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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 תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 1 Showing Delocalized Electrons תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 2 Resonance Contributors Resonance Hybrid תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 3 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 © 2014 Pearson Education, Inc. תרכובות ארומטיות-10 4 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. תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 5 Nonaromatic and Aromatic Compounds מימנים יחסית חומציים-נוצר אניון ארומטי-יציב 6 -10תרכובות ארומטיות © 2014 Pearson Education, Inc. Resonance Contributors and the Resonance Hybrid תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 7 Aromatic Compounds 10 e 14 e תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 18 e 8 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) תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 9 Orbital Structure of Pyridine תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 10 Orbital Structure of Pyrrole and Furan תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 11 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. תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 12 Resonance Contributors of Furan תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 13 More Aromatic Heterocyclic Compounds תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 14 Relative Stabilities תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 15 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 תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 16 An MO Description of Aromaticity and Antiaromaticity תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 17 Many Substituted Benzenes are Found in Nature מרחיב סימפונות מצוי בעיקר בצמח הקקטוס, וגורם לשיכרון חושים ,הזיות חזותיות ושינויים קיצוניים במצב ההכרה 18 -10תרכובות ארומטיות © 2014 Pearson Education, Inc. Some Drugs Have Substituted Benzene Rings אנטי היסטמין הוא חומר המיועד לטיפול בתגובות אלרגיות ,כך שהוא אמור להפחיתן באופן משמעותי. 19 -10תרכובות ארומטיות © 2014 Pearson Education, Inc. 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 20 -10תרכובות ארומטיות © 2014 Pearson Education, Inc. Some Substituted Benzenes are Toxic 21 © 2014 Pearson Education, Inc. תרכובות ארומטיות-10 The Nomenclature of Substituted Benzenes some monosubstituted benzenes are named just by adding the name of the substituent to “benzene” תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 22 The Nomenclature of Substituted Benzenes some monosubstituted benzenes have names that incorporate the substituent תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 23 Phenyl and Benzyl Substituents תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 24 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. תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 25 Aryl Means Benzene or Substituted Benzene Each of the structures above could be abbreviated as ArOH. תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 26 How Benzene Reacts Aromatic compounds such as benzene undergo electrophilic aromatic substitution reactions. תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 27 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. תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 28 Benzene Undergoes Substitution, Not Addition Aromaticity is restored in the product from electrophilic substitution. תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 29 Benzene Undergoes Substitution, Not Addition The reaction of benzene with an electrophile forms the aromatic substitution product, not the nonaromatic addition product. 30 © 2014 Pearson Education, Inc. תרכובות ארומטיות-10 An Electrophilic Aromatic Substitution Reaction An electrophile (Y+) substitutes for H+. תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 31 The Mechanism for Electrophilic Aromatic Substitution תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 32 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. תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 33 Generating the Electrophile for Halogenation Donating a lone pair to a Lewis acid weakens the Br—Br or Cl—Cl bond. תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 34 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. תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 35 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). תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 36 Iodination of Benzene The electrophile is generated differently. Hydrogen peroxide is commonly used as the oxidizing agent. תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 37 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. תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 38 Nitration of Benzene תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 39 Generating the Electrophile for Nitration • Sulfuric acid protonates nitric acid. • Protonated nitric acid loses water to form the electrophile (the nitronium ion). תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 40 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. תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 41 Sulfonation of Benzene תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 42 Generating the Electrophile for Sulfonation • Sulfuric acid protonates sulfuric acid. • Protonated sulfuric acid loses water to form the electrophile (the sulfonium ion). תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 43 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. תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 44 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 תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 45 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. תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 46 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. תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 47 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. תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 48 Friedel–Crafts Acylation An acyl chloride or an acid anhydride is the source of the acyl group. A Lewis acid (AlCl3) is required. תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 49 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. תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 50 Generating the Electrophile for Friedel–Crafts Acylation The acylium ion is the electrophile. תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 51 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. תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 52 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. תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 53 Friedel–Crafts Alkylation An alky halide is the source of the alkyl group. A Lewis acid (AlCl3) is required. תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 54 Generation of the Electrophile for Friedel–Crafts Alkylation A carbocation is the electrophile. תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 55 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. תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 56 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. תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 57 Carbocation Rearrangement Leads to an Undesired Product תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 58 Carbocation Rearrangement Leads to an Undesired Product תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 59 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. תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 60 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] תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 61 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.) תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 62 Other Ways to Convert a Carbonyl Group to a Methylene Group תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 63 Alkylation of Benzene by Acylation–Reduction אלקילצית פרידל-קרפטס -יש שחלוף אצילצית פרידל-קרפטס -אין שחלוף -10תרכובות ארומטיות 64 התמרה אלקטרופילית ארומטית -10תרכובות ארומטיות 65 סיכום -10תרכובות ארומטיות 66 Alkyl Substituents are Oxidized to Carboxyl Groups תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 67 Alkyl Substituents Without Benzylic Hydrogens Cannot Be Oxidized (The first step in the oxidation reaction is removal of a benzylic hydrogen.) תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 68 Nitro Substituents are Reduced by Catalytic Hydrogenation תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 69 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 תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 70 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. תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 71 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.” תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 72 Nomenclature of Disubstituted Benzenes Some disubstituted benzenes have both substituents incorporated into the name. תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 73 Nomenclature of Polysubstituted Benzenes List substituents alphabetically and number so that the lowest possible numbers result. An incorporated substituent is given the 1-position. תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 74 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. תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 75 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. תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 76 Resonance Electron Donation A lone pair on an atom directly attached to the ring donates electrons by resonance. תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 77 Resonance Electron Withdrawal An atom directly attached to the ring that is doubly or triply bonded to an electronegative atom withdraws electrons by resonance. תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 78 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. תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 79 Moderately Activating Substituents Moderately activating substituents donate electrons by resonance. תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 80 Moderately Activating Substituents Moderately activating substituents are less effective than strongly activating substituents because they donate electrons in two competing directions. © 2014 Pearson Education, Inc. תרכובות ארומטיות-10 81 Weakly Activating Substituents Alkyl substituents donate electrons by hyperconjugation. Aryl and CH═CHR donate and withdraw electrons by resonance: electron donation is more important. תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 82 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. תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 83 Moderately Deactivating Substituents All the moderately deactivating substituents withdraw electrons by resonance and withdraw electrons inductively. תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 84 Strongly Deactivating Substituents All the strongly deactivating substituents (except ammonium ions) withdraw electrons inductively and by resonance. Ammonium ions strongly withdraw electrons inductively. תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 85 86 © 2014 Pearson Education, Inc. תרכובות ארומטיות-10 The Effect of Substituents on Orientation When an electrophilic aromatic substitution reaction occurs on a substituted benzene, where does the new substituent attach itself? תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 87 Ortho–Para Directors • All activating substituents are ortho–para directors. • Weakly deactivating substituents (halogens) are ortho–para directors. תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 88 Meta Directors All moderately and strongly deactivating substituents are meta directors. תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 89 Which Carbocation Intermediate is the Most Stable? For substituents that donate electrons by resonance, ortho or para substitution forms a relatively stable resonance contributor. © 2014 Pearson Education, Inc. תרכובות ארומטיות-10 90 Which Carbocation Intermediate is the Most Stable? For substituents that donate electrons inductively, ortho or para substitution forms a relatively stable resonance contributor. © 2014 Pearson Education, Inc. תרכובות ארומטיות-10 91 Which Carbocation Intermediate is the Most Stable? For substituents that withdraw electrons, ortho or para substitution forms a relatively unstable resonance contributor. © 2014 Pearson Education, Inc. תרכובות ארומטיות-10 92 השפעת המתמיר על ההכוונה -10תרכובות ארומטיות 93 Substituents on the Benzene Ring Affect the pKa תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 94 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) תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 95 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) תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 96 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) תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 97 The Ortho–Para Ratio The size of a substituent affects the ortho–para ratio. © 2014 Pearson Education, Inc. תרכובות ארומטיות-10 98 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 99 © 2014 Pearson Education, Inc. תרכובות ארומטיות-10 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. תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 100 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. תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 101 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. תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 102 The Order of the Reactions is Important The order of the reactions is important. תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 103 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. תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 104 סדר התגובות חשוב -10תרכובות ארומטיות 105 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 -10תרכובות ארומטיות 106 The Order of the Reactions is Important תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 107 Designing a Synthesis הפיכה-סולפונציה תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 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. תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 109 The Synthesis of Trisubstituted Benzenes Both substituents direct to equivalent positions. Addition between two substituents is a minor product because of steric hindrance. תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 110 The Synthesis of Trisubstituted Benzenes Both substituents direct to different positions. The strong activator wins out over the weak activator. תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 111 The Synthesis of Trisubstituted Benzenes Both substituents direct to different positions. The similar directing ability of the groups leads to addition at both positions. תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 112 The Synthesis of Cyclic Compounds Cyclic compounds are formed from intramolecular reactions. Formation of five- and six-membered rings are favored. תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 113 Arene Oxides העשרה תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 114 Two Pathways for Reaction תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 115 Mechanism for Formation of Phenol תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 116 The Epoxide Opens Preferentially in the Direction That Forms the More Stable Carbocation תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 117 Addition Products Can Be Carcinogenic If formation of the addition products is faster than formation of the phenol, the arene oxide can be carcinogenic. תרכובות ארומטיות-10 © 2014 Pearson Education, Inc. 118 BENZO[a]PYRENE AND CANCER 119 תרכובות ארומטיות-10