Vollhardt  Capítulo 15 (Benzenos e Aromaticidade)
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Vollhardt Capítulo 15 (Benzenos e Aromaticidade)

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planar \ufffd aromatic

1,4-Cyclohexadiene:
nondelocalized
\ufffd nonaromatic

[16]Annulene:
planar \ufffd antiaromatic

1,3-Cyclopentadiene:
noncyclically delocalized

\ufffd nonaromatic

The alternating behavior of the annulenes between aromatic and antiaromatic had been
predicted earlier by the theoretical chemist Hückel, who formulated this (4n 1 2) rule in
1931. Hückel\u2019s rule expresses the regular molecular-orbital patterns calculated for planar,
cyclic conjugated polyenes. The p orbitals mix to give an equal number of p molecular
orbitals, as shown in Figure 15-18. For example, the four p orbitals of cyclobutadiene result
in four molecular orbitals, the six p orbitals of benzene in six molecular orbitals, and so
on. These orbitals occur in symmetrically disposed degenerate pairs, except for the lowest
bonding and highest antibonding ones, which are unique. Thus, cyclobutadiene has one such
degenerate pair of molecular orbitals, benzene has two, and so on. A closed-shell system
is possible only if all bonding molecular orbitals are occupied (see Section 1-7), that is,
only if there are (4n 1 2)p electrons. On the other hand, 4n p cycles always contain a pair
of singly occupied orbitals, an unfavorable electronic arrangement.

E

A B C

)) x

Cyclopolyene with
x \ufffd 1 conjugated

double bonds

Cyclobutadiene
(x \ufffd 1)

Benzene
(x \ufffd 2)

Highest antibonding
molecular orbital

Pairs of degenerate
molecular orbitals

Lowest bonding
molecular orbital

Figure 15-18 (A) Hückel\u2019s
4n 1 2 rule is based on the regular
pattern of the p molecular orbitals
in cyclic conjugated polyenes.
They are equally spaced and
 degenerate, except for the highest
and lowest ones, which are unique.
(B) Molecular-orbital levels in 1,3-
cyclobutadiene. Four p electrons
are not enough to result in a closed
shell (in other words, doubly fi lled
molecular orbitals), so the mole-
cule is not aromatic. (C) The six
p electrons in benzene produce
a closed-shell confi guration, so
benzene is aromatic.

Exercise 15-16

On the basis of Hückel\u2019s rule, label the following molecules as aromatic or antiaromatic.
(a) [30]Annulene; (b) [20]annulene; (c) trans-15,16-dihydropyrene; (d) the deep blue (see
Figure 14-17) azulene; (e) S-indacene.

2
1

10

3

8 6

7

4

9 5

15
16

H

H

trans-15,16-Dihydropyrene

0

´

1
8 7

62

3
4 5

Azulene S-indacene

 C h a p t e r 1 5 699

In Summary Cyclic conjugated polyenes are aromatic if their p electron count is 4n 1 2.
This number corresponds to a completely fi lled set of bonding molecular orbitals. Con-
versely, 4n p systems have open-shell, antiaromatic structures that are unstable, are reactive,
and lack aromatic ring-current effects in 1H NMR. Finally, when steric constraints impose
nonplanarity, cyclic polyenes behave as nonaromatic alkenes.

15-7 Hückel\u2019s Rule and Charged Molecules
Hückel\u2019s rule also applies to charged molecules, as long as cyclic delocalization can occur.
Their aromaticity is refl ected in relative thermodynamic and kinetic stability, the observation
of ring currents in the NMR experiment, and the absence of bond alternation in crystal
structures. This section shows how charged aromatic systems can be prepared.

The cyclopentadienyl anion and the cycloheptatrienyl cation
are aromatic
1,3-Cyclopentadiene is unusually acidic [pKa < 16; comparable to alcohols (Section 8-3)]
because the cyclopentadienyl anion resulting from deprotonation contains a delocalized,
aromatic system of six p electrons. The negative charge is equally distributed over all fi ve
carbon atoms. For comparison, the pKa of propene is 40. An electrostatic potential map of
the molecule is shown in the margin, on a scale that attenuates the otherwise overwhelming
effect of the negative charge.

Aromatic Cyclopentadienyl Anion
H

pKa \ufffd 16

H\ufffd etc. or
\ufffd

\ufffd

H

\u161 \ufffd
\ufffd

\u161

\ufffd
\ufffd

In contrast, the cyclopentadienyl cation, a system of four p electrons, can be produced only
at low temperature and is extremely reactive.

When 1,3,5-cycloheptatriene is treated with bromine, a stable salt is formed, cyclohepta-
trienyl bromide. In this molecule, the organic cation contains six delocalized p electrons,
and the positive charge is equally distributed over seven carbons (as shown in the electrostatic
potential map in the margin). Even though it is a carbocation, the system is remarkably
unreactive, as is expected for an aromatic system. In contrast, the cycloheptatrienyl anion is
antiaromatic, as indicated by the much lower acidity of cycloheptatriene (pKa 5 39) com-
pared with that of cyclopentadiene.

Br\ufffd etc. or

\ufffd

\ufffd
Br2, \ufffd
\ufffdHBr \ufffd

\ufffd

Aromatic Cycloheptatrienyl Cation

H H

Cyclopentadienyl anion:
aromatic

Exercise 15-17

Draw an orbital picture of (a) the cyclopentadienyl anion and (b) the cycloheptatrienyl cation
(consult Figure 15-2).

Cycloheptatrienyl cation:
aromatic

1 5 - 7 H ü c k e l \u2019 s R u l e a n d C h a r g e d M o l e c u l e s

700 C h a p t e r 1 5 B e n z e n e a n d A r o m a t i c i t y

Nonaromatic cyclic polyenes can form aromatic dianions
and dications
Cyclic systems of 4n p electrons can be converted into their aromatic counterparts by
two-electron oxidations and reductions. For example, cyclooctatetraene is reduced by
alkali metals to the corresponding aromatic dianion. This species is planar, contains
fully delocalized electrons, and is relatively stable. It also exhibits an aromatic ring current
in 1H NMR.

Nonaromatic Cyclooctatetraene Forms an Aromatic Dianion

Eight \ufffd \ufffdelectrons,
nonaromatic

2\ufffd=

Ten electrons,
aromatic

K, THF

Reduction
by two electrons

K\ufffd

Similarly, [16]annulene can be either reduced to its dianion or oxidized to its dication, both
products being aromatic. On formation of the dication, the confi guration of the molecule
changes.

Aromatic [16]Annulene Dication and Dianion from Antiaromatic [16]Annulene

+
+

=
CF3SO3H, SO2, CH2Cl2, \u201380°C

Oxidation by two electrons

Eighteen electrons,
aromatic
\ufffd

[16]Annulene
\ufffdSixteen electrons,

antiaromatic
Fourteen electrons,

aromatic
\u3c0

K, THF

Reduction
by two electrons

Exercise 15-18

The rate of solvolysis of compound A in 2,2,2-trifl uoroethanol at 258C exceeds that of compound B
by a factor of 1014. Explain.

A B

CH3

H3C OCCF3
O
B

C(CH3)3

(CH3)3C
H3C OCCF3

O
B

Exercise 15-19

On the basis of Hückel\u2019s rule, label the following molecules aromatic or antiaromatic.

(a) Cyclopropenyl cation; (b) cyclononatetraenyl anion; (c) cycloundecapentaenyl anion.

 C h a p t e r 1 5 701

In Summary Charged species may be aromatic, provided they exhibit cyclic delocalization
and obey the 4n 1 2 rule.

15-8 Synthesis of Benzene Derivatives: Electrophilic
Aromatic Substitution

In this section we begin to explore the reactivity of benzene, the prototypical aromatic com-
pound. The aromatic stability of benzene makes it relatively unreactive, despite the presence
of three formal double bonds. As a result, its chemical transformations require forcing

Exercise 15-20

Working with the Concepts: Recognizing Aromaticity in Charged Molecules

Azulene (see Exercise 15-16d) is readily attacked by electrophiles at C1, by nucleophiles at C4. Explain.
Strategy
We need to formulate fi rst the various resonance forms of the species resulting from these two modes of attack. Inspection of these struc-
tures might then provide the answer.
Solution
\u2022 Attack by electrophiles at C1:

\ufffd \ufffd

H
EE\ufffd

A cycloheptatrienyl cation:
aromatic

H
E

\ufffdetc.
\ufffd

H
E

Attack by E1 at C1 generates a fused, aromatic cycloheptatrienyl cation framework.
\u2022 Attack by nucleophiles at C4:

\ufffd

Nuð\ufffd

A cyclopentadienyl anion:
aromatic

\ufffdetc.
\ufffd

H Nu

ð

H Nu

Attack by Nu2