21 Benzene

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Chapter 21, Benzeneand and the Concept of

Aromaticity

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Benzene - Kekul

In 1872, August Kekul proposed the following

structure for benzene.

This structure, however, did not account for the

unusual chemical reactivity of benzene.

CH

CH

CH

CHC

H

C

HC

C

CC

C

C

H

HHH

HH

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Benzene - Resonance

We often represent benzene as a hybrid of two

equivalent Kekul structures. Each makes an equal contribution to the hybrid and

thus the C-C bonds are neither double nor single, butsomething in between.

Benzene as a hybrid of two equ ivalen tcontributing structures

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Benzene - Resonance Model

The concepts of hybridization of atomic orbitals

and the theory of resonance, developed in the1930s, provided the first adequate description ofbenzenes structure.

The carbon skeleton is a planar regular hexagon.

All C-C-C and H-C-C bond angles 120.

sp2-sp

2

sp2-1s109 pm

120

120

120

139 pm

C

C

C

C

C

C H

H H

H

H H

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The Pi System of Benzene

(a) The carbon framework with the six 2p orbitals.

(b) Overlap of the parallel 2p orbitals forms one torusabove the plane of the ring and another below it

this orbital represents the lowest-lying pi-bondingmolecular orbital.

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Benzene-Molecular Orbital Model

The molecular orbital representation of the pi

bonding in benzene.

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Orbitals of the pi System of BenzeneNumber ofnodal surfaces

0

1

2

3

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Benzene - Resonance

Resonance energy:The difference in energy

between a resonance hybrid in which theelectrons are delocalized

and

the most stable one of its hypotheticalcontributing structures in which electrons arelocalized on particular atoms and in particularbonds.

One way to estimate the resonance energy of benzeneis to compare the heats of hydrogenation of benzeneand cyclohexene.

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Benzene- Resonance Energy

Experimentaldata

Model

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Concept of Aromaticity

The underlying criteria for aromaticity were

recognized in the early 1930s by Erich Hckel,based on molecular orbital (MO) calculations.

To be aromatic, a compound must

Be cyclic. Have one p orbital on each atom of the ring.

Be planar or nearly planar so that there is continuousor nearly continuous overlap of all p orbitals of the

ring. Have a closed loop of (4n + 2) pi electrons in the cyclic

arrangement of p orbitals.

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Frost Circles

Frost circle: A graphic method for determining

the relative order of pi MOs in planar, fullyconjugated monocyclic compounds.

Inscribe in a circle a polygon of the same number ofsides as the ring to be examined such that one of the

vertices of the polygon is at the bottom of the circle.

The relative energies of the MOs in the ring are givenby where the vertices of the polygon touch the circle.

Those MOs

Below the horizontal line through the center of the ringare bonding MOs.

on the horizontal line are nonbonding MOs.

above the horizontal line are antibonding MOs.

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Frost Circles

Frost circles describing the MOs for monocyclic,

planar, fully conjugated four-, five-, and six-memberedrings.

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Relationship of hexa-1,3,5-triene to benzene

How does the linear triene relate

to benzene?

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Relationship of hexa-1,3,5-triene to benzene

?

R l i hi f h 3 i b

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Relationship of hexa-1,3,5-triene to benzene

?

Look at orbitals 2 and 3.

p2

p3

Curvearound

Antibonding,destabilizing

Bonding,stabilizing

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Aromatic Hydrocarbons

Annulene:A cyclic hydrocarbon with a

continuous alternation of single and doublebonds.

[14]Annulene is aromatic according to Hckels

criteria.

[14]Annulene

(aromatic)

HH

H H

H

H

H

H

HH

H

H

H H

n = 3

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Aromatic Hydrocarbons

[18]Annulene is also aromatic.

[18]Annulene

(aromatic)

H

H

H

H

H

H

H

H

HH

H

H

H

H

H

H

H

H

n = 4

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Aromatic Hydrocarbons

According to Hckels criteria, [10]annulene should be

aromatic; it has been found, however, that it is not. Nonbonded interactions between the two hydrogens

that point inward toward the center of the ring forcethe ring into a nonplanar conformation in which

overlap of the ten 2p orbitals is no longer continuous.

[10]Annulene

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Aromatic Hydrocarbons

What is remarkable relative to [10]annulene is that if

the two hydrogens facing inward toward the center ofthe ring are replaced by a methylene (CH2) group, thering is able to assume a conformation close enough toplanar that it becomes aromatic.

CH2

Bridged [10]annulene

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Antiaromatic Hydrocarbons

Antiaromatic hydrocarbon:A monocyclic, planar,

fully conjugated hydrocarbon with 4n pielectrons (4, 8, 12, 16, 20...).

An antiaromatic hydrocarbon is especially unstablerelative to an open-chain fully conjugated hydrocarbon

of the same number of carbon atoms.

Cyclobutadiene is antiaromatic.

In the ground-state electron configuration of thismolecule, two electrons fill the p

1

bonding MO.

The remaining two electrons lie in the p2 and p3nonbonding MOs.

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Cyclobutadiene

The ground state of planar cyclobutadiene has two

unpaired electrons, which make it highly unstable andreactive.

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Cyclooctatetraene

Cyclooctatetraene, with 8 pi electrons is not aromatic;

it shows reactions typical of alkenes. X-ray studies show that the most stable conformation

is a nonplanar tub conformation.

Although overlap of 2p orbitals occurs to form pi

bonds, there is only minimal overlap between sets of2p orbitals because they are not parallel.

viewed from above viewed through an edge

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Cyclooctatetraene

MO energy diagram for a planar conformation of

cyclooctatetraene.

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Heterocyclic Aromatics

Heterocyclic compound: A compound that

contains more than one kind of atom in a ring. In organic chemistry, the term refers to a ring with one

or more atoms that differ from carbon.

Pyridine and pyrimidine are heterocyclic analogsof benzene; each is aromatic.

Pyridine

N

N

N

Pyrimidine

1

2

3

4

5

6

12

3 4

5

6

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Pyridine

The nitrogen atom of

pyridine is sp2

hybridized.

The unshared pair ofelectrons lies in an sp2

hybrid orbital and is nota part of the six pielectrons of thearomatic system (thearomatic sextet).

Resonance energy ofpyridine is134 kJ (32kcal)/mol.

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Furan and Pyrrole

The oxygen atom of furan is sp2 hybridized.

one unshared pairs of electrons on oxygen lies in anunhybridized 2p orbital and is a part of the aromaticsextet.

The other unshared pair lies in an sp2 hybrid orbital

and is not a part of the aromatic system. The resonance energy of furan is 67 kJ (16 kcal)/mol.

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Other Heterocyclics

Purine

Indole

N

N

NN

N

H

H

N

H

CH2 CH2 NH 2

Serotonin(a neurotransmitter)

HO

Caffeine

N

NN

N

O

O

H3 C

CH3

CH3

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Aromatic Hydrocarbon Ions

Any neutral, monocyclic, unsaturated

hydrocarbon with an odd number of carbonsmust have at least one CH2 group and, therefore,cannot be aromatic.

Cyclopropene, for example, has the correct number of

pi electrons to be aromatic, 4(0) + 2 = 2, but does nothave a closed loop of 2p orbitals.

Cyclopropene Cyclopentadiene Cycloheptatriene

CH2 CH2CH2

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Cyclopropenyl Cation

If, however, the CH2 group of cyclopropene is

transformed into a CH+

group in which carbon