Vollhardt  Capítulo 15 (Benzenos e Aromaticidade)

Vollhardt Capítulo 15 (Benzenos e Aromaticidade)

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at C4 generates a fused, aromatic cyclopentadienyl anion framework.

Exercise 15-21

Try It Yourself

The triene A can be readily deprotonated twice to give the stable dianion B. However, the neutral
analog of B, the tetraene C (pentalene), is extremely unstable. Explain.

�

�

ð ð

Base

A B C

H

H

H
H

1 5 - 8 E l e c t r o p h i l i c A r o m a t i c S u b s t i t u t i o n

702 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

 conditions and proceed through new pathways. Not surprisingly, however, most of the chem-
istry of benzene features attack by electrophiles. We shall see in Section 22-4 that attack by
nucleophiles is rare but possible, provided that a suitable leaving group is present.

Benzene undergoes substitution reactions with electrophiles
Benzene is attacked by electrophiles, but, in contrast to the corresponding reactions of alkenes,
this reaction results in substitution of hydrogens — electrophilic aromatic substitution — not
addition to the ring.

Electrophilic Aromatic Substitution

� E�X�
Electrophile

� H�X�
H

H

H

H

H

E

H

H

H

H

H

H

Under the conditions employed for these processes, nonaromatic conjugated polyenes would
rapidly polymerize. However, the stability of the benzene ring allows it to survive. Let us
begin with the general mechanism of electrophilic aromatic substitution.

Electrophilic aromatic substitution in benzene proceeds by
addition of the electrophile followed by proton loss
The mechanism of electrophilic aromatic substitution has two steps. First, the electrophile E1
attacks the benzene nucleus, much as it would attack an ordinary double bond. The reso-
nance stabilized cationic intermediate thus formed then loses a proton to regenerate the
aromatic ring. Note two important points in the formulation of this general mechanism.
First, always show the hydrogen at the site of the initial electrophilic attack. Second, the
positive charge in the resulting cation is indicated by three resonance forms and is located
ortho and para to the carbon that has been attacked, a result of the rules for drawing reso-
nance forms (Sections 1-5 and 14-1).

REACTION

MECHANISM

Mechanism of Electrophilic Aromatic Substitution

Step 1. Electrophilic attack

E
H

EE
�

� �

�H H H

orE E

Step 2. Proton loss

� H�
� �

� EH H H

EEE

The fi rst step in this mechanism is not favored thermodynamically. Although charge is
delocalized in the cationic intermediate, the formation of the C – E bond generates an
sp3-hybridized carbon in the ring, which interrupts cyclic conjugation: The intermediate is

 C h a p t e r 1 5 703

not aromatic (Figure 15-19). However, the next step, loss of the proton at the sp3-hybridized
carbon, regenerates the aromatic ring. This process is more favored than nucleophilic
 trapping by the anion that accompanies E1. Such trapping would give a nonaromatic addi-
tion product. The overall substitution is exothermic, because the bonds formed are stronger
than the bonds broken.

Figure 15-20 depicts a potential energy diagram in which the fi rst step is rate determining,
a kinetic fi nding that applies to most electrophiles that we shall encounter. The subsequent
loss of proton is much faster than initial electrophilic attack because it leads to the aromatic
product in an exothermic step, which furnishes the driving force for the overall sequence.

The following sections look more closely at the most common reagents employed in
this transformation and the details of the mechanism.

Intermediate
cation

Not aromatic,
endothermic

Slow

Rate-determining
transition state

Ea

Fast

Aromatic

Aromatic, exothermic

+ E+ ΔH˚

E + H+

E

H

E �+

H �+

�+

�+

E

H
E

‡

‡

Reaction coordinate

+

Figure 15-20 Potential-energy
diagram describing the course of
the reaction of benzene with an
electrophile. The fi rst transition
state is rate determining. Proton
loss is relatively fast. The overall
rate of the reaction is controlled
by Ea; the amount of exothermic
energy released is given by DH8.

In Summary The general mechanism of electrophilic aromatic substitution begins with
electrophilic attack by E1 to give an intermediate, charge-delocalized but nonaromatic cation
in a rate-determining step. Subsequent fast proton loss regenerates the (now substituted)
 aromatic ring.

1 5 - 8 E l e c t r o p h i l i c A r o m a t i c S u b s t i t u t i o n

Exercise 15-22

We learned in Section 11-5 that the DH8 of the hydrogenation of cis-2-butene is 228.6 kcal mol – 1.
Taking this system as a model for a double bond in benzene, estimate the corresponding DH8 of
the hydrogenation of benzene to 1,3-cyclohexadiene. What is the difference between these two
double bonds? (Hint: Consult Figure 15-3.)

Figure 15-19 (A) Orbital picture
of the cationic intermediate result-
ing from attack by an electrophile
on the benzene ring. Aromaticity is
lost because cyclic conjugation is
interrupted by the sp3-hybridized
carbon. The four electrons in
the p system are not shown.
(B) Dotted-line notation to indicate
delocalized nature of the charge
in the cation.

H

E

H

E

BA

+

+

sp3

704 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

15-9 Halogenation of Benzene: The Need for a Catalyst
An example of electrophilic aromatic substitution is halogenation. Benzene is normally
unreactive in the presence of halogens, because halogens are not electrophilic enough to
disrupt its aromaticity. However, the halogen may be activated by Lewis acidic catalysts,
such as ferric halides (FeX3) or aluminum halides (AlX3), to become a much more power-
ful electrophile.

How does this activation work? Lewis acids have the ability to accept electron pairs.
When a halogen such as bromine is exposed to FeBr3, the two molecules combine in a
Lewis acid-base reaction.

Br Br FeBr3Ošð ð� š� Br

Activation of Bromine by the Lewis Acid FeBr3
�

�Br FeBr3O Ošð� š�
�Br FeBr3Oš�

�

Brš�

In this complex, the Br – Br bond is polarized, thereby imparting electrophilic character to
the bromine atoms. Electrophilic attack on benzene is at the terminal bromine, allowing the
other bromine atom to depart with the good leaving group FeBr4

2. In terms of electron
fl ow, you can also view this process as a nucleophilic substitution of [Br2FeBr3] by the
benzene double bond, not unlike an SN2 reaction (see also Figure 15-21).

Electrophilic Attack on Benzene by Activated Bromine

Br
�

�Br FeBr3 FeBr4��O Ošð� š�

�H H

Br

The FeBr4
2 formed in this step now functions as a base, abstracting a proton from the

cyclohexadienyl cation intermediate. This transformation not only furnishes the two prod-
ucts of the reaction, bromobenzene and hydrogen bromide, but also regenerates the FeBr3
catalyst.

Bromobenzene Formation

Br HBr FeBr3
�FeBr3Ošð�

�

� �

Br
H

Br

A quick calculation confi rms that electrophilic bromination of benzene is exothermic.
A phenyl – hydrogen bond (approximately 112 kcal mol21, Table 15-1) and a bromine
molecule (46 kcal mol21) are lost in the process. Counterbalancing this loss is the
 formation of a phenyl – bromine bond (DH 8 5 81 kcal mol21) and an H – Br bond (DH 8 5
87.5 kcal mol21). Thus, the overall reaction is exothermic by 158 2 168.5 5 210.5 kcal
mol21 (43.9 kJ mol21).

As in the radical halogenation of alkanes (Section 3-7), the exothermic nature of aro-
matic halogenation decreases down the periodic table. Fluorination is so exothermic that
direct reaction of fl uorine with benzene is explosive. Chlorination, on the other hand, is
controllable but requires the presence of an activating catalyst, such as aluminum chloride
or ferric chloride. The mechanism of