Organic chemistry
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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

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Organic chemistry

Organic Chemistry

Aromatic compounds


Organic chemistry

Aromatic Compounds Are Unusually Stable

Benzene is an aromatic compound


Organic chemistry

Benzene is unusually stable because of electron

delocalization

Compounds with unusually large resonance energies,

like benzene, are called aromatic compounds


Organic chemistry

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


Organic chemistry

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


Organic chemistry

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


Organic chemistry

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


Organic chemistry

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


Organic chemistry

These compounds are aromatic


Organic chemistry

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


Organic chemistry

Pyridine Is Aromatic


Organic chemistry

Pyrrole Is Aromatic

The lone-pair electrons on the nitrogen atom of pyrrole

are p electrons


Organic chemistry

Furan and thiophene are aromatic compounds like pyrrole


Organic chemistry

Examples of Heterocyclic

Aromatic Compounds


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The Effect of Aromaticity on the pKa

Values of Some Compounds


Organic chemistry

Why is the pKa of cyclopentadiene so much lower than

that of ethane?

The conjugate base is aromatic


Organic chemistry

Aromaticity influences chemical reactivity

The cycloheptatrienyl cation is aromatic


Organic chemistry

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


Organic chemistry

A Molecular Orbital Description of Aromaticity and Antiaromaticity

Aromatic compounds are stable because they have filled

bonding p molecular orbitals


Organic chemistry

Nomenclature of Monosubstituted Benzenes

Some are named by attaching “benzene” after the name

of the substituent


Organic chemistry

Some have to be memorized


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