1. Aromaticity of Benzenoid and Non-benzenoid compounds
PART-II By
Dr. Atul Prasad Sikdar
Associate Professor
Department of Chemistry
Mangaldai College : Assam

2. Aromaticity
Polycyclic benzenoid aromatic compounds have two or more benzene rings fused together

3. Aromaticity
Naphthalene can be represented by three resonance structures
The most important resonance structure is shown below
Calculations show that the 10 p electrons of napthalene are delocalized and that it has substantial resonance energy

4. Aromaticity
Pyrene has 16 p electrons, a non-Huckel number, yet is known to be aromatic
Ignoring the central double bond, the periphery of pyrene has 14 p electrons, a Huckel number, and on this basis it resembles the aromatic [14]annulene

5. Aromaticity Nonbenzenoid Aromatic Compounds
Nonbenzenoid aromatic compounds do not contain benzene rings
Examples are cyclopentadienyl anion and the aromatic annulenes (except [6] annulene)
Azulene has substantial resonance energy and also substantial separation of charge, as shown in the electrostatic potential map

6. Aromaticity of the Cyclopentadienyl Anion
Not fully conjugated and not aromatic
Unstable and nonaromatic
Stable and aromatic

7. Cyclopentadiene is relatively acidic (pKa = 16) because its conjugate base, the aromatic cyclopentadienyl anion, is so stable.
Other hydrocarbons have pKa > 45

8. Aromaticity of the Cyclopentadienyl Anion
1,3-Cyclopentadiene contains conjugated double bonds joined by a CH2 that blocks delocalization
Removal of H+ at the CH2 produces a cyclic 6-electron system, which is stable
Removal of H- or H• generate nonaromatic 4 and 5 electron systems
Relatively acidic (pKa = 16) because the anion is stable

9. Aromaticity of the Cycloheptatrienyl Cation
Not fully conjugated and not aromatic
Stable and aromatic
Unstable and nonaromatic

10. The cycloheptatrienyl cation (six p electrons) is aromatic and very stable
Reaction of cycloheptatriene with Br2 yields cycloheptatrienylium bromide, an ionic substance containing the cycloheptatrienyl cation

11. Aromaticity of the Cycloheptatrienyl Cation
Cycloheptatriene has 3 conjugated double bonds joined by a CH2
Removal of H- at the CH2 produces the cycloheptatrienyl cation
The cation is a cyclic 6-electron system, which is stable and is aromatic
Removal of H+ or H• generate nonaromatic 7 and 8 electron systems

12. Thanking You