Introduction to Aromaticity Lecture Supplement page 45

Historical Background Street lamps fueled by gas derived from whale and cod oils Liquid named benzene Vapor density experiment reveals MW = 78; formula = C 6 H 6 C:H ratio 1:1 implies high reactivity like acetylene (HC  CH) However benzene is fairly inert:  Stable at room temperature  More resistant to catalytic hydrogenation (H 2 /Pt) than related substances Analyzed by Michael Faraday Conclusion: Benzene is not like related substances London, 1825 Unknown liquid condensed in pipes

Benzene Structure? Problem remained unsolved for 40 years, until two key ideas formulated: 328 C 6 H 6 isomers possible 1857: Fredrich Kekulé suggests carbon can form four bonds 1864: Alexander Crum Brown suggests carbon can form multiple bonds

The C 6 H 4 Cl 2 Isomer Problem None of these are the correct structure for benzene Dewar benzeneFulvene 2,4-Hexadiyne X Prismane X X X Structure must be consistent with experimental facts Fact: C 6 H 6 C 6 H 4 Cl 2 Three isomers Cl 2 Some benzene structure candidates: Six C 6 H 4 Cl 2 isomersFour C 6 H 4 Cl 2 isomersNine C 6 H 4 Cl 2 isomers Two C 6 H 4 Cl 2 isomers Thinkbook p.83 Question 2

Benzene Structure? Kekulé’s Dream 1866: Solution to the benzene problem comes to Kekulé in a dream. “During my stay in Ghent [Belgium] I resided in elegant bachelor quarters in the main thoroughfare. My study, however, faced a narrow side alley and no daylight penetrated it. For a chemist who spends his day in the laboratory this mattered little.” “I was sitting writing at my textbook but the work did not progress; my thoughts were elsewhere. I turned my chair to the fire and dozed. Again the atoms were gamboling before my eyes. This time the smaller groups kept modestly in the background. My mental eye, rendered more acute by repeated visions of the kind, could now distinguish larger structures of manifold conformation: long rows, sometimes more closely fitted together all twining and twisting in snakelike motion.”

Benzene Structure? Kekulé’s Dream “But look! What was that? One of the snakes had seized hold of its own tail, and the form whirled mockingly before my eyes. As if by a flash of lightning I awoke; and this time I also spent the rest of the night in working out the consequences of the hypothesis.” Kekulé benzene The Ouroboros

Problems with Kekulé Benzene Structure The Isomer Problem Observation: These 1,2-dichlorobenzene isomers never isolated or detected Conclusion: Kekulé structure cannot be accurate Bond length: C-C > C=C Therefore Kekulé structure suggests two isomers for 1,2-dichlorobenzene: Longer C-C bondShorter C-C bond

Problems with Kekulé Benzene Structure Kekulé’s solution to the isomer problem Separate structures cannot be isolated or detected Test: Regardless of equilibrium rate, C=C present  Expect typical C=C reactions very fast Isomers in very fast equilibrium

Problems with Kekulé Benzene Structure Test benzene in typical alkene reaction Addition reaction : Product has all atoms of reactant plus new groups Prediction for Kekulé benzene: Similar functional groups = similar reactions Alkene reaction: Both have pi bonds

Problems with Kekulé Benzene Structure But in fact... X No reaction occurs Catalyst required: Benzene less reactive than alkene Substitution (not addition) occurs Substitution reaction : Portion of reactant is replaced

Revised Kekulé Benzene Structure Very rapid equilibrium reminiscent of resonance Kekulé benzene “isomers” = resonance contributors These "isomers” have no discrete existence Benzene is not “just three alkenes in a ring” Resonance hybrid: or Prediction: All C-C bond lengths equal Verified by x-ray diffraction (Kathleen Lonsdale; 1928) How to revise Kekulé structure to be consistent with the actual properties?

Revised Kekulé Benzene Structure How can we explain its special stability? Resonance? Conjugation? How do we measure this special stability? Catalytic hydrogenation What is so special about Kekulé benzene?

Vollhardt, Figure 15.3 Calculations: Expected  H hyd, 1,3-cyclohexadiene = x 2 = kcal/mol Expected vs. Actual: = 2.3 kcal/mol (stabilization through conjugation) Expected  H hyd, “1,3,5-cyclohexatriene” = (-28.6 x 3) = kcal/mol Expected vs. Actual: 78.9 – 49.3 = 29.6 kcal/mol (stabilization through _________ ?????) Benzene is NOT simply a cyclic, conjugated triene! AROMATICITY Heats of Hydrogenation (  H hyd )

Measuring the Special Stability of Benzene  H = kcal mol -1 Compare cyclohexene and benzene via catalytic hydrogenation Observations Benzene +3 H 2 requires more heat and more pressure than cyclohexene + H 2  H (benzene + 3 H 2 ) = kcal mol -1 Prediction: If benzene is “just three alkenes”...  H = 3 x (cyclohexene + H 2 ) = 3 x = kcal mol -1 (~10 kcal mol -1 less if conjugation is included) Conclusions  H (benzene + 3 H 2 ) <  H [3 x (cyclohexene + H 2 )], so benzene more stable Extra stability = = 36.0 kcal mol -1 = Resonance energy = Aromaticity

Why Called “Aromatic”? Why is this special stability called “aromaticity”? Toluene glue Methyl salicylate oil of wintergreen Vanillin vanilla flavor Benzoic acid odorless Thiophenol skunky First molecules known to contain benzene ring have pleasant aromas; hence “aromatic” But not all benzene-containing molecules have pleasant odors

Is Benzene Ring the Only Aromatic Structure? Observation: Aromatic stability due to resonance of C=C in ring Cyclobutadiene C 4 H 4 Conclusion: Other rings with resonance might also be aromatic Examine other cyclic C n H n isomers with alternating pi bonds Many synthesis attempts failed 1965: Isolated in matrix at 4 K Cyclobutadiene is very reactive 1991: Isolated in a “carcerand” molecule (Donald Cram, UCLA) Unstable...not aromatic? Instability due to ring strain? Quantum mechanics: Instability due to two unpaired electrons Conclusion: No special stability; not aromatic

Is Benzene Ring the Only Aromatic Structure? Cyclooctatetraene C 8 H 8 Synthesis: 1911, Richard Willstätter Conclusion: No special stability; not aromatic Hydrogenation: Reacts easily with H 2 /Pt: Shape = tub

Is Benzene Ring the Only Aromatic Structure? Conclusion: Not all cyclic C n H n molecules are aromatic Aromatic Not aromatic Conclusion?

How to Predict Aromaticity? Benzene is not the only aromatic molecule Aromatic: Benzene Naphthalene Pyridine FuranCyclopentadienyl anion 1,3-Cyclohexadiene Cyclobutadiene Cyclooctatetraene Cyclopentadienyl cation Not aromatic: All the structures on this page are conjugated, but not all of them are aromatic!:

How to Predict Aromaticity? Studies on many molecules reveal three requirements for aromaticity Required: Closed loop of p orbitals (loop of sp 2 or sp atoms) Required: Atoms of closed loop must be planar ( p orbital overlap) ~30 kcal mol -1 in benzene To override planarity (and aromaticity) strain must be severe = 6 pi electrons (4n+2 = 6; n = 1) Required: Closed loop must contain 2, 6, 10, pi electrons (Hückel’s Rule) Series described by 4n+2 = pi electrons (where n = integer: 0, 1, 2, 3...) Six pi electrons is most common number (benzene) (n =1)

How to Predict Aromaticity? How much strain is too much? Stabilization from aromaticity Aromatic Not aromatic Destabilization from strain

Aromaticity: A Definition Aromaticity: Special stability possessed by a molecule which has 4n+2 (n = an integer) pi electrons contained within a closed loop of adjacent, parallel, overlapping p orbitals.

How to Predict Aromaticity? Additional examples Naphthalene Closed p orbital loop? Planar? No significant reason to be nonplanar  Pi electron count?  5 2 e - each = 10 (4n+2 = 10 when n = 2)  Conclusion: Naphthalene is predicted to be aromatic

How to Predict Aromaticity? Additional examples Furan Closed p orbital loop? Planar? No significant reason to be nonplanar Pi electron count?  2 2 e - each = 4 1 lone pair in oxygen p = 2 Total = 6 sp 3 ; no p orbital Build a model!  sp 2 ; one lone pair in p one lone pair in sp 2  Oxygen sp 3 Oxygen sp 2

How to Predict Aromaticity? Additional examples and requirements Example: Tropylium cation Unoccupied p orbital Example: Cyclopentadienone Oxygen not in closed loop These pi electrons do not add to aromaticity Is cyclopentadienone aromatic? Atoms must all lie in closed loop for their pi electrons to participate in aromaticity Is tropylium cation aromatic? Closed loop p orbitals do not have to be occupied p orbitals in a closed loop  p orbitals aligned (molecule is planar)  Pi electron count: 4n+2 = 6, where n = 1 (an integer)  YES p orbitals arranged in a closed loop  p orbitals aligned (molecule is planar)  Pi electron count: 4n+2 = 4, where n = integer X NO

Consequences of Aromaticity Aromatic rings are planar Aromatic rings are more stable than similar nonaromatic structures Biological example: DNA nucleobases are aromatic Planarity: More bases (more genetic information) into smaller space Chemical stability: Not easily degraded; effectively preserves DNA function Pi stacking: A noncovalent interaction; adds stability to the molecule

How to Predict Aromaticity? Benzene is not the only aromatic molecule Aromatic: Benzene Naphthalene Pyridine FuranCyclopentadienyl anion 1,3-Cyclohexadiene Cyclobutadiene Cyclooctatetraene Cyclopentadienyl cation Not aromatic: All the structures on this page are conjugated, but not all of them are aromatic!: