1. Organic Chemistry
Aromatic compounds

2. Aromatic Compounds Are Unusually Stable
Benzene is an aromatic compound

3. Benzene is unusually stable because of electron
delocalization
Compounds with unusually large resonance energies,
like benzene, are called aromatic compounds

4. Criteria for Aromaticity
1. A compound must have an uninterrupted cyclic cloud of p electrons above and below the plane of the molecule
2. The p cloud must contain an odd number of pairs
of p electrons

5. Hückel’s Rule For a planar, cyclic compound to be aromatic, its
uninterrupted p cloud must contain (4n + 2) p electrons,
where n is any whole number

6. Monocyclic hydrocarbons with alternating single and
double bonds are called annulenes
Cyclobutadiene and cyclooctatetraene are not aromatic
because they have an even number of p electron pairs
Furthermore, cyclooctatetraene is nonplanar

7. Cyclopentadiene does not have an uninterrupted ring of
aromatic
not
aromatic
aromatic
Cyclopentadiene does not have an uninterrupted ring of
p orbital-bearing atoms
Cyclopentadienyl cation has an even number of p
electron pairs
Cyclopentadienyl anion has an uninterrupted ring of p
orbital-bearing atoms and an odd number p electron pairs

8. The resonance hybrid shows that all the carbons in the
cyclopentadienyl anion are equivalent
Each carbon has exactly one-fifth of the negative charge
associated with the anion

9. These compounds are aromatic

10. Aromatic Heterocyclic Compounds
A heterocyclic compound is a cyclic compound in which
one or more of the ring atoms is an atom other than
carbon

11. Pyridine Is Aromatic

12. Pyrrole Is Aromatic
The lone-pair electrons on the nitrogen atom of pyrrole
are p electrons

13. Furan and thiophene are aromatic compounds like pyrrole

14. Examples of Heterocyclic
Aromatic Compounds

15. The Effect of Aromaticity on the pKa Values of Some Compounds

16. Why is the pKa of cyclopentadiene so much lower than
that of ethane?
The conjugate base is aromatic

17. Aromaticity influences chemical reactivity
The cycloheptatrienyl cation is aromatic

18. Antiaromaticity A compound is antiaromatic if it is a planar, cyclic
compound with an uninterrupted ring of p orbital-bearing
atoms, and the p cloud must contain an even number of
pairs of p electrons
Antiaromatic compounds are highly unstable

19. A Molecular Orbital Description of Aromaticity and Antiaromaticity
Aromatic compounds are stable because they have filled
bonding p molecular orbitals

20. Nomenclature of Monosubstituted Benzenes
Some are named by attaching “benzene” after the name
of the substituent

21. Some have to be memorized