Chemistry of Benzene. Aromatic Electrophilic Substitution

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Chemistry of Benzene. Aromatic Electrophilic
Substitution Chapters 15
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Outline 1. Nomenclature and physical properties
of substituted benzenes 2. Reactivity of benzene
versus alkenes 3. Electrophilic substitution.
Halogenation. 4. Nitration 5. Sulfonation 6.
Alkylation 7. Acylation 8. Electrophilic
substitution of substituted benzenes.
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Outline 1. Nomenclature and physical properties
of substituted benzenes 2. Reactivity of benzene
versus alkenes 3. Electrophilic substitution.
Halogenation. 4. Nitration 5. Sulfonation 6.
Alkylation 7. Acylation 8. Electrophilic
substitution of substituted benzenes.
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1. Nomenclature and physical properties of
substituted benzenes
Substituents in the benzene ring are ranked and
numbered in the following order -COOH gt -CHO gt
-OH gt -Alkyl gt others Examples
Note Hydroxy-derivatives of benzene are called
phenols, not alcohols. Some derivatives also
have common names
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Boiling and points of substituted benzenes and
cyclohexenes are very close. Benzene
m.p. 5.5 oC, b.p. 80.1 oC Cyclohexene m.p. 6.6
oC, b.p. 80.7 oC Melting points of
para-derivatives usually much higher, than of
meta- and ortho-derivatives, because the
molecules of para-derivatives are packed easier
in the crystal lattice.
2. Reactivity of benzene versus alkenes
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Outline 1. Nomenclature and physical properties
of substituted benzenes 2. Reactivity of benzene
versus alkenes 3. Electrophilic substitution.
Halogenation. 4. Nitration 5. Sulfonation 6.
Alkylation 7. Acylation 8. Electrophilic
substitution of substituted benzenes.
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3. Electrophilic substitution (SEAr).
Halogenation.
a. Bromination
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b. Chlorination (same mechanism as for
bromination) Iodination does not occur this
way. Fluorination proceeds through a different
mechanism.
4. Nitration
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Outline 1. Nomenclature and physical properties
of substituted benzenes 2. Reactivity of benzene
versus alkenes 3. Electrophilic substitution.
Halogenation. 4. Nitration 5. Sulfonation 6.
Alkylation 7. Acylation 8. Electrophilic
substitution of substituted benzenes.
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SO3 can also be generated in situ by heating of
H2SO4. Sulfonation is reversible
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Outline 1. Nomenclature and physical properties
of substituted benzenes 2. Reactivity of benzene
versus alkenes 3. Electrophilic substitution.
Halogenation. 4. Nitration 5. Sulfonation 6.
Alkylation 7. Acylation 8. Electrophilic
substitution of substituted benzenes.
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Alkylation, catalyzed by aluminium chloride, can
be complicated by carbocation rearrangements
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Outline 1. Nomenclature and physical properties
of substituted benzenes 2. Reactivity of benzene
versus alkenes 3. Electrophilic substitution.
Halogenation. 4. Nitration 5. Sulfonation 6.
Alkylation 7. Acylation 8. Electrophilic
substitution of substituted benzenes.
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Outline 1. Nomenclature and physical properties
of substituted benzenes 2. Reactivity of benzene
versus alkenes 3. Electrophilic substitution.
Halogenation. 4. Nitration 5. Sulfonation 6.
Alkylation 7. Acylation 8. Electrophilic
substitution of substituted benzenes.
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Electronic effect of substituents at the benzene
ring
a. Inductive acceptor. The atom of the
substituent, connected to the benzene ring, has
higher electronegativity, than H. Examples
-OCH3, -NH2, -Cl, -NO2
b. Resonance acceptor. Conjugation between
p-orbitals is depicted by resonance structures
with the positive charge in the benzene ring.
Examples -COR, -NO2, -SO3H
c. Resonance donor. Conjugation between
p-orbitals is depicted by resonance structures
with the negative charge in the benzene ring.
Examples -OCH3, -NH2, -Cl, -phenyl
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d. Hyperconjugative donor. Conjugation, involving
s-orbitals, is depicted by non-classical
resonance structures (allowing breaking s-bonds)
with the negative charge in the benzene ring.
Examples -CH3, -Alkyl
e. Hyperconjugative acceptor. Conjugation,
involving s-orbitals, is depicted by
non-classical resonance structures (allowing
breaking s-bonds) with the positive charge in
the benzene ring. Examples -CF3
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Substituent effect on reactivity of substituted
benzenes
Electron donors increase reactivity in
SEAr Examples -CH3, -NR2, -OR, -CHCH2 Electron
acceptors decrease reactivity in SEAr Examples
-NO2, -NH3, -COR, -Cl
For substituents with opposite effects, the
resonance effect overrides all other
effects, except for chlorine and bromine, where
the inductive effect is the strongest.
Example of activation
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Directing effect of substituents (already present
at the benzene ring before substitution) on
substitution (Note the signs of resonance
refers to the structures before the substituent
is added)
All electron donors direct the incoming
substituent to the ortho- and para-positions
(regardless of any accepting effects) Examples
-CH3, -NR2, -OR, -Cl, -Br, -CHCH2
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(Note the signs of resonance refers to the
structures before the substituent is added)
Pure electron acceptors direct the incoming
substituent to the meta-position Examples -NO2,
-NH3, -COR, -CF3
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Examples of orientation
Consistent orientation

Inconsistent orientation A
donor substituent overrides an acceptor
substituent. A resonance
donor overrides a hyperconjugative donor