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
p orbital-bearing atoms
not
aromatic
not
aromatic
aromatic
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
18. 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
Antiaromaticity
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
22. When a benzene ring is a substituent, it is called a
phenyl group
A benzene ring with a methylene group is called a benzyl
group
23. With the exception of toluene, benzene rings with an alkyl
substituent are named as alkyl-substituted benzenes or
as phenyl-substituted alkanes
Aryl group (Ar) is the general term for either a phenyl
group or a substituted phenyl group
26. Benzene is a nucleophile that reacts with an electrophile
An electrophilic substitution yields an aromatic product
which is significantly more stable than the addition
reaction
28. There are five common electrophilic aromatic substitution
reactions
1. Halogenation
2. Nitration
3. Sulfonation
4.Friedel-Crafts acylation
5.Friedel-Crafts alkylation
46. However, a 100% of the 2-methyl-2-phenylbutane
product can be obtained if a bulky alkyl halide is used
47.
48. It is not possible to obtain a good yield of an
alkylbenzene containing a straight-chain group via
Friedel–Crafts alkylation due to carbocation
rearrangement
Acylium ions, however, do not rearrange
