Vollhardt  Capítulo 15 (Benzenos e Aromaticidade)

Vollhardt Capítulo 15 (Benzenos e Aromaticidade)


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this reaction is identical with that of bromination. 
Finally, electrophilic iodination with iodine is endothermic and thus not normally possible. 
Much like the radical halogenation of alkanes, electrophilic chlorination and bromination 
of benzene (and substituted benzenes, Chapter 16) introduces functionality that can be 
REACTION
Br2, FeBr3
Bromination
of Benzene
Bromobenzene
Br
MECHANISM
Br
Figure 15-21 X-ray structure 
of the carbocation arising from 
 attack of bromine on hexamethyl-
benzene. This cation can be 
isolated because it is unusually 
stabilized by the methyl substitu-
ents and because it lacks a proton 
on the sp3-hybridized carbon, thus 
disabling bromoarene formation.
Table 15-1
Dissociation Energies [DH 8] 
of Bonds A \u2013 B [kcal mol21 
(kJ mol21)]
A B DH8
F F 38 (159)
Cl Cl 58 (243)
Br Br 46 (192)
I I 36 (151)
F C6H5 126 (527)
Cl C6H5 96 (402)
Br C6H5 81 (339)
I C6H5 65 (272)
C6H5 H 112 (469)
F H 135.8 (568)
Cl H 103.2 (432)
Br H 87.5 (366)
I H 71.3 (298)
 C h a p t e r 1 5 705
In Summary The halogenation of benzene becomes more exothermic as we proceed from 
I2 (endothermic) to F2 (exothermic and explosive). Chlorinations and brominations are 
achieved with the help of Lewis acid catalysts that polarize the X \u2013 X bond and activate the 
halogen by increasing its electrophilic power.
15-10 Nitration and Sulfonation of Benzene
In two other typical electrophilic substitutions of benzene, the electrophiles are the nitronium 
ion (NO2
1), leading to nitrobenzene, and sulfur trioxide (SO3), giving benzenesulfonic acid.
Benzene is subject to electrophilic attack by the nitronium ion
To bring about nitration of the benzene ring at moderate temperatures, it is not suffi cient 
just to treat benzene with concentrated nitric acid. Because the nitrogen in HNO3 has little 
electrophilic power, it must somehow be activated. Addition of concentrated sulfuric acid 
serves this purpose by protonating the nitric acid. Loss of water then yields the nitronium 
ion, NO2
1, a strong electrophile, with much of its positive charge residing on nitrogen, as 
shown in the electrostatic potential map.
O O
>)>
>
p>
>
>
p
p
)
HOO
l
i
N
O
O
>)\ufffd
\ufffd
>
p
)
l
i
N
O
O
>)\ufffd
\ufffd
>
p
)
l
i
N
O
O
>)\ufffd
\ufffd
\ufffd H OSO3H HO
H
\ufffd HSO4\ufffd
\ufffd
A
O p>O
Nitric acid
HOO
\ufffd
A
H
H2O \ufffd PN
\ufffd
P
Activation of Nitric Acid by Sulfuric Acid
Nitronium ion
*Professor George A. Olah (b. 1927), University of Southern California, Los Angeles, Nobel Prize 1994 
(chemistry).
Exercise 15-23
When benzene is dissolved in D2SO4, its 
1H NMR absorption at d 5 7.27 ppm disappears and a 
new compound is formed having a molecular weight of 84. What is it? Propose a mechanism for 
its formation. (Caution: In all mechanisms of electrophilic aromatic substitution, always draw the 
H atom at the site of electrophilic attack.)
Exercise 15-24
Professor G. Olah* and his colleagues exposed benzene to the especially strong acid system 
HF \u2013 SbF5 in an NMR tube and observed a new 
1H NMR spectrum with absorptions at d 5 
5.69 (2 H), 8.22 (2 H), 9.42 (1 H), and 9.58 (2 H) ppm. Propose a structure for this species.
REACTION
HNO3, H2SO4
Nitration of Benzene
Nitrobenzene
NO2
utilized in further reactions, in particular C \u2013 C bond formations through organometallic 
reagents (see Problem 54, Section 13-9, and Chemical Highlight 13-1).
1 5 - 1 0 N i t r a t i o n a n d S u l f o n a t i o n o f B e n z e n e
706 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
The nitronium ion, with its positively charged nitrogen, then attacks benzene.
\ufffd \ufffdOOP PN
\ufffd
\ufffd
NO2
OSO3H\ufffd
HOSO3H
Nitrobenzene
H NO2H
Mechanism of Aromatic Nitration
Aromatic nitration is the best way to introduce nitrogen-containing substituents into 
the benzene ring. The nitro group functions as a directing group in further substitutions 
(Chapter 16) and as a masked amino function (Section 16-5), as unraveled in benzenamines 
(anilines; Section 22-10).
Sulfonation is reversible
Concentrated sulfuric acid does not sulfonate benzene at room temperature. However, a 
more reactive form, called fuming sulfuric acid, permits electrophilic attack by SO3. Com-
mercial fuming sulfuric acid is made by adding about 8% of sulfur trioxide, SO3, to the 
concentrated acid. Because of the strong electron-withdrawing effect of the three oxygens, 
the sulfur in SO3 is electrophilic enough to attack benzene directly. Subsequent proton 
transfer results in the sulfonated product, benzenesulfonic acid.
Mechanism of Aromatic Sulfonation
S
\ufffd
B
NKS
ðð
\u161\ufffdO \u161\ufffdO
O
H\ufffd
ð\u161O
\u161\ufffdOHð\ufffdO
Benzenesulfonic acid
B
B
ð
S ð\u161\ufffdO
ð\ufffdO
ðO
\ufffd
B
B
O
H
\ufffdH\ufffd \ufffdH\ufffd
Aromatic sulfonation is readily reversible. The reaction of sulfur trioxide with water to 
give sulfuric acid is so exothermic that heating benzenesulfonic acid in dilute aqueous acid 
completely reverses sulfonation.
SOH C4O, catalytic H , 100 22
H3HOSO\ufffd
Reverse Sulfonation: Hydrolysis
\ufffd
HSO3H
Hydration of SO3
H
O O
P
ð
OOH\ufffd H
\ufffd \ufffd
\ufffd
\ufffd
\ufffd
\ufffd O
\ufffd
\ufffd
O
S
ðð
ð
O\ufffdð
H heat\ufffd
B
NKS
ððO
The reversibility of sulfonation may be used to control further aromatic substitution pro-
cesses. The ring carbon containing the substituent is blocked from attack, and electrophiles 
are directed to other positions. Thus, the sulfonic acid group can be introduced to serve as 
a directing blocking group and then removed by reverse sulfonation. Synthetic applications 
of this strategy will be discussed in Section 16-5.
SO3, H2SO4
SO3H
Sulfonation of Benzene
Benzenesulfonic
acid
MECHANISM
MECHANISM
REACTION
 C h a p t e r 1 5 707
Benzenesulfonic acids have important uses
Sulfonation of substituted benzenes is used in the synthesis of detergents. Thus, long-chain 
branched alkylbenzenes are sulfonated to the corresponding sulfonic acids, then converted 
into their sodium salts. Because such detergents are not readily biodegradable, they have 
been replaced by more environmentally acceptable alternatives. We shall examine this class 
of compounds in Chapter 19.
R
R \ufffd branched alkyl group
Aromatic Detergent Synthesis
SO3H SO3\ufffdNa\ufffd
SO3, H2SO4 NaOH
 \ufffd H2O
R R
Another application of sulfonation is to the manufacture of dyes, because the sulfonic 
acid group imparts water solubility (Chapter 22).
Sulfonyl chlorides, the acid chlorides of sulfonic acids (see Section 9-4), are usually 
prepared by reaction of the sodium salt of the acid with PCl5 or SOCl2.
Preparation of Benzenesulfonyl Chloride
O
S O\ufffdNa\ufffdO SO2Cl
\ufffd POCl3\ufffdPCl5 Na\ufffdCl\ufffd\ufffd
O
O
Sulfonyl chlorides are frequently employed in synthesis. For example, recall that the hydroxy 
group of an alcohol may be turned into a good leaving group by conversion of the alcohol 
into the 4-methylbenzenesulfonate (p-toluenesulfonate, tosylate; Sections 6-7 and 9-4).
Sulfonyl chlorides are important precursors of sulfonamides, many of which are 
 chemotherapeutic agents, such as the sulfa drugs discovered in 1932 (Section 9-11). Sulfon-
amides are derived from the reaction of a sulfonyl chloride with an amine. Sulfa drugs 
specifi cally contain the 4-aminobenzenesulfonamide (sulfanilamide) function. Their mode 
of action is to interfere with the bacterial enzymes that help to synthesize folic acid (Chemical 
Highlight 25-3), thereby depriving them of an essential nutrient and thus causing bacterial 
cell death.
RHN
Sulfa Drugs
SO2NHR\u2032
General structure
H2N SO2NH
Sulfalene
(Kelfizina)
CH3O
N
N
(Antiparasitic)
Sulfamethoxazole
(Gantanol)
(Antibacterial, used to treat urinary