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  1. 1. Arenes and Aromaticity
  2. 2. 11.1 Benzene
  3. 3. Some history <ul><li>1825 Michael Faraday isolates a new hydrocarbon from illuminating gas. </li></ul><ul><li>1834 Eilhardt Mitscherlich isolates same substance and determines its empirical formula to be C n H n . Compound comes to be called benzene . </li></ul><ul><li>1845 August W. von Hofmann isolates benzene from coal tar. </li></ul><ul><li>1866 August Kekul י proposes structure of benzene. </li></ul>
  4. 4. בנזן - נוסחה אמפירית C 6 H 6 דוגמה לנוסחה אמפירית כזו בה היחס בין הפחמן למימן הוא 1:1 היא זו של האצטילן - C 2 H 2 HC=CH שוני גדול בין השניים - בניגוד לאצטילן בנזן לא מגיב ל : HCl, KMnO 4 , Br 2 , H 2 O/acid , אינו עובר תגובות של סיפוח שראינו באלקנים .
  5. 5. Kekul י and the Structure of Benzene
  6. 6. <ul><li>Kekul י proposed a cyclic structure for C 6 H 6 with alternating single and double bonds. </li></ul>Kekul י Formulation of Benzene H H H H H H
  7. 7. <ul><li>Later, Kekul י revised his proposal by suggesting a rapid equilibrium between two equivalent structures . </li></ul>Kekul י Formulation of Benzene H H H H H H H H H H H H
  8. 8. <ul><li>However, this proposal suggested isomers of the kind shown were possible. Yet, none were ever found. </li></ul>Kekul י Formulation of Benzene H X X H H H H X X H H H
  9. 9. <ul><li>Structural studies of benzene do not support the Kekul י formulation. Instead of alternating single and double bonds, all of the C—C bonds are the same length. </li></ul>Structure of Benzene Benzene has the shape of a regular hexagon.
  10. 10. All C—C bond distances = 140 pm 140 pm 140 pm 140 pm 140 pm 140 pm 140 pm
  11. 11. All C—C bond distances = 140 pm <ul><li>140 pm is the average between the C—C single bond distance and the double bond distance in 1,3-butadiene. </li></ul>140 pm 140 pm 140 pm 140 pm 140 pm 140 pm 146 pm 134 pm
  12. 12. The Stability of Benzene <ul><li>benzene is the best and most familiar example of a substance that possesses &quot;special stability&quot; or &quot;aromaticity&quot; </li></ul><ul><li>aromaticity is a level of stability that is substantially greater for a molecule than would be expected on the basis of any of the Lewis structures written for it </li></ul>
  13. 13. <ul><li>heat of hydrogenation: compare experimental value with &quot;expected&quot; value for hypothetical &quot;cyclohexatriene&quot; </li></ul>Thermochemical Measures of Stability  H°= – 208 kJ + 3H 2 Pt
  14. 14. Figure 11.2 (p 404) <ul><li>&quot;expected&quot; heat of hydrogenation of benzene is 3 x heat of hydrogenation of cyclohexene </li></ul>120 kJ/mol 360 kJ/mol 3 x cyclohexene
  15. 15. Figure 11.2 (p 404) <ul><li>observed heat of hydrogenation is 152 kJ/mol less than &quot;expected&quot; </li></ul><ul><li>benzene is 152 kJ/mol more stable than expected. 152 kJ/mol (36K cal/mol) is the resonance energy of benzene </li></ul>208 kJ/mol 360 kJ/mol 3 x cyclohexene
  16. 16. אנטלפית השריפה של בנזן חום השריפה מחושב - Kcal/mol -827 חום השריפה שמתקבל בפועל - 789 Kcal/mol - הפרש של 38 ק”קל למול
  17. 17. A Resonance Picture of Bonding in Benzene
  18. 18. <ul><li>Instead of Kekul י 's suggestion of a rapid equilibrium between two structures: </li></ul>Kekul י Formulation of Benzene H H H H H H H H H H H H
  19. 19. <ul><li>express the structure of benzene as a resonance hybrid of the two Lewis structures. Electrons are not localized in alternating single and double bonds, but are delocalized over all six ring carbons. </li></ul>Resonance Formulation of Benzene H H H H H H H H H H H H
  20. 20. <ul><li>Circle-in-a-ring notation stands for resonance description of benzene (hybrid of two Kekul י structures) </li></ul>Resonance Formulation of Benzene
  21. 21. Orbital Hybridization Model of Bonding in Benzene <ul><li>Planar ring of 6 sp 2 hybridized carbons </li></ul>Figure 11.3
  22. 22. Orbital Hybridization Model of Bonding in Benzene <ul><li>Each carbon contributes a p orbital </li></ul><ul><li>Six p orbitals overlap to give cyclic  system; six  electrons delocalized throughout  system </li></ul>Figure 11.3
  23. 23. Orbital Hybridization Model of Bonding in Benzene <ul><li>High electron density above and below plane of ring </li></ul>Figure 11.3
  24. 24. מושג הארומטיות <ul><li>ההשערה שהיציבות הארומטית של הבנזן נובעת מקיום מצבי רזוננס </li></ul><ul><li>מקרה הציקלובוטן והציקלו אוקטן . </li></ul><ul><li>תגובות של הלוגנציה , תגובה עם חומצה מלחית , תגובה עם פרמנגנט </li></ul><ul><li>תיאורית Huckel – מבנים עם : </li></ul><ul><li>4n + 2 , 4n  - electrons </li></ul>
  25. 25. תרכובות ארומטיות
  26. 26. Examples of Aromatic Hydrocarbons Benzene Toluene Naphthalene H H H H H H CH 3 H H H H H H H H H H H H H
  27. 27. 11.8 Polycyclic Aromatic Hydrocarbons
  28. 28. <ul><li>resonance energy = 255 kJ/mol </li></ul>Naphthalene most stable Lewis structure; both rings correspond to Kekulé benzene
  29. 29. Anthracene and Phenanthrene Anthracene Phenanthrene resonance energy: 347 kJ/mol 381 kJ/mol
  30. 30. An Orbital Hybridization View of Bonding in Benzene
  31. 31. Substituted Derivatives of Benzene and Their Nomenclature
  32. 32. <ul><li>1) Benzene is considered as the parent and comes last in the name. </li></ul>General Points
  33. 33. Examples Bromobenzene tert -Butylbenzene Nitrobenzene NO 2 C(CH 3 ) 3 Br
  34. 34. <ul><li>1) Benzene is considered as the parent and comes last in the name. </li></ul><ul><li>2) List substituents in alphabetical order </li></ul><ul><li>3) Number ring in direction that gives lowest locant at first point of difference </li></ul>General Points
  35. 35. Ortho, Meta, and Para alternative locants for disubstituted derivatives of benzene 1,2 = ortho (abbreviated o -) 1,3 = meta (abbreviated m -) 1,4 = para (abbreviated p -)
  36. 36. Examples o -ethylnitrobenzene NO 2 CH 2 CH 3 m -dichlorobenzene (1-ethyl-2-nitrobenzene) (1,3-dichlorobenzene) Cl Cl
  37. 37. Table 11.1 (p 407) Certain monosubstituted derivatives of benzene have unique names
  38. 38. Table 11.1 (p 407) Benzaldehyde CH O
  39. 39. Table 11.1 (p 407) Benzoic acid COH O
  40. 40. Table 11.1 (p 407) Styrene CH 2 CH
  41. 41. Table 11.1 (p 407) Anisole OCH 3
  42. 42. Table 11.1 (p 407) Acetophenone CCH 3 O
  43. 43. Table 11.1 (p 407) Phenol OH
  44. 44. Table 11.1 (p 407) Anisole OCH 3
  45. 45. Table 11.1 (p 407) Aniline NH 2
  46. 46. Easily confused names phenyl phenol benzyl OH CH 2 —
  47. 47. 11.9 Physical Properties of Arenes
  48. 48. <ul><li>Resemble other hydrocarbons </li></ul><ul><li>nonpolar </li></ul><ul><li>insoluble in water </li></ul><ul><li>less dense than water </li></ul>Physical Properties
  49. 49. Reactions of Arenes: Electrophilic Aromatic Substitution H E + E Y + H Y  +  –
  50. 50. 12.2 Mechanistic Principles of Electrophilic Aromatic Substitution
  51. 51. מנגנון התמרה אלקטרופילית ב - Alkenes
  52. 52. Step 1: attack of electrophile on  -electron system of aromatic ring <ul><li>highly endothermic </li></ul><ul><li>carbocation is allylic, but not aromatic </li></ul>H H H H H H E + H H H H H H E +
  53. 53. Step 2: loss of a proton from the carbocation intermediate <ul><li>highly exothermic </li></ul><ul><li>this step restores aromaticity of ring </li></ul>H H H E H H H + H H H H H H E +
  54. 54. H H H H H + E + H E H H H H + H + H H H H H H H E +
  55. 55. תגובות של בנזן
  56. 56. Electrophilic aromatic substitutions include: <ul><li>Nitration </li></ul><ul><li>Sulfonation </li></ul><ul><li>Halogenation </li></ul><ul><li>Friedel-Crafts Alkylation </li></ul><ul><li>Friedel-Crafts Acylation </li></ul>H E + E Y + H Y  +  –
  57. 57. Nitration of Benzene
  58. 58. Table 12.1: Nitration of Benzene + + H 2 O H 2 SO 4 HO NO 2 Nitrobenzene (95%) H NO 2
  59. 59. Nitration of Benzene + + H 2 O H 2 SO 4 HO NO 2 Electrophile is nitronium ion H NO 2 O N O •• + •• • • • •
  60. 60. Where does nitronium ion come from? H 2 SO 4 + O N H O O + •• •• •• • • • • • • •• – O N H O O + •• •• •• • • • • •• – H + O N O •• + •• • • • • H O •• H ••
  61. 61. Step 1: attack of nitronium cation on  -electron system of aromatic ring H H H H H H NO 2 + H H H H H H NO 2 +
  62. 62. Step 2: loss of a proton from the carbocation intermediate H H H NO 2 H H H + H H H H H H NO 2 + מצבים רזונטביים
  63. 63. Sulfonation of Benzene
  64. 64. Table 12.1: Sulfonation of Benzene + + H 2 O heat HO SO 2 OH Benzenesulfonic acid (100%) H SO 2 OH
  65. 65. Sulfonation of Benzene + + H 2 O heat HO SO 2 OH Several electrophiles present: a major one is sulfur trioxide H SO 2 OH O S O O + •• •• •• • • • • • • •• –
  66. 66. Step 1: attack of sulfur trioxide on  -electron system of aromatic ring H H H H H H SO 3 H H H H H H SO 3 – +
  67. 67. Step 2: loss of a proton from the carbocation intermediate H H H SO 3 – H H H + H H H H H H SO 3 – +
  68. 68. Step 3: protonation of benzenesulfonate ion H 2 SO 4 H H H SO 3 – H H H H H SO 3 H H H
  69. 69. Halogenation of Benzene
  70. 70. Table 12.1: Halogenation of Benzene + + H Br FeBr 3 Br 2 Bromobenzene (65-75%) H Br
  71. 71. Halogenation of Benzene + + H Br FeBr 3 Br 2 Electrophile is a Lewis acid-Lewis base complex between FeBr 3 and Br 2 . H Br
  72. 72. The Br 2 -FeBr 3 Complex <ul><li>The Br 2 -FeBr 3 complex is more electrophilic than Br 2 alone. </li></ul>+ Lewis base Lewis acid FeBr 3 Complex • • Br Br • • •• •• •• •• Br Br • • •• •• •• •• FeBr 3 – +
  73. 73. Step 1: attack of Br 2 -FeBr 3 complex on  -electron system of aromatic ring H H H H H H Br Br FeBr 3 – + + FeBr 4 – H H H H H H Br +
  74. 74. Step 2: loss of a proton from the carbocation intermediate H H H Br H H H + H H H H H H Br +
  75. 75. Friedel-Crafts Alkylation of Benzene
  76. 76. Table 12.1: Friedel-Crafts Alkylation of Benzene + + H Cl AlCl 3 tert -Butylbenzene (60%) (CH 3 ) 3 C Cl H C(CH 3 ) 3
  77. 77. <ul><li>acts as a Lewis acid to promote ionization of the alkyl halide </li></ul>Role of AlCl 3 (CH 3 ) 3 C Cl •• •• + AlCl 3 • • + (CH 3 ) 3 C Cl •• •• AlCl 3 –
  78. 78. Friedel-Crafts Alkylation of Benzene + + H Cl AlCl 3 Electrophile is tert -butyl cation (CH 3 ) 3 CCl H C(CH 3 ) 3 C CH 3 H 3 C H 3 C +
  79. 79. <ul><li>acts as a Lewis acid to promote ionization of the alkyl halide </li></ul>Role of AlCl 3 (CH 3 ) 3 C Cl •• •• + AlCl 3 + (CH 3 ) 3 C + • • + (CH 3 ) 3 C Cl •• •• AlCl 3 – Cl •• •• AlCl 3 – • •
  80. 80. Step 1: attack of tert-butyl cation on  -electron system of aromatic ring H H H H H H H H H H H H C(CH 3 ) 3 + C(CH 3 ) 3 +
  81. 81. Step 2: loss of a proton from the carbocation intermediate H H H C(CH 3 ) 3 H H H + H H H H H H C(CH 3 ) 3 +
  82. 82. Friedel-Crafts Acylation of Benzene
  83. 83. Table 12.1: Friedel-Crafts Acylation of Benzene + + H Cl AlCl 3 1-Phenyl-1-propanone (88%) H O CH 3 CH 2 C Cl CCH 2 CH 3 O
  84. 84. Friedel-Crafts Acylation of Benzene + + H Cl AlCl 3 O CH 3 CH 2 CCl CCH 2 CH 3 O Electrophile is an acyl cation H •• CH 3 CH 2 C O • • + CH 3 CH 2 C O • • +
  85. 85. Step 1: attack of the acyl cation on  -electron system of aromatic ring H H H H H H H H H H H H + O CCH 2 CH 3 + O CCH 2 CH 3
  86. 86. Step 2: loss of a proton from the carbocation intermediate H H H H H H + H H H H H H + O CCH 2 CH 3 O CCH 2 CH 3
  87. 87. Reactions Related to Friedel-Crafts Alkylation <ul><li>Cyclohexene is protonated by sulfuric acid, giving cyclohexyl cation which attacks the benzene ring </li></ul>H 2 SO 4 + Cyclohexylbenzene (65-68%) H
  88. 88. Substituent Effects in Electrophilic Aromatic Substitution: הכנסת מתמיר שני לטבעת
  89. 89. הכנסת מתמיר שני לטבעת <ul><li>שתי שאלות – </li></ul><ul><li>לאן המתמיר השני יכנס ? </li></ul><ul><li>האם כניסת המתמיר השני תהיה מהירה או איטית מכניסה של אותו מתמיר לבנזן עצמו ?? </li></ul>
  90. 90. Table 12.2 <ul><ul><li>Very strongly activating </li></ul></ul><ul><ul><li>Strongly activating </li></ul></ul><ul><ul><li>Activating </li></ul></ul><ul><ul><li>Standard of comparison is H </li></ul></ul><ul><ul><li>Deactivating </li></ul></ul><ul><ul><li>Strongly deactivating </li></ul></ul><ul><ul><li>Very strongly deactivating </li></ul></ul><ul><ul><li>Classification of Substituents in Electrophilic Aromatic Substitution Reactions </li></ul></ul>
  91. 91. <ul><li>1. All activating substituents are – OH, -OR, ortho-para directors. - R, NHR, NR2 </li></ul><ul><li>2. Halogen substituents are slightly deactivating but ortho-para directing. </li></ul><ul><li>3. Strongly deactivating substituents are meta directors. NO2, C=O, SO3H, </li></ul>Generalizations
  92. 92. <ul><li>are ortho-para directing and strongly activating </li></ul>Electron-Releasing Groups OR such as —OH, and —OR are strongly activating
  93. 93. <ul><li>occurs about 1000 times faster than nitration of benzene </li></ul>Nitration of Phenol HNO 3 + 44% 56% OH OH NO 2 OH NO 2
  94. 94. <ul><li>FeBr 3 catalyst not necessary </li></ul>Bromination of Anisole Br 2 90% acetic acid OCH 3 OCH 3 Br
  95. 95. <ul><li>All of these are ortho-para directing and strongly to very strongly activating </li></ul>Examples ERG = • • • • OH •• OR • • •• • • NH 2 • • NHR • • NR 2
  96. 96. מדוע קבוצות אלה גורמות לאקטיבציה ולהכוונה לעמדות ortho, para הסבר – שתי סיבות מולקולת האם מצבי הביניים לאחר כניסת המתמיר
  97. 97. Substituent Effects in Electrophilic Aromatic Substitution: Strongly Deactivating Substituents
  98. 98. <ul><li>All of these are meta directing and strongly deactivating </li></ul>Many EWGs Have a Carbonyl Group Attached Directly to the Ring — EWG = — NO 2 — SO 3 H O — CH O — CR O — COH O — COR O — CCl — C N
  99. 99. הסיבות להכוונה לעמדת meta ודאקטיבציה
  100. 100. Nitration of Benzaldehyde HNO 3 75-84% H 2 SO 4 CH O CH O O 2 N
  101. 101. Problem 12.14(a); page 468 Cl 2 62% FeCl 3 Cl CCl O CCl O
  102. 102. Disulfonation of Benzene SO 3 90% H 2 SO 4 SO 3 H HO 3 S
  103. 103. Bromination of Nitrobenzene Br 2 60-75% Fe Br NO 2 NO 2
  104. 104. 12.14 Substituent Effects in Electrophilic Aromatic Substitution: Halogens <ul><li>F, Cl, Br, and I are ortho-para directing, but deactivating </li></ul>גורמים לדאקטיבציה חלשה והכוונה לעמדות ortho, para
  105. 105. Nitration of Chlorobenzene <ul><li>The rate of nitration of chlorobenzene is about 30 times slower than that of benzene. </li></ul>HNO 3 + + 69% 1% 30% H 2 SO 4 Cl Cl NO 2 Cl NO 2 Cl NO 2
  106. 106. Multiple Substituent Effects
  107. 107. <ul><li>all possible EAS sites may be equivalent </li></ul>The Simplest Case AlCl 3 + CH 3 CH 3 CCH 3 99% O CH 3 COCCH 3 O CH 3 CH 3 O
  108. 108. <ul><li>directing effects of substituents reinforce each other; substitution takes place ortho to the methyl group and meta to the nitro group </li></ul>Another Straightforward Case CH 3 NO 2 Br 86-90% CH 3 NO 2 Br 2 Fe
  109. 109. Generalization <ul><li>regioselectivity is controlled by the most activating substituent </li></ul>
  110. 110. <ul><li>all possible EAS sites may be equivalent </li></ul>The Simplest Case Br 2 87% acetic acid strongly activating NHCH 3 Cl NHCH 3 Cl Br
  111. 111. <ul><li>position between two substituents is last position to be substituted </li></ul>Steric effects control regioselectivity when electronic effects are similar 98% HNO 3 H 2 SO 4 CH 3 CH 3 NO 2 CH 3 CH 3
  112. 112. 12.16 Regioselective Synthesis of Disubstituted Aromatic Compounds
  113. 113. Factors to Consider <ul><li>order of introduction of substituents to ensure correct orientation </li></ul>
  114. 114. Synthesis of m-Bromoacetophenone <ul><li>Which substituent should be introduced first? </li></ul>Br CCH 3 O CCH 3 O Br
  115. 115. Synthesis of m-Bromoacetophenone para meta Br CCH 3 O
  116. 116. Synthesis of m-Bromoacetophenone AlCl 3 Br 2 AlCl 3 CCH 3 O O CH 3 COCCH 3 O CCH 3 O Br
  117. 117. Factors to Consider <ul><li>order of introduction of substituents to ensure correct orientation </li></ul><ul><li>sometimes electrophilic aromatic substitution must be combined with a functional group transformation </li></ul>
  118. 118. Synthesis of p-Nitrobenzoic Acid from Toluene <ul><li>Which first? (oxidation of methyl group or nitration of ring) </li></ul>CH 3 CO 2 H NO 2 CH 3
  119. 119. Synthesis of p-Nitrobenzoic Acid from Toluene nitration gives m -nitrobenzoic acid oxidation gives p -nitrobenzoic acid CH 3 CO 2 H NO 2 CH 3
  120. 120. Synthesis of p-Nitrobenzoic Acid from Toluene HNO 3 H 2 SO 4 Na 2 Cr 2 O 7 , H 2 O H 2 SO 4 , heat CH 3 NO 2 CH 3 NO 2 CO 2 H

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