Aromatics H H H H H H

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2 Aromatics

3 Some istory 1825 Michael Faraday isolates a new hydrocarbon from illuminating gas Eilhardt Mitscherlich isolates same substance and determines its empirical formula to be C n n. Compound comes to be called benzene August W. von ofmann isolates benzene from coal tar August Kekulé proposes structure of benzene.

4 Greatest Scientist in istory?? Michael Faraday ( )

5 What in the World is Benzene?? C 6 6 discovered by Michael Faraday in 1825 Synthesized in 1834 from benzoic acid Remarkable chemical stability Unsaturation number is very high but. Does not add Bromine Substitutiuon with Br 2 / FeBr 3 Not oxidized by Permanganate or ozone No reaction with strong Br (aq) No reaction with ydrogen on Pd..??????

6 Addition vs Substitution a Mystery Br + Br 2 Br Addition of Bromine to Cyclohexene + Br 2 FeBr 3 Br + Br Substitution of Bromine for ydrogen in Benzene C Br 2 C 6 5 Br + Br

7 of ydrogenation Name ethylene propene 1-butene cis-2-butene trans-2-butene 2-methyl-2-butene 2,3-dimethyl-2-butene Structural Formula C 2 =C 2 C 3 C=C 2 C 3 C 2 C=C 2 C 3 C=CC 3 C 3 C=CC 3 (C 3 ) 2 C=CC 3 (C 3 ) 2 C=C(C 3 ) 2 (kcal/mol)

8 A quantitative measure of the special stability of benzene

9 Friedrich August Kekulé ( )

10 Friedrich August von Kekule had a dream of whirling snakes, and the structure of benzene. e reported the dream in the following words in 1890, in a speech at a dinner commemorating his discovery that reportedly occurred during a reverie on a London bus. Again the atoms were gamboling before my eyes. This time the smaller groups kept modestly to the background. My mental eye, rendered more acute by repeated vision of this kind, could not distinguish larger structures, tures, of manifold conformation; long rows, sometimes more closely fitted together; all twining and twisting in snakelike motion. But look! What was that? One of the snakes had seized hold of its own n tail, and the form whirled mockingly before my eyes. As if by a flash of lighting I awoke... Let us learn to dream, gentlemen. Arthur Koestler (in "The Act of Creation") called this incident "probably the most important dream in history since Joseph's seven fat and seven lean cows.

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12 QUESTION: Can you tell me more about this vision to which you have referred? KEKULÉ: Surely! Let me read to you the remarks I am about to make to the assembly today: During my stay in London I resided in Clapham Road...I frequently, however, spent my evenings with my friend ugo Mueller...We talked of many things but most often of our beloved chemistry. One fine summer evening I was returning by the last bus, riding outside as usual, through the deserted streets of the city...i fell into a reverie, and lo, the atoms were gamboling before my eyes. Whenever, hitherto, these diminutive beings had appeared to me, they had always been in motion. Now, however, I saw how, frequently, two smaller atoms united to form a pair: how a larger one embraced the two smaller ones; how still larger ones kept hold of three or even four of the smaller: whilst the whole kept whirling in a giddy dance. I saw how the larger ones formed a chain, dragging the smaller ones after them but only at the ends of the chains...the cry of the conductor: "Clapham Road," awakened me from my dreaming; but I spent a part of the night in putting on paper at least sketches of these dream forms.

13 Contributors to our understanding of the Structure of Benzene

14 Kekulé Formulation of Benzene Note bond lengths

15 Kekulé Formulation of Benzene Later, Kekulé revised his proposal by suggesting a rapid equilibrium between two equivalent structures.

16 Kekulé Formulation of Benzene owever, this proposal suggested isomers of the kind shown were possible. Yet, none were ever found. X X X X

17 All C C C C bond distances = 140 pm 140 pm 140 pm 146 pm 140 pm 140 pm 140 pm 140 pm 134 pm 140 pm is the average between the C C C C single bond distance and the double bond distance in 1,3- butadiene.

18 A Resonance Picture of Bonding in Benzene

19 Kekulé Formulation of Benzene Instead of Kekulé's suggestion of a rapid equilibrium between two structures:

20 Resonance Formulation of Benzene Pauling described 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.

21 Resonance Formulation of Benzene Circle-in in-a-ring notation stands for resonance description of benzene (hybrid of two Kekulé structures)

22 Unusual Stability of Benzene benzene is the best and most familiar example of a substance that possesses "special stability" or "aromaticity" aromaticity" Aromatic molecules have stability that is substantially greater for a molecule than would be expected on the basis of any of the Lewis structures written for it

23 Thermochemical Measures of Stability heat of hydrogenation: compare experimental value with "expected" value for hypothetical "cyclohexatriene" Pt = 208 kj [ kj / kcal ]

24 3 x cyclohexene 231 kj/mol 360 kj/mol 120 kj/mol 208 kj/mol

25 3 x cyclohexene 120 kj/mol "expected" heat of hydrogenation of benzene is 3 x heat of hydrogenation of cyclohexene 360 kj/mol

26 observed heat of hydrogenation is 152 kj/mol less than "expected" benzene is 152 kj/mol more stable than expected 152 kj/mol is the resonance energy of benzene 208 kj/mol?! 3 x cyclohexene 360 kj/mol

27 231 kj/mol hydrogenation of 1,3-cyclohexadiene (2 2 ) gives off more heat than hydrogenation of benzene (3 2 )!!! 208 kj/mol

28 Cyclic conjugation versus noncyclic conjugation 3 2 Pt heat of hydrogenation = 208 kj/mol 3 2 Pt heat of hydrogenation = 337 kj/mol

29 Resonance Energy of Benzene compared to localized 1,3,5-cyclohexatriene 152 kj/mol compared to 1,3,5-hexatriene 129 kj/mol exact value of resonance energy of benzene depends on what it is compared to, but regardless of model, benzene is more stable than expected by a substantial amount

30 Molecular Orbital View of Bonding in Benzene Dr. Anslyn will cover this in detail

31 Molecular Orbital Model of Bonding in Benzene igh electron density above and below plane of ring

32 Another Depiction of Benzene

33 Electron Density Map of Benzene

34 Benzene MOs

35 The Three Bonding π MOs of Benzene

36 Benzene MOs Antibonding orbitals Energy Bonding orbitals 6 p AOs combine to give 6 π MOs 3 MOs are bonding; 3 are antibonding

37 Benzene MOs Antibonding orbitals Energy Bonding orbitals All bonding MOs are filled No electrons in antibonding orbitals

38 The special stability of benzene results from the fact that these three bonding MOs are much lower in energy than the six uncombined 2p atomic orbitals.and the bonding orbitals are full

39 Frost Circles a Great Trick Inscribe a polygon of the same number of sides as the ring to be examined such that one of the vertices is at the bottom of the ring The relative energies of the MOs in the ring are given by where the vertices touch the circle The MOs below the horizontal line through the center of the ring are bonding MOs on the horizontal line are nonbonding MOs above the horizontal line are antibonding MOs

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41 π-mos of Cyclobutadiene (square planar) Antibonding Cyclo- butadiene Bonding 4 π electrons; bonding orbital is filled; other 2 π electrons singly occupy two nonbonding orbitals

42 Structure of Cyclobutadiene structure of a stabilized derivative is characterized by alternating short bonds and long bonds (C 3 ) 3 C C(C 3 ) 3 (C 3 ) 3 C 151 pm 138 pm CO 2 C 3

43 π-mos of Cyclooctatetraene (square planar) Antibonding Cyclo- octatetraene Bonding 8 p orbitals give 8 π orbitals 3 orbitals are bonding, 3 are antibonding,, and 2 are nonbonding

44 π-mos of Cyclooctatetraene (square planar) Antibonding Cyclo- octatetraene Bonding 8 π electrons; 3 bonding orbitals are filled; 2 nonbonding orbitals are each half-filled filled

45 Structure of Cyclooctatetraene cyclooctatetraene is not planar has alternating long (146 pm) and short (133 pm) bonds

46 eats of ydrogenation to give cyclohexane (kj/mol) heat of hydrogenation of benzene is 152 kj/mol less than 3 times heat of hydrogenation of cyclohexene

47 eats of ydrogenation to give cyclooctane (kj/mol) heat of hydrogenation of cyclooctatetraene is more than 4 times the heat of hydrogenation of cyclooctene.no special stability here!

48 Requirements for Aromaticity cyclic conjugation is necessary, but not sufficient not aromatic aromatic not aromatic

49 Conclusion there must be some factor in addition to cyclic conjugation that determines whether a molecule is aromatic or not

50 ückel's Rule the additional factor that influences aromaticity is the number of π electrons

51 among planar, monocyclic, completely conjugated polyenes,, only those with 4n π electrons possess special stability (are aromatic) n 4n benzene! ückel's Rule

52 ückel's Rule for Aromaticity To be Aromatic a compound must : 1. be Cyclic 2. have one P orbital on each atom in the ring 3. be planar or nearly so to give orbital overlap 4. have a closed loop of 4n+2 pi electrons in the cyclic arrangement of p orbitals

53 ückel's Rule defines a condition for cyclic molecules in which the bonding molecular orbitals are filled and there are no electrons in non-bonding or antibonding orbitals.roughly analogous to the rare gas condition fpr atomic orbitals

54 π-mos of Benzene Antibonding Benzene 6 π electrons fill all of the bonding orbitals all π antibonding orbitals are empty Bonding

55 π-mos of Cyclobutadiene (square planar) Antibonding Cyclo- butadiene Bonding 4 π electrons; bonding orbital is filled; other 2 π electrons singly occupy two nonbonding orbitals

56 π-mos of Cyclooctatetraene (square planar) Antibonding Cyclo- octatetraene Bonding 8 π electrons; 3 bonding orbitals are filled; 2 nonbonding orbitals are each half-filled filled

57 π-electron Requirement for Aromaticity 4 π electrons 6 π electrons 8 π electrons not aromatic aromatic not aromatic

58 Completely Conjugated Polyenes 6 π electrons; completely conjugated 6 π electrons; not completely conjugated aromatic not aromatic

59 Annulenes Annulenes are planar, monocyclic, completely conjugated polyenes.. That is, they are the kind of hydrocarbons treated by ückel's rule.

60 [10]Annulene predicted to be aromatic by ückel's rule, but too much angle strain when planar and all double bonds are cis 10-sided regular polygon has angles of 144

61 [10]Annulene strain between these two hydrogens incorporating two trans double bonds into the ring relieves angle strain but introduces another kind of strain into the structure and causes the ring to be distorted from planarity

62 ow about [14]Annulene?? 14 π electrons satisfies ückel's rule There is some strain between hydrogens inside the ring

63 [16]Annulene?? 16 π electrons does not satisfy ückel's rule alternating short (134 pm) and long (146 pm) bonds not aromatic

64 [18]Annulene The protons resonate at -3.0 and at 9.3δ!!! 18 π electrons satisfies ückel's rule resonance energy = 418 kj/mol bond distances range between pm

65 What about??