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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 10-תרכובות ארומטיות
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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 תרכובת ארומטית היא תרכובת יציבה במיוחד 10-תרכובות ארומטיות
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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. 10-תרכובות ארומטיות
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Nonaromatic and Aromatic Compounds
מימנים יחסית חומציים-נוצר אניון ארומטי-יציב 10-תרכובות ארומטיות
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Resonance Contributors and the Resonance Hybrid
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Aromatic Compounds 10 e 14 e 18 e 10-תרכובות ארומטיות
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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) 10-תרכובות ארומטיות
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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. 10-תרכובות ארומטיות
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Resonance Contributors of Furan
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More Aromatic Heterocyclic Compounds
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Relative Stabilities 10-תרכובות ארומטיות
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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 תרכובת אנטי-ארומטית-לא יציבה במיוחד 10-תרכובות ארומטיות
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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 10-תרכובות ארומטיות
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Substituted Benzene Rings
Some Drugs Have Substituted Benzene Rings אנטי היסטמין הוא חומר המיועד לטיפול בתגובות אלרגיות, כך שהוא אמור להפחיתן באופן משמעותי. 10-תרכובות ארומטיות
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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 10-תרכובות ארומטיות
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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” 10-תרכובות ארומטיות
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The Nomenclature of Substituted Benzenes
some monosubstituted benzenes have names that incorporate the substituent 10-תרכובות ארומטיות
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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. 10-תרכובות ארומטיות
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or Substituted Benzene
Aryl Means Benzene or Substituted Benzene Each of the structures above could be abbreviated as ArOH. 10-תרכובות ארומטיות
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How Benzene Reacts Aromatic compounds such as benzene undergo electrophilic aromatic substitution reactions. 10-תרכובות ארומטיות
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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. 10-תרכובות ארומטיות
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Benzene Undergoes Substitution, Not Addition
Aromaticity is restored in the product from electrophilic substitution. 10-תרכובות ארומטיות
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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. 10-תרכובות ארומטיות
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An Electrophilic Aromatic Substitution Reaction
An electrophile (Y+) substitutes for H+. 10-תרכובות ארומטיות
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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. 10-תרכובות ארומטיות
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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. 10-תרכובות ארומטיות
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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. 10-תרכובות ארומטיות
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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). 10-תרכובות ארומטיות
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Iodination of Benzene The electrophile is generated differently.
Hydrogen peroxide is commonly used as the oxidizing agent. 10-תרכובות ארומטיות
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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. 10-תרכובות ארומטיות
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Nitration of Benzene 10-תרכובות ארומטיות
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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). 10-תרכובות ארומטיות
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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. 10-תרכובות ארומטיות
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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). 10-תרכובות ארומטיות
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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. 10-תרכובות ארומטיות
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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 10-תרכובות ארומטיות
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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. 10-תרכובות ארומטיות
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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. 10-תרכובות ארומטיות
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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. 10-תרכובות ארומטיות
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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. 10-תרכובות ארומטיות
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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. 10-תרכובות ארומטיות
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Generating the Electrophile for Friedel–Crafts Acylation
The acylium ion is the electrophile. 10-תרכובות ארומטיות
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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. 10-תרכובות ארומטיות
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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. 10-תרכובות ארומטיות
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Friedel–Crafts Alkylation
An alky halide is the source of the alkyl group. A Lewis acid (AlCl3) is required. 10-תרכובות ארומטיות
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Generation of the Electrophile for Friedel–Crafts Alkylation
A carbocation is the electrophile. 10-תרכובות ארומטיות
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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. 10-תרכובות ארומטיות
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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. 10-תרכובות ארומטיות
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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. 10-תרכובות ארומטיות
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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] 10-תרכובות ארומטיות
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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.) 10-תרכובות ארומטיות
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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 10-תרכובות ארומטיות
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התמרה אלקטרופילית ארומטית
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סיכום 10-תרכובות ארומטיות
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Alkyl Substituents are Oxidized
to Carboxyl Groups 10-תרכובות ארומטיות
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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.) 10-תרכובות ארומטיות
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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 10-תרכובות ארומטיות
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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. 10-תרכובות ארומטיות
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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.” 10-תרכובות ארומטיות
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Nomenclature of Disubstituted Benzenes
Some disubstituted benzenes have both substituents incorporated into the name. 10-תרכובות ארומטיות
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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. 10-תרכובות ארומטיות
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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. 10-תרכובות ארומטיות
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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. 10-תרכובות ארומטיות
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Resonance Electron Donation
A lone pair on an atom directly attached to the ring donates electrons by resonance. 10-תרכובות ארומטיות
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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. 10-תרכובות ארומטיות
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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. 10-תרכובות ארומטיות
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Moderately Activating Substituents
Moderately activating substituents donate electrons by resonance. 10-תרכובות ארומטיות
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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. 10-תרכובות ארומטיות
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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. 10-תרכובות ארומטיות
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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. 10-תרכובות ארומטיות
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Moderately Deactivating Substituents
All the moderately deactivating substituents withdraw electrons by resonance and withdraw electrons inductively. 10-תרכובות ארומטיות
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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. 10-תרכובות ארומטיות
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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? 10-תרכובות ארומטיות
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Ortho–Para Directors All activating substituents are ortho–para directors. Weakly deactivating substituents (halogens) are ortho–para directors. 10-תרכובות ארומטיות
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Meta Directors All moderately and strongly deactivating substituents are meta directors. 10-תרכובות ארומטיות
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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. 10-תרכובות ארומטיות
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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. 10-תרכובות ארומטיות
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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. 10-תרכובות ארומטיות
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השפעת המתמיר על ההכוונה
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Substituents on the Benzene Ring
Affect the pKa 10-תרכובות ארומטיות
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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) 10-תרכובות ארומטיות
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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) 10-תרכובות ארומטיות
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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) 10-תרכובות ארומטיות
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The size of a substituent affects the ortho–para ratio.
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Strongly Activating Group Present
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 10-תרכובות ארומטיות
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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. 10-תרכובות ארומטיות
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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. 10-תרכובות ארומטיות
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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. 10-תרכובות ארומטיות
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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. 10-תרכובות ארומטיות
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סדר התגובות חשוב 10-תרכובות ארומטיות
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חיזור ניטרו שאלה: פתרון: 10-תרכובות ארומטיות
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The Order of the Reactions is Important
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Designing a Synthesis סולפונציה-הפיכה 10-תרכובות ארומטיות
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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. 10-תרכובות ארומטיות
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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. 10-תרכובות ארומטיות
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The Synthesis of Trisubstituted Benzenes
Both substituents direct to different positions. The strong activator wins out over the weak activator. 10-תרכובות ארומטיות
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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. 10-תרכובות ארומטיות
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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. 10-תרכובות ארומטיות
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Arene Oxides העשרה 10-תרכובות ארומטיות
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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 10-תרכובות ארומטיות
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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. 10-תרכובות ארומטיות
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BENZO[a]PYRENE AND CANCER
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