Vollhardt  Capítulo 15 (Benzenos e Aromaticidade)

Vollhardt Capítulo 15 (Benzenos e Aromaticidade)


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one of the two possible unique hydrogens gives rise to this signal? 
Hint: Look at the coupling pattern of the 1H NMR signal.) Similarly, this isomer contains four distinct 
ring carbons, as seen in the 13C NMR spectrum.
Armed with the knowledge of the structure of A, we can now formulate the possible products of 
its electrophilic aromatic bromination:
Br
CH3
CH3CH3CH3
Br
CH3 Br CH3
B
The Three Possible Structures for B
Which one is B? Symmetry (or lack thereof ) provides the answer. The 1H NMR spectrum of 
B reveals two distinct methyl groups and three separate ring hydrogen resonances, whereas the 13C 
NMR spectrum exhibits two methyl carbon peaks, in addition to six aromatic carbon absorptions. The 
combined data are only compatible with the center structure above, 1-bromo-2,4-dimethylbenzene. 
The reaction of A to provide B is therefore:
CH3
CH3CH3
Br
CH3
A B
Br2, FeBr3
\ufffdHBr
Why does single bromination of A give only isomer B? Read on to Chapter 16! But fi rst try Problem 43 
for more practice with spectroscopy.
15-35. The insecticide DDT (see Chemical Highlight 3-2) has been made in ton quantities by treat-
ment of chlorobenzene with 2,2,2-trichloroacetaldehyde in the (required) presence of concentrated 
H2SO4. Formulate a mechanism for this reaction.
Chlorobenzene
2
Cl
Cl Cl
Cl
CCH3\ufffd
ðð
2,2,2-Trichloro-
acetaldehyde
DDT
3
H
98%
C Cl
CO
99% H2SO4, 15°C, 5 h
 C h a p t e r 1 5 719
SOLUTION
Let us fi rst take an inventory of what is given:
1. The product is composed of two subunits derived from chlorobenzene and one subunit derived 
from the aldehyde.
2. In the process, the starting materials \u2014 two chlorobenzenes and one trichloroacetaldehyde, which 
amount to a combined atomic total of C14H11Cl5O \u2014 turn into DDT, with the molecular formula 
C14H9Cl5. Conclusion: The elements of H2O are extruded.
3. Topologically, the transformation constitutes a replacement of an aromatic hydrogen by an 
alkyl carbon substituent, strongly implicating the occurrence of a Friedel-Crafts alkylation 
 (Section 15-12).
We can now address the details of a possible mechanism. Friedel-Crafts alkylations require pos-
itively polarized or cationic carbon electrophiles. In our case, inspection of the product suggests that 
the carbonyl carbon in the aldehyde is the electrophilic aggressor. This carbon is positively polarized 
to begin with, because of the presence of the electron-withdrawing oxygen and chlorine substituents. 
A nonoctet dipolar resonance form (Section 1-5) illustrates this point.
Activation of 2,2,2-Trichloroacetaldehyde as an Electrophile
Octet Dipolar
nonoctet
H\ufffd
\u161ð ð
C\ufffd
\ufffd
O
HCl C3
HE
Hydroxycarbocation Protonated
aldehyde
ð ð
C
O
HCl C3
HE
\ufffd
ð
C
OH
HCl C3
HE
\u161ð
C\ufffd
OH
HCl C3
HE
In the presence of strong acid, protonation of the negatively polarized oxygen of the neutral species 
generates a positively charged intermediate in which the electrophilic character of the carbonyl carbon 
is further accentuated in the resonance form depicting a hydroxy carbocation. The stage is now set 
for the fi rst of two electrophilic aromatic substitution steps (Section 15-11).
Cl H Cl C
OH
CCl3
HCl\ufffd CCl3C
\ufffd \ufffd
OH
First Electrophilic Aromatic Substitution Step
A A
A
\u161ð
ZZ
ZZ
y
y
y
y
C
HHO
H \ufffdH
\ufffd
G
A A
HCl C3
HE
The product of this step is an alcohol, which can be readily converted into the corresponding carbo-
cation by acid (Section 9-2), partly because the ensuing charge is resonance stabilized by the adjacent 
benzene ring (benzylic resonance, Section 22-1, related to allylic resonance, Sections 14-1 and 14-3). 
The cation subsequently enables the second electrophilic aromatic substitution step to provide DDT. 
Another more advanced mechanism is the subject of Problem 66.
Cl C
OH
CCl3
H
H\ufffd
\ufffdH2O
Alcohol Activation and Second Electrophilic Aromatic Substitution Step
A
A
Cl
Cl C DDT
Cl3C
H
A
\ufffd
ZZ
ZZ
y
yyy
Cl
H
\ufffdH\ufffd
Cl C
CCl3
H
\ufffd
i
f
Cl C etc.
CCl3
H
\ufffd
C h a p t e r I n t e g r a t i o n P r o b l e m s
720 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
New Reactions
1. Hydrogenation of Benzene (Section 15-2)
H2, catalyst
\ufffdH° \ufffd \ufffd49.3 kcal mol\ufffd1
Resonance energy: ~\ufffd30 kcal mol\ufffd1
Electrophilic Aromatic Substitution
2. Chlorination, Bromination, Nitration, and Sulfonation (Sections 15-9 and 15-10)
C6H6 C6H5X HX X \ufffd Cl, Br\ufffd
X2, FeX3
C6H6 C6H5NO2 H2O\ufffd
HNO3, H2SO4
C6H6 C6H5SO3H Reversible
SO3, H2SO4
H2SO4, H2O, \u394
3. Benzenesulfonyl Chlorides (Section 15-10)
C6H5SO3Na C6H5SO2ClPCl5\ufffd POCl3 NaCl\ufffd \ufffd
4. Friedel-Crafts Alkylation (Section 15-11)
C6H6 C6H5RRX
AlCl3
\ufffd HX\ufffd overalkylated product
R\ufffd is subject to carbocation rearrangements
\ufffd
Intramolecular
Cl
AlCl3
\ufffd HCl
Alcohols and alkenes as substrates
C6H6 C6H5CHRRCHR	
OH
BF3, 60°C
\ufffdH2O
\ufffd
A
R	
A
C6H6 C6H5CHCH3CH2RCH
HF, 0°C
\ufffd
R
A
P
5. Friedel-Crafts Acylation (Section 15-13)
Acyl halides
C6H6
Requires at least one full equivalent of Lewis acid
RCCl\ufffd C6H5CR
O
HCl\ufffd
1. AlCl3
2. H2O B
O
B
Anhydrides
C6H6 CH3COCCH3 C6H5CCH3 CH3COOH\ufffd
O
B
O
B
O
\ufffd
B1. AlCl32. H2O
Re
ac
tio
ns
 of
 Be
nz
en
e a
nd
 Su
bs
tit
ut
ed
 Be
nz
en
es
se
cti
on
 nu
mb
er
H
2,
 
Pt
(R
\ufffd
H
)
14
-7
, 1
5-
2
R
X
2,
 
Fe
X
3
(R
\ufffd
H
)
15
-9X
SO
3,
 
H
2S
O
4
(R
\ufffd
H
)
15
-1
0
SO
3H
R
	X
, A
lX
3
(R
\ufffd
H
)
15
-1
1
R
	
Se
c.
 
o
r 
te
rt
.
RO
H
,
 
H
\ufffd
 o
r 
B
F 3
(R
\ufffd
H
)
15
-1
1
R
	
R
	C
H
P
CH
2,
 
H
F
(R
\ufffd
H
)
15
-1
1R
	
E
H
H
3C
15
-1
3
N
R
	
H
O
R
	C
Cl
, A
lC
l 3
(R
\ufffd
H
)
BO
15
-1
3
N
R
	
H
O
R
	C
O
CR
	,
 A
lC
l 3
(R
\ufffd
H
)
BO
BO
E\ufffd
(R
\ufffd
do
no
r
o
r 
ha
lo
ge
n)
16
-2
, 1
6-
3
R
E
H
R E
\ufffd
E\ufffd
(R
\ufffd
ac
ce
pt
or
)
16
-2
, 1
6-
3
R
E
H
H
Cl
, Z
n(H
g)
(R
\ufffd
N
O
2)
16
-5N
H
2
CF
3C
O
3H
(R
\ufffd
N
H
2)
16
-5N
O
2
16
-5
H
Cl
, Z
n(H
g)
(R
\ufffd
R
	C
)BOR	
H
H
2O
, \ufffd
(R
\ufffd
SO
3H
)
16
-5
(R
\ufffd
R
	C
H
2)
X
2,
 
h
22
-1
R
	
E
H
X
(R
\ufffd
R
	C
H
L)
R
\ufffdO
H
,
so
lv
o
ly
sis
22
-1 O
R\ufffd
E
H
R
	
22
-1 N
u
H
(R
\ufffd
CH
2L
)
\ufffd
 
 N
u,
 S
N
2
)
(R
\ufffd
CH
3)
R
	L
i
22
-1 CH
2
)
\ufffd
(R
\ufffd
R
	C
H
2)
K
M
nO
4,
 
\ufffd
O
H
22
-2
CO
O
H
(R
\ufffd
R
	C
H
O
H
)
M
nO
2
22
-2
N
R
	
H
O
(R
\ufffd
CH
2O
R	
)
H
2,
 
Pd
\u2013C
22
-2
CH
3
R
	O
H
\ufffd
Es
pe
ci
al
ly
w
ith
 e
-
po
or
ar
en
es
22
-4
, 2
6-
5
N
u
(R
\ufffd
L)
\ufffd
 
 N
u
)
22
-4
(R
\ufffd
N
2\ufffd
)
H
2O
, \ufffd O
H
22
-6
(R
\ufffd
O
H
)
N
aO
H
, C
O
2
H
O
CO
O
H
H
22
-7
(R
\ufffd
O
CH
2C
H
P
CH
2)
\ufffd
H
O
22
-1
0
X
(C
N)
(R
\ufffd
N
2\ufffd
)
Cu
X
 o
r C
uC
N
, \ufffd
 
22
-1
0
F 
(R
\ufffd
N
2\ufffd
B
F 4
\ufffd
)
\ufffd
 
22
-1
0
(R
\ufffd
N
2\ufffd
)
H
3P
O
2 
22
-1
1
O
H
(R
\ufffd
N
2\ufffd
)
 
O
H
N
N
N
(R
\ufffd
al
ky
l)
Cl
CH
2O
CH
2C
H
3,
Sn
Cl
4 
R
 
 
CH
2C
l
26
-7
H
N