peaks closely after

28. NMR • 1H NMR: Aromatic H’s strongly deshielded by ring and absorb between  6.5 and  8.0

29. Aromatic ring oriented perpendicular to a strong magnetic field • Delocalized  electrons producing a small local magnetic field • Opposes applied field in middle of ring • Reinforces applied field outside of ring • Aromatic H’s experience greater effective mag field • Deshielded