History of Benzene

History of Benzene 1824 - 1825 Isolated from burnt whale oil by Michael Faraday 1834 Eilhardt Mitscherlich finds benzene has formula of C6H6

History of Benzene 1824 - 1825 Isolated from burnt whale oil by Michael Faraday 1834 Eilhardt Mitscherlich finds benzene has formula of C6H6 1861 Josef Loschmidt proposes structure for benzoic acid and aniline

History of Benzene 1824 - 1825 Isolated from burnt whale oil by Michael Faraday 1834 Eilhardt Mitscherlich finds benzene has formula of C6H6 1861 Josef Loschmidt proposes structure for benzoic acid and aniline

History of Benzene 1865 Friedrich August Kekulé steps into the picture

History of Benzene 1865 Kekulé has a dream

History of Benzene 1865 Kekule has a dream 1865 Kekule invokes sausage diagrams Bulletin de la Société Chimique de France, 3, 98 (1865)

History of Benzene 1865 Kekule has a dream 1865 Kekule invokes sausage diagrams Bulletin de la Société Chimique de France, 3, 98 (1865) 1866 Kekule introduces ring model for benzene +

History of Benzene 1865 Kekule has a dream 1865 Kekule invokes sausage diagrams Bulletin de la Société Chimique de France, 3, 98 (1865) 1866 Kekule introduces ring model for benzene +

Alternate Benzene Structures

Figure 15.2: A comparison of the heats of hydrogenation for cyclohexene, 1,3-cyclohexadiene, and benzene. Benzene is 150 kJ/mol (36 kcal/mol) more stable than might be expected for “cyclohexatriene.”

Hückel’s Rules for aromaticity 1. 2 3. 4.

Benzene electron distribution map

Figure 15.12: An orbital picture and electrostatic potential map of naphthalene, showing that the ten π electrons are fully delocalized throughout both rings.

Hückel’s Rules for aromaticity Aromaticity vs. antiaromaticity

Why 4n + 2 for aromatic and 4n for antiaromatic? Hückel’s Rules for aromaticity Aromaticity vs. antiaromaticity Why 4n + 2 for aromatic and 4n for antiaromatic? Frost’s cycle

Benzene Molecular Orbitals Figure 15.3: The six benzene π molecular orbitals. The bonding orbitals ψ2 and ψ3 have the same energy and are said to be degenerate, as are the antibonding orbitals ψ4* and ψ5*. The orbitals ψ3 and ψ4* have no electron density on two carbons because of a node passing through these atoms.

Cyclopentadienyl ions

Figure 15.11: Energy levels of the five cyclopentadienyl molecular orbitals. Only the six-π-electron cyclopentadienyl anion has a filled-shell configuration leading to aromaticity.

Figure 15.5: An orbital view of the aromatic cyclopentadienyl anion, showing the cyclic conjugation and six π electrons in five p orbitals. The electrostatic potential map further indicates that the ion is symmetrical and that all five carbons are electron-rich (red).

1. methoxycyclobutadiene Aromatic Antiaromatic Neither

2. tropyllium Aromatic Antiaromatic Neither

Figure 15.7: Reaction of cycloheptatriene with bromine yields cycloheptatrienylium bromide, an ionic substance containing the cycloheptatrienyl cation. The electrostatic potential map shows that all seven carbon atoms are equally charged and electron-poor (blue).

3. pyrrole Aromatic Antiaromatic Neither

Figure 15.9: Pyrrole and imidazole are five-membered, nitrogen-containing heterocycles but have six π electron arrangements, much like that of the cyclopentadienyl anion. Both have a lone pair of electrons on nitrogen in a p orbital perpendicular to the ring.

4. imidazole Aromatic Antiaromatic Neither

Figure 15.9: Pyrrole and imidazole are five-membered, nitrogen-containing heterocycles but have six π electron arrangements, much like that of the cyclopentadienyl anion. Both have a lone pair of electrons on nitrogen in a p orbital perpendicular to the ring.

Figure 15. 14: The origin of aromatic ring-current Figure 15.14: The origin of aromatic ring-current. Aromatic protons are deshielded by the induced magnetic field caused by delocalized π electrons circulating in the molecular orbitals of the aromatic ring.

5. 14-annulene Aromatic Antiaromatic Neither

x x

6. 12-Annulene Aromatic Antiaromatic Neither

More than an electronic effect. Not the full show, just an introduction. Yes, there is a lot going on here.

Inside: d – 0.5 ppm Outside: d 6.9 – 7.3 ppm

CHE 311 Starts Here Section 9.6

Reactivity of Benzene Electrophilic Aromatic Substitution

Reactivity of Benzene Electrophilic Aromatic Substitution The Mechanism

Reactivity of Benzene Electrophilic Aromatic Substitution The electrophiles 1. Cl+ 2. Br+ 3. I+ 4. “F+” 5. NO2+ 6. SO3H+ 7. R+ (carbocation) 8. R-C O+

Reactivity of Benzene Substituent Group Effects on Electrophilic Aromatic Substitution Activators and deactivators

Reactivity of Benzene Substituent Group Effects on Electrophilic Aromatic Substitution Figure 9.16 Classification of substituent effects in electrophilic aromatic substitution. All activating groups are ortho- and para-directing, and all deactivating groups other than halogen are meta-directing. The halogens are unique in being deactivating but ortho- and para-directing.

Reactivity of Benzene Substituent Group Effects on Electrophilic Aromatic Substitution Directing Group Effects

Activation energy vs. substituent for EAS Y, directing effect

p. 340

p. 345

p. 338

Aromatic Biomolecules Amino acids Nucleic acids

Figure 15.8: Pyridine and pyrimidine are nitrogen-containing aromatic heterocycles with π electron arrangements much like that of benzene. Both have a lone pair of electrons on nitrogen in an sp2 orbital in the plane of the ring. Fig. 15-8, p. 528

Figure 15.9: Pyrrole and imidazole are five-membered, nitrogen-containing heterocycles but have six π electron arrangements, much like that of the cyclopentadienyl anion. Both have a lone pair of electrons on nitrogen in a p orbital perpendicular to the ring. Fig. 15-9, p. 529

p. 533

p. 529

p. 533

End