Vollhardt  Capítulo 16 (Ataque Eletrofílico + Benzeno)

Vollhardt Capítulo 16 (Ataque Eletrofílico + Benzeno)


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and 
1-bromo-4-methylbenzene. Explain.
In Summary Electron-donating substituents inductively activate the benzene ring and direct 
electrophiles ortho and para; their electron-accepting counterparts deactivate the benzene 
ring and direct electrophiles to the meta positions.
16-3 Directing Effects of Substituents in Conjugation with the 
Benzene Ring
What is the infl uence of substituents whose electrons are in conjugation with those of the 
benzene ring? We can answer this question by again comparing the resonance forms of the 
intermediates resulting from the various modes of electrophilic attack.
Groups that donate electrons by resonance activate and direct
ortho and para
Benzene rings bearing the groups \u2013NH2 and \u2013OH are strongly activated. For example, 
halogenations of benzenamine (aniline) and phenol not only take place in the absence of 
catalysts but also are diffi cult to stop at single substitution. The reactions proceed very 
rapidly and, as in inductive activation (Section 16-2), furnish exclusively ortho- and para-
substituted products.
Br
Br
100%
Br
NH2
Electrophilic Brominations of Benzenamine (Aniline) and Phenol Give Ortho and Para Substitution
NH2
Benzenamine
(Aniline)
2,4,6-Tribromobenzenamine
(2,4,6-Tribromoaniline)
Br
Br
100%
Br
OH
2,4,6-Tribromophenol
3 Br
\ufffd3 HBr
Br, H2OO
OH
Phenol
3 Br
\ufffd3 HBr
Br, H2OO
 C h a p t e r 1 6 739
Better control of monosubstitution is attained with modifi ed amino and hydroxy sub-
stituents, such as in N-phenylacetamide (acetanilide) and methoxybenzene (anisole). These 
groups are ortho and para directing but less strongly activating (Section 16-5).
NO2
NO2
21%
Electrophilic Nitration of N-Phenylacetamide (Acetanilide)
HNCCH3
O
N-Phenylacetamide
(Acetanilide)
N-(2-Nitrophenyl)-
acetamide
(o-Nitroacetanilide)
Trace
\ufffd \ufffd
N-(3-Nitrophenyl)-
acetamide
(m-Nitroacetanilide)
NO2
79%
N-(4-Nitrophenyl)-
acetamide
(p-Nitroacetanilide)
HNO3, H2SO4, 20°C
\ufffdH2O
HNCCH3
O
HNCCH3
O
HNCCH3
O
Both the activated nature of these compounds and the observed regioselectivity on 
electrophilic substitution can be explained by writing resonance forms for the various inter-
mediate cations.
Ortho, Meta, and Para Attack on Benzenamine (Aniline)
Ortho attack
NH2
E\ufffd
\ufffd
Strongly stabilized cation
) NH2)
\ufffd
NH2)
\ufffd
NH2)
Strongly
contributing
all-octet form
\ufffdNH2
E
H
E
H
E
H
E
H
Meta attack
NH2) NH2) NH2) NH2)
E\ufffd
\ufffd \ufffd
\ufffd
E
H
E
H
E
H
Para attack
E\ufffd
\ufffd
\ufffd
\ufffd
Strongly stabilized cation
Strongly
contributing
all-octet form
NH2) NH2) NH2) NH2)\ufffdNH2
HE HE HE HE
MECHANISM
ANIM
ATION ANIMATED MECHANISM: 
Electrophilic aromatic 
substitution of benzenamine 
(ortho vs meta vs para)
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
740 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
Because nitrogen is more electronegative than carbon, the amino group in benzenamine 
(aniline) is inductively electron withdrawing (Section 16-1). However, the lone electron pair 
on the nitrogen atom may participate in resonance, thereby stabilizing the intermediate cations 
resulting from ortho and para (but not meta) substitutions. This resonance contribution out-
weighs the inductive effect. Compared to inductively directing alkyl groups (Section 16-2), the 
amino function provides not only an extra resonance contributor to the intermediate cation, but 
one that features an all-important Lewis octet. The result is a much reduced activation barrier 
for ortho or para attack. Consequently, benzenamine (aniline) is strongly activated toward 
electrophilic substitution relative to alkylbenzenes (and benzene), and the reaction is more 
highly regioselective as well, giving ortho and para substitution products almost exclusively.
Exercise 16-6
Working with the Concepts: Predicting Regiochemistry in Electrophilic Aromatic Substitutions
Predict the outcome of electrophilic aromatic substitution of methoxybenzene (anisole) by a general electrophile E1.
Strategy
In all problems addressing the potential regioselectivity of electrophilic substitution on substituted benzenes, there is a golden rule: When 
in doubt, write down all intermediates from all possible modes of attack before attempting to formulate a solution.
Solution
\u2022 In the case of methoxybenzene, this procedure results in the following cations:
Ortho attack
OCH3
E\ufffd
\ufffd \ufffd
\ufffdE
H
E E E
H H H
OCH3 OCH3 OCH3 \ufffdOCH3
Meta attack
E\ufffd
\ufffd \ufffd
\ufffd
E
H
E
H
E
H
OCH3 OCH3OCH3OCH3
Para attack
E\ufffd
\ufffd
\ufffd
\ufffd
HE HE HE HE
OCH3\ufffdOCH3OCH3OCH3OCH3
\u2022 How do they differ? You can immediately tell that ortho and para attacks give intermediates with four resonance forms, respectively, 
one of which in each case involves the lone electron pair on the methoxy group.
\u2022 In contrast, meta substitution proceeds through a cation described by only three resonance forms, none of which benefi ts from the 
 participation of the oxygen lone pair of electrons.
\u2022 Thus, substitution will occur exclusively ortho and para to the directing methoxy substituent in the starting material.
Exercise 16-7
Try It Yourself
In strongly acidic solution, benzenamine (aniline) becomes less reactive to electrophilic attack, 
and, while ortho, para attack is still dominant, increased meta substitution is observed. Explain. 
(Hint: The nitrogen atom in benzenamine may behave as a base. How would you classify the 
resulting substituent within the framework of the discussion in Section 16-1?)
 C h a p t e r 1 6 7411 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
CHEMICAL HIGHLIGHT 16-1
Explosive Nitroarenes: TNT and Picric Acid
Complete ortho, para nitration of methylbenzene (toluene) or 
phenol furnishes the corresponding trinitro derivatives, both 
of which are powerful explosives: TNT (discovered in 1863) 
and picric acid (1771). Both compounds have long histories 
as military and industrial explosives.
NO2
CH3
NO2
O2N NO2
OH
NO2
O2N
2-Methyl-1,3,5-trinitrobenzene
(2,4,6-Trinitrotoluene, TNT)
2,4,6-Trinitrophenol
(Picric acid)
Explosives are generally high-energy-density compounds 
capable of extremely rapid decomposition. In contrast to 
propellants (such as rocket fuel), they do not burn, but 
detonate under their own power. They frequently generate 
high heat and a large quantity of gaseous products, producing 
a (usually destructive) shock wave: TNT has a detonation 
velocity of 6940 m s21 (|4.3 mi s21). An explosion can be 
initiated by impact (including blasting caps), friction, heat 
and fl ame, electrical discharge (including static), and ultraviolet 
irradiation, depending on the compound. The nitro group 
features prominently in these materials, because it functions 
as an oxidizer to the surrounding carbon framework (producing 
the gases CO and CO2) and as a precursor to N2.
TNT is the most widely used military explosive in history. 
The reasons for its popularity are its low cost and simplicity 
of preparation, safe handling (low sensitivity to impact and 
friction), relatively high explosive power (yet good chemical 
and thermal stability), low volatility and toxicity, compatibility 
with other explosives, and a low melting point, allowing for 
melt-casted formulations.
TNT has become such a standard, particularly in military 
uses, that the destructive power of other explosives, especially 
in bombs, is often compared to that of an equivalent of 
TNT. For example, the fi rst atomic bomb\u2014detonated on 
July 16, 1945, in New Mexico\u2014had the equivalent power of 
19,000 tons of TNT. The device exploded over Hiroshima, 
Japan, which killed more than 140,000 people, had the 
power of 13,000 tons of TNT. Although these numbers 
appear huge, comparison with the hydrogen bomb\u2014with the 
destructive equivalent of 10 million tons of TNT\u2014dwarfs 
them. For further calibration, all of the explosions of World 
War II combined amounted