Vollhardt  Capítulo 16 (Ataque Eletrofílico + Benzeno)

Vollhardt Capítulo 16 (Ataque Eletrofílico + Benzeno)


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to the equivalent of \u201conly\u201d 
2 million tons of TNT.
Picric acid has some commercial applications other than 
as an explosive\u2014in matches, in the leather industry, in electric 
batteries, and in colored glass. It is called an acid because of 
the unusually high acidity of its hydroxy group (pKa 0.38; 
Section 22-3), which is increased beyond that of acetic acid 
(pKa 4.7) and even hydrogen fl uoride (pKa 3.2; Table 2-2) by 
the electron-withdrawing effect of the three nitro groups. This 
property was in part responsible for its replacement by TNT 
in military uses. For example, in artillery shells, it would 
corrode the casing and cause leakage, thus creating a hazard.
TNT is now rarely used pure, but more commonly in 
mixtures with other high-energy compounds, such as tetryl 
and RDX. In modern commercial applications, particularly 
mining and building demolition, TNT and picric acid have 
been replaced by nitroglycerine (Section 9-11). On the 
research front, chemists are continuing to explore novel struc-
tures. A case in point is octanitrocubane, synthesized in 2000, 
in which ring strain adds to the brisance of the compound. Its 
molecular formula, C8N8O16, indicates a composition condu-
cive to gaseous product formation (e.g., 8 CO2 1 4 N2) with 
an associated |1150-fold volume expansion and an estimated 
energy release of 830 kcal mol21 (3470 kJ mol21).
Spherical shock waves generated by the \ufb01 ring of the huge guns of 
the USS Iowa are clearly visible on the ocean surface.
NO2
NO2
O2N
2,4,6-Trinitrophenyl-
N-methylnitramine
(Tetryl)
N
NO2
NN
NO2
O2N
1,3,5-Trinitro-1,3,5-
triazacyclohexane
(RDX)
Octanitrocubane
NO2
NO2
NO2
NO2
O2N
O2N
O2N
O2N
N
O2N CH3H E
742 C h a p t e r 1 6 E l e c t r o p h i l i c A t t a c k o n D e r i v a t i v e s o f B e n z e n e
Groups that withdraw electrons by resonance deactivate and 
direct meta
Several groups deactivate the benzene ring by resonance (Section 16-1). An example is the 
carboxy group in benzoic acid, C6H5CO2H. Nitration of benzoic acid takes place at only 
about 1/1000th the rate of benzene nitration and gives predominantly meta substitution. The 
CO2H group is deactivating and, as in inductive deactivation (Section 16-2), meta directing.
CO2H
HNO3, H2SO4, \ufffd
\ufffdH2O
CO2H
Electrophilic Meta Nitration of Benzoic Acid
CO2H CO2H
NO2
\ufffd \ufffd
NO2
NO2
1.5%80%18.5%
2-Nitrobenzoic acid
(o-Nitrobenzoic acid)
3-Nitrobenzoic acid
(m-Nitrobenzoic acid)
4-Nitrobenzoic acid
(p-Nitrobenzoic acid)
Let us see how conjugation with the CO2H function affects the resonance forms of the 
cations resulting from electrophilic attack on benzoic acid.
Ortho, Meta, and Para Attack on Benzoic Acid
REACTION
ANIM
ATION ANIMATED MECHANISM: 
Electrophilic aromatic 
substitution of benzoic acid 
(ortho vs meta vs para)
Para attack
\ufffd
E\ufffd
E H
\ufffd \ufffd
E H E H
\ufffd
E H
Poor Poor
Strongly destabilized cation
C
HO Oð\u161\u161\ufffdH K C
HO Oð\u161\u161\ufffdH K C
HO Oð\u161\u161\ufffdH K\ufffd
\ufffd
C
HO\u161\ufffdH EOð\u161\ufffd
C
HO Oð\u161\u161\ufffdH HH
Meta attack
E\ufffd
\ufffd \ufffd
Less destabilized cation
None is poor
\ufffd
E
H
E
H
E
H
C
HO Oð\u161\u161\ufffdH K C
HO Oð\u161\u161\ufffdH K C
HO Oð\u161\u161\ufffdH K C
HO Oð\u161\u161\ufffdH K
Ortho attack
E\ufffd
\ufffd
Strongly destabilized cation
\ufffd
\ufffd \ufffd
\ufffd
\ufffd
PoorPoor
C
HO Oð\u161\u161\ufffdH K C
HO Oð\u161\u161\ufffdH K C
HO Oð\u161\u161\ufffdH K C
HO Oð\u161\u161\ufffdH C
HO\u161\ufffdH EOð\u161\ufffd
E
H
E
H
E
H
E
H
HH
 C h a p t e r 1 6 743
Attack at the meta position avoids placing the positive charge next to the electron-
withdrawing carboxy group, whereas ortho and para attacks necessitate the formulation of 
poor resonance contributors. Thus, while the substituent deactivates all positions, it does so 
to a greater extent at the ortho and para positions than at the meta positions. One might say, 
meta \u201cwins by default.\u201d
Exercise 16-8
Electrophilic nitration of nitrobenzene gives almost exclusively 1,3-dinitrobenzene. Formulate the 
(poor) resonance forms of the intermediate cations resulting from attack by NO2
1 at the ortho and 
para positions that explain this result.
There is always an exception: halogen substituents, although 
deactivating, direct ortho and para
Halogen substituents inductively withdraw electron density (Section 16-1); however, they 
are donors by resonance. On balance, the inductive effect wins out, rendering haloarenes 
deactivated. Nevertheless, the electrophilic substitution that does take place is mainly at the 
ortho and para positions.
Electrophilic Bromination of Bromobenzene Results
in ortho- and para-Dibromobenzene
Br Br Br Br
Br
Br
Br
13%
1,2-Dibromobenzene
(o-Dibromobenzene)
2%
1,3-Dibromobenzene
(m-Dibromobenzene)
85%
1,4-Dibromobenzene
( p-Dibromobenzene)
\ufffd \ufffd
Br\ufffdBr, FeBr3
\ufffdHBr
The competition between resonance and inductive effects explains this seemingly contra-
dictory reactivity. Again, we must examine the resonance forms for the various possible 
intermediates.
Ortho, Meta, and Para Attack on a Halobenzene
Ortho attack
X
E\ufffd
\ufffd
More stable cation
\ufffd
\ufffd
\ufffd
Strongly
contributing
all-octet form
ð\u161ð Xð\u161ð Xð\u161ð Xð\u161 Xð\u161ð
E
H
E
H
E
H
E
H
Meta attack
Xð\u161ð Xð\u161ð Xð\u161ð Xð\u161ð
E\ufffd
\ufffd
Less stable cation
\ufffd
\ufffd
H
E
H
E
H
E
REACTION
MECHANISM
1 6 - 3 D i r e c t i n g E f f e c t s o f S u b s t i t u e n t s i n C o n j u g a t i o n
744 C h a p t e r 1 6 E l e c t r o p h i l i c A t t a c k o n D e r i v a t i v e s o f B e n z e n e
Para attack
Xð\u161ð Xð\u161ðXð\u161ðXð\u161ð
E\ufffd
\ufffd
\ufffd
\ufffd
More stable cation
Strongly
contributing
all-octet form
Xð\u161
\ufffd
HE HE HE HE
Note that ortho and para attack lead to resonance forms in which the positive charge is placed 
next to the halogen substituent. Although this might be expected to be unfavorable, because 
the halogen is inductively electron withdrawing, resonance with the lone electron pairs allows 
the charge to be delocalized. Therefore, ortho and para substitutions become the preferred 
modes of reaction. The inductive effect of the halogen is still strong enough to make all 
three possible cations less stable than the one derived from benzene itself. Therefore, we 
have the unusual result that halogens are ortho and para directing, but deactivating.
This section completes the survey of the regioselectivity of electrophilic attack on mono-
substituted benzenes, summarized in Table 16-1. Table 16-2 ranks various substituents by 
their activating power and lists the product distributions obtained on electrophilic nitration 
of the benzene ring.
Exercise 16-9
Explain why (a) \u2013NO2, (b) 2N
1
R3, and (c) \u2013SO3H are meta directing. (d) Why should phenyl 
be activating and ortho and para directing (Table 16-1)? [Hint: Draw resonance forms for the 
appropriate cationic intermediates of electrophilic attack on phenylbenzene (biphenyl).]
Table 16-1 Effects of Substituents in Electrophilic Aromatic Substitution
Ortho and para directors
Moderate and strong activators
O
B
\u161
ðð
\ufffd \ufffdNH2\ufffd \ufffd \ufffdO\u161NR2O\u161NHR O\u161OH O\u161ORO\u161NHCR
O
\ufffd \ufffd
Weak activators Weak deactivators
Alkyl * phenyl O ð\u161\ufffdF O ð\u161\ufffdCl O ð\u161\ufffdBr O ð\u161\ufffdI\ufffd\ufffd\ufffd
Meta directors
Strong deactivators
ONO2 ONR3
\ufffd
\ufffd \ufffd
B
B
O
O
OSOH\ufffdOC N\ufffdOCF3 c\ufffd
B
OCOH
O
\ufffdO
B
COR
O
O
B
CR
O
\ufffd
Increasing activation
Increasing deactivation
 C h a p t e r 1 6 745
In Summary Activating groups, whether operating by induction or resonance, direct incom-
ing electrophiles to the ortho and para positions, whereas deactivating groups direct to the 
meta carbons. This statement is true for all classes of substituents except one\u2014the halogens. 
They are deactivating by induction, but they stabilize positive charges by resonance, there-
fore effecting ortho, para substitution.
16-4 Electrophilic Attack on Disubstituted Benzenes
Do the rules developed so far in this chapter predict the reactivity and regioselectivity of 
still higher substitution? We shall see that they do, provided we take into account the indi-
vidual effect of each substituent. Let us investigate the reactions of disubstituted benzenes 
with electrophiles.
The strongest activator wins out
In trying to predict the regioselectivity of electrophilic