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The Structure of Benzene
Compounds like benzene, which have relatively few hydrogens in relation to the
number of carbons, are typically found in oils produced by trees and other plants.
Early chemists called such compounds aromatic compounds because of their pleasing fragrances
The chemical meaning of the word “aromatic” now signifies certain kinds of chemical structures
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Showing Delocalized Electrons
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Resonance Contributors
Resonance Hybrid
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Criteria for a Compound to Be Aromatic
Hückel’s Rule
• It must have an uninterrupted cloud of π electrons.
(cyclic, planar, every ring atom must have a p orbital).
• The π cloud must have an odd number of pairs of π electrons.
For a planar, cyclic compound to be aromatic, its uninterrupted  cloud must contain
(4n + 2)  electrons, where n is any whole number
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Examples of Compounds
That are Not Aromatic
Cyclobutadiene has an even number of pairs of π electrons.
Cyclooctatetraene has an even number of pairs of π electrons and
it is not planar.
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‫‪Nonaromatic and Aromatic Compounds‬‬
‫מימנים יחסית‬
‫חומציים‪-‬נוצר אניון‬
‫ארומטי‪-‬יציב‬
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Resonance Contributors and the
Resonance Hybrid
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Aromatic Compounds
10 e
14 e
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Heterocyclic Aromatic Compounds
A heterocyclic compound is a cyclic compound in which one or more of
the ring atoms is an atom other than carbon ( N,O,S)
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Orbital Structure of Pyridine
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Orbital Structure of Pyrrole and Furan
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Resonance Contributors of Pyrrole
The aromatic ring of pyridine is less electron dense than benzene
because the nitrogen withdraws electrons from the ring.
The aromatic ring of pyrrole is more electron dense than benzene
because the nitrogen donates electrons into the ring.
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Resonance Contributors of Furan
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More Aromatic
Heterocyclic Compounds
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Relative Stabilities
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Antiaromatic Compounds
A compound is antiaromatic if it is a planar, cyclic
compound with an uninterrupted ring of  cloud,
but it contains even number of pairs of  electrons
‫לא יציבה במיוחד‬-‫ארומטית‬-‫תרכובת אנטי‬
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An MO Description of
Aromaticity and Antiaromaticity
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‫‪Many Substituted Benzenes‬‬
‫‪are Found in Nature‬‬
‫מרחיב סימפונות‬
‫מצוי בעיקר בצמח הקקטוס‪,‬‬
‫וגורם לשיכרון חושים‪ ,‬הזיות חזותיות‬
‫ושינויים קיצוניים במצב ההכרה‬
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‫‪Some Drugs Have‬‬
‫‪Substituted Benzene Rings‬‬
‫אנטי היסטמין הוא חומר‬
‫המיועד לטיפול בתגובות‬
‫אלרגיות‪ ,‬כך שהוא אמור‬
‫להפחיתן באופן משמעותי‪.‬‬
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‫‪Substituted Benzene Rings Appear in‬‬
‫‪Many Commercial Compounds‬‬
‫הוא גורם לאיבוד התיאבון‪ ,‬אנרגיה ופעילות‬
‫מוגברות‪ ,‬ולתחושת בטחון והרגשת רווחה‪.‬‬
‫מדכא תיאבון‬
‫ממתיק מלאכותי‬
‫בכדור נפטלין ו‪-‬במטהר אוויר‬
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Some Substituted Benzenes are Toxic
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The Nomenclature of
Substituted Benzenes
some monosubstituted benzenes are named
just by adding the name of the substituent to “benzene”
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The Nomenclature of
Substituted Benzenes
some monosubstituted benzenes have names
that incorporate the substituent
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Phenyl and Benzyl Substituents
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Alkyl-Substituted Benzenes
• Name as alkyl-substituted benzenes when the alkyl group has a name.
• Name as phenyl-substituted alkanes otherwise.
• Toluene (methyl substituent on benzene) is an exception.
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Aryl Means Benzene
or Substituted Benzene
Each of the structures above could be abbreviated as ArOH.
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How Benzene Reacts
Aromatic compounds such as benzene undergo
electrophilic aromatic substitution reactions.
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Benzene Reacts with Electrophiles
The  electrons above and below the ring make benzene a nucleophile.
Benzene attacking an electrophile is like an
alkene attacking an electrophile.
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Benzene Undergoes Substitution,
Not Addition
Aromaticity is restored in the product from electrophilic substitution.
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Benzene Undergoes Substitution,
Not Addition
The reaction of benzene with an electrophile forms the
aromatic substitution product, not the nonaromatic addition product. 30
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An Electrophilic Aromatic Substitution
Reaction
An electrophile (Y+) substitutes for H+.
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The Mechanism for
Electrophilic Aromatic Substitution
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Halogenation of Benzene
Bromination or chlorination of benzene requires a Lewis acid catalyst
because benzene’s aromaticity causes it to be less reactive than an
alkene.
Ferric bromide (FeBr3) or ferric chloride (FeCl3) is usually used.
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Generating the Electrophile for Halogenation
Donating a lone pair to a Lewis acid weakens the Br—Br or Cl—Cl bond.
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The Mechanism for Halogenation
•
•
•
The electrophile adds to the benzene ring.
A base in the reaction mixture removes the proton from the carbon that
attacked the electrophile.
The catalyst is regenerated.
Chlorination occurs by an analogous mechanism.
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Catalysts for Halogenation
Because ferric halides react readily with moisture in the air,
they are generated the reaction mixture
from iron filings and bromine (or chlorine).
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Iodination of Benzene
The electrophile is generated differently.
Hydrogen peroxide is commonly used as the oxidizing agent.
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The Mechanism for Iodination
• The electrophile adds to the benzene ring.
• A base in the reaction mixture removes the proton from
the carbon that formed the bond with the electrophile.
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Nitration of Benzene
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Generating the Electrophile for Nitration
•
Sulfuric acid protonates nitric acid.
•
Protonated nitric acid loses water
to form the electrophile (the nitronium ion).
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The Mechanism for Nitration
• The electrophile adds to the benzene ring.
• A base in the reaction mixture removes the proton from
the carbon that formed the bond with the electrophile.
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Sulfonation of Benzene
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Generating the Electrophile for Sulfonation
• Sulfuric acid protonates sulfuric acid.
• Protonated sulfuric acid loses water to form the electrophile
(the sulfonium ion).
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The Mechanism for Sulfonation
• The electrophile adds to the benzene ring.
• A base in the reaction mixture removes the proton from the carbon
that formed the bond with the electrophile.
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Sulfonic Acids are Strong Acids
a sulfonic acid is a strong acid because
its conjugate base is particularly stable
the negative charge in the benzenesulfonate ion
is delocalized over three oxygens
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Sulfonation of Benzene is Reversible
If benzenesulfonic acid is heated in dilute acid,
an H+ adds to the ring and the sulfonic acid group comes off the ring.
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The Mechanism for Desulfonation
• The electrophile adds to the benzene ring.
• A base in the reaction mixture removes the proton from
the carbon that formed the bond with the electrophile.
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Friedel–Crafts Substitutions
Two electrophilic substitutions are named for the chemists
Charles Friedel and James Crafts.
• Friedel–Crafts acylation places an acyl group on a benzene ring.
• Friedel–Crafts alkylation places an alkyl group on a benzene ring.
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Friedel–Crafts Acylation
An acyl chloride or an acid anhydride is the source of the acyl group.
A Lewis acid (AlCl3) is required.
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The Mechanism for
Friedel–Crafts Acylation
• The electrophile adds to the benzene ring.
• A base in the reaction mixture removes the proton from
the carbon that formed the bond with the electrophile.
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Generating the Electrophile for
Friedel–Crafts Acylation
The acylium ion is the electrophile.
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AlCl3 Reacts with the Product of
Friedel–Crafts Acylation
More than one equivalent of AlCl3 must be used in a Friedel–Crafts acylation.
AlCl3 forms a complex with the carbonyl group in the product.
Water is added to the reaction mixture
to liberate the product from the complex.
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The Gatterman–Koch Reaction
Benzaldehyde cannot be made by a Friedel–Crafts acylation
because the needed acyl chloride (formyl chloride) is unstable.
Formyl chloride is generated in the reaction mixture.
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Friedel–Crafts Alkylation
An alky halide is the source of the alkyl group.
A Lewis acid (AlCl3) is required.
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Generation of the Electrophile for
Friedel–Crafts Alkylation
A carbocation is the electrophile.
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The Mechanism for
Friedel–Crafts Alkylation
• The electrophile adds to the benzene ring.
• A base in the reaction mixture removes the proton from the
carbon that formed the bond with the electrophile.
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There are Limits to Friedel–Crafts Alkylation
The alkylated benzene product is more reactive than benzene.
Therefore, a large excess of benzene is needed to avoid multiple
alkylations of the ring.
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Carbocation Rearrangement Leads to
an Undesired Product
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Carbocation Rearrangement Leads to
an Undesired Product
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Should We Draw Primary Carbocations?
Friedel–Crafts alkylation mechanisms are written as if a primary carbocation
is formed, although we know that primary carbocations are too unstable to
be formed.
A true primary carbocation is never formed.
Instead, an incipient carbocation is formed.
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A Biological Friedel–Crafts Alkylation
A Friedel–Crafts alkylation is one of the steps in the biosynthesis of
vitamin KH2, the coenzyme required to form blood clots.
[Pg. 922]
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Putting a Straight Chain Alkyl Group
on a Ring
• Add the correct number of carbons with a Friedel–Crafts acylation
•
reaction.
Reduce the carbonyl group to a methylene group.
(Catalytic hydrogenation reduces only a carbonyl group adjacent to a benzene ring.)
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Other Ways to Convert a Carbonyl Group
to a Methylene Group
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‫‪Alkylation of Benzene by Acylation–Reduction‬‬
‫אלקילצית פרידל‪-‬קרפטס‪ -‬יש שחלוף‬
‫אצילצית פרידל‪-‬קרפטס‪ -‬אין שחלוף‬
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‫התמרה אלקטרופילית ארומטית‬
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‫סיכום‬
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Alkyl Substituents are Oxidized
to Carboxyl Groups
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Alkyl Substituents Without Benzylic
Hydrogens Cannot Be Oxidized
(The first step in the oxidation reaction is removal of a benzylic hydrogen.)
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Nitro Substituents are Reduced
by Catalytic Hydrogenation
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Nomenclature of Disubstituted Benzenes
The relative positions of two substituents can be indicated
by numbers or by prefixes.
ortho = adjacent
meta = separated by one carbon
para = opposite one another
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Nomenclature of Disubstituted Benzenes
•
The substituents are listed in alphabetical order.
•
The first substituent is given the 1-position.
•
The ring is numbered to give the second substituent the lowest possible number.
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Nomenclature of Disubstituted Benzenes
•
Some substituents can be incorporated into a name.
•
An incorporated substituent is given the 1-position.
•
With a second substituent, “methylbenzene” is used rather than “toluene.”
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Nomenclature of Disubstituted Benzenes
Some disubstituted benzenes have both substituents incorporated into the name.
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Nomenclature of Polysubstituted Benzenes
List substituents alphabetically and number so that the
lowest possible numbers result.
An incorporated substituent is given the 1-position.
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The Effect of Substituents on Reactivity
•
Substituents that donate electron density to the benzene ring increase
benzene’s nucleophilicity and stabilize the carbocation intermediate.
•
Substituents that withdraw electron density to the benzene ring
decrease benzene’s nucleophilicity and destabilize the carbocation intermediate.
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Inductive Electron Withdrawal and Donation
•
A substituent more electronegative than a hydrogen withdraws σ electrons
inductively from the benzene ring more than a hydrogen will.
•
An alkyl group donates electrons by hyperconjugation.
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Resonance Electron Donation
A lone pair on an atom directly attached to the ring
donates electrons by resonance.
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Resonance Electron Withdrawal
An atom directly attached to the ring that is doubly or triply bonded to
an electronegative atom withdraws electrons by resonance.
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Strongly Activating Substituents
All the strongly activating substituents donate electrons by resonance.
All the strongly activating substituents withdraw electrons inductively.
Because the substituents are activating, electron donation by resonance is more
significant than inductive electron withdrawal.
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Moderately Activating Substituents
Moderately activating substituents donate electrons by resonance.
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Moderately Activating Substituents
Moderately activating substituents are less effective than
strongly activating substituents because they
donate electrons in two competing directions.
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Weakly Activating Substituents
Alkyl substituents donate electrons by hyperconjugation.
Aryl and CH═CHR donate and withdraw electrons by resonance:
electron donation is more important.
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Weakly Deactivating Substituents
All the weakly activating substituents donate electrons by resonance.
All the weakly activating substituents withdraw electrons inductively.
Because the substituents are deactivating, electron withdrawal
is more important than electron donation.
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Moderately Deactivating Substituents
All the moderately deactivating substituents withdraw electrons by
resonance and withdraw electrons inductively.
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Strongly Deactivating Substituents
All the strongly deactivating substituents (except ammonium ions)
withdraw electrons inductively and by resonance.
Ammonium ions strongly withdraw electrons inductively.
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The Effect of Substituents on Orientation
When an electrophilic aromatic substitution reaction occurs on a
substituted benzene, where does the new substituent attach itself?
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Ortho–Para Directors
• All activating substituents are ortho–para directors.
• Weakly deactivating substituents (halogens) are ortho–para directors.
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Meta Directors
All moderately and strongly deactivating substituents are meta directors.
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Which Carbocation Intermediate
is the Most Stable?
For substituents that donate electrons by resonance,
ortho or para substitution forms a relatively stable resonance contributor.
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Which Carbocation Intermediate
is the Most Stable?
For substituents that donate electrons inductively,
ortho or para substitution forms a relatively stable resonance contributor.
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Which Carbocation Intermediate
is the Most Stable?
For substituents that withdraw electrons,
ortho or para substitution forms a relatively unstable resonance contributor.
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‫השפעת המתמיר על ההכוונה‬
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Substituents on the Benzene Ring
Affect the pKa
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Substituents on the Benzene Ring
Affect the pKa of Phenol
electron donating groups decrease the acidity
(destabilize the conjugate base)
electron withdrawing groups increase the acidity
(stabilize the conjugate base)
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Substituents on the Benzene Ring
Affect the pKa of Benzoic Acid
electron donating groups decrease the acidity
(destabilize the conjugate base)
electron withdrawing groups increase the acidity
(stabilize the conjugate base)
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Substituents on the Benzene Ring
Affect the pKa of Protonated Aniline
electron donating groups decrease the acidity
(destabilize the conjugate base)
electron withdrawing groups increase the acidity
(stabilize the conjugate base)
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The Ortho–Para Ratio
The size of a substituent affects the ortho–para ratio.
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Halogenation with a
Strongly Activating Group Present
halogenation of a ring with a strongly activating substituent
does not require a catalyst
if a catalyst is used,
substitution occurs at all ortho and para positions
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Friedel–Crafts Reactions Do Not
Occur with Meta Directors
Friedel–Crafts reactions are the slowest of the electrophilic aromatic substitution
reactions and do not occur if the ring is moderately or strongly deactivated.
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Anilines Do Not Undergo
Friedel–Crafts Reactions
The lone pair on the amino group forms a complex with the Lewis acid
catalyst, which converts the substituent to a meta director.
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Aniline Must Be Protected in
Order to Be Nitrated
•
Aniline cannot be nitrated directly because nitric acid will oxidize an NH2 group.
•
If the amino group is protected by acetylation, the ring can be nitrated.
•
An acetyl group is removed by acid-catalyzed hydrolysis.
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102
The Order of the Reactions is Important
The order of the reactions is important.
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The Order of the Reactions is Important
The acetyl group must be added first because a
Friedel–Crafts acylation will not occur with a meta director on the ring.
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‫סדר התגובות חשוב‬
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‫חיזור ניטרו‬
‫‪N‬‬
‫‪H‬‬
‫‪2‬‬
‫שאלה‪:‬‬
‫‪Br‬‬
‫פתרון‪:‬‬
‫‪2‬‬
‫‪2‬‬
‫‪NH‬‬
‫‪1. Sn, H C l‬‬
‫‪-‬‬
‫‪Br‬‬
‫‪NO‬‬
‫‪2‬‬
‫‪F e B r3‬‬
‫‪2,‬‬
‫‪Br‬‬
‫‪2. H O‬‬
‫‪NO‬‬
‫‪3‬‬
‫‪H NO‬‬
‫‪4‬‬
‫‪H 2S O‬‬
‫‪Br‬‬
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The Order of the Reactions is Important
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Designing a Synthesis
‫הפיכה‬-‫סולפונציה‬
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108
The Synthesis of Trisubstituted Benzenes
The directing effects of both substituents on a disubstituted benzene
must be considered in deciding where the third group will add.
Both substituents direct to equivalent positions.
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The Synthesis of Trisubstituted Benzenes
Both substituents direct to equivalent positions.
Addition between two substituents is a minor product
because of steric hindrance.
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The Synthesis of Trisubstituted Benzenes
Both substituents direct to different positions.
The strong activator wins out over the weak activator.
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The Synthesis of Trisubstituted Benzenes
Both substituents direct to different positions.
The similar directing ability of the groups leads to
addition at both positions.
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The Synthesis of Cyclic Compounds
Cyclic compounds are formed from intramolecular reactions.
Formation of five- and six-membered rings are favored.
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Arene Oxides
‫העשרה‬
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Two Pathways for Reaction
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Mechanism for Formation of Phenol
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The Epoxide Opens Preferentially in the Direction
That Forms the More Stable Carbocation
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Addition Products Can Be Carcinogenic
If formation of the addition products is faster than formation of the phenol,
the arene oxide can be carcinogenic.
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BENZO[a]PYRENE AND CANCER
119
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