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
A B C 
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 \u2014 electrophilic aromatic substitution \u2014 not 
addition to the ring.
Electrophilic Aromatic Substitution
\ufffd E\ufffdX\ufffd
\ufffd H\ufffdX\ufffd
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).
Mechanism of Electrophilic Aromatic Substitution
Step 1. Electrophilic attack
\ufffd \ufffd
\ufffdH H H
orE E
Step 2. Proton loss
\ufffd H\ufffd
\ufffd \ufffd
\ufffd EH H H
The fi rst step in this mechanism is not favored thermodynamically. Although charge is 
delocalized in the cationic intermediate, the formation of the C \u2013 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.
Not aromatic,
transition state
Aromatic, exothermic
+ E+ \u394H\u2da
E + H+
E \ufffd+
H \ufffd+
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 \u2013 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.
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 FeBr3O\u161ð ð\ufffd \u161\ufffd Br
Activation of Bromine by the Lewis Acid FeBr3
\ufffdBr FeBr3O O\u161ð\ufffd \u161\ufffd
\ufffdBr FeBr3O\u161\ufffd
In this complex, the Br \u2013 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
\ufffdBr FeBr3 FeBr4\ufffd\ufffdO O\u161ð\ufffd \u161\ufffd
\ufffdH H
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 
Bromobenzene Formation
Br HBr FeBr3
\ufffd \ufffd
A quick calculation confi rms that electrophilic bromination of benzene is exothermic. 
A phenyl \u2013 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 \u2013 bromine bond (DH 8 5 81 kcal mol21) and an H \u2013 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