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Organic Chemistry . Aromatic compounds. Aromatic Compounds Are Unusually Stable. Benzene is an aromatic compound. Benzene is unusually stable because of electron delocalization. Compounds with unusually large resonance energies, like benzene, are called aromatic compounds.

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Organic chemistry

Organic Chemistry

Aromatic compounds


Aromatic Compounds Are Unusually Stable

Benzene is an aromatic compound


Benzene is unusually stable because of electron

delocalization

Compounds with unusually large resonance energies,

like benzene, are called aromatic compounds


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


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


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


not

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


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



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



Pyrrole Is Aromatic

The lone-pair electrons on the nitrogen atom of pyrrole

are p electrons



Examples of Heterocyclic

Aromatic Compounds


The Effect of Aromaticity on the pKa

Values of Some Compounds


Why is the pKa of cyclopentadiene so much lower than

that of ethane?

The conjugate base is aromatic


Aromaticity influences chemical reactivity

The cycloheptatrienyl cation is aromatic


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


A Molecular Orbital Description of Aromaticity and Antiaromaticity

Aromatic compounds are stable because they have filled

bonding p molecular orbitals


Nomenclature of Monosubstituted Benzenes Antiaromaticity

Some are named by attaching “benzene” after the name

of the substituent


Some have to be memorized Antiaromaticity


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