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Transcript

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