1. Aromatic Compounds
15.7 Introduction to Aromatic Compounds

2. Benzene, C6H6, a Puzzling “Alkene”
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. Each carbon in benzene is trigonal planar and sp2 hybridized
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/mol per CH group
COT: DH°f = kJ/mol
37.3 kJ/mol per CH group
Therefore, benzene is 23.5 kJ/mol ( ) 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
Anti-aromatic
Aromatic n 4n
4n + 2 2 1 4 6 8 10 3 12 14 16 18
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
Vinylic electrons
Electrons on doubly bonded atoms
Not included in π electron count
Allylic electrons
Electrons on atom next to a doubly bonded atom
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 and cm1
Ordinary Alkene
C-H absorbance at cm-1
C=C at 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