Dr. Wolf's CHM 201 & Chapter 11 Arenes and Aromaticity
Dr. Wolf's CHM 201 & BenzeneToluene Naphthalene Examples of Aromatic Hydrocarbons HHH HH H CH 3 H H HH H HH H HH HHH
Dr. Wolf's CHM 201 & Benzene
Dr. Wolf's CHM 201 & Some history 1834 Eilhardt Mitscherlich isolates a new hydrocarbon and determines its empirical formula to be C n H n. Compound comes to be called benzene August W. von Hofmann isolates benzene from coal tar August Kekulé proposes structure of benzene.
Dr. Wolf's CHM 201 & Kekulé and the Structure of Benzene
Dr. Wolf's CHM 201 & Kekulé proposed a cyclic structure for C 6 H 6 with alternating single and double bonds. Kekulé Formulation of Benzene HHH HH H
Dr. Wolf's CHM 201 & Later, Kekulé revised his proposal by suggesting a rapid equilibrium between two equivalent structures. Kekulé Formulation of Benzene HHH HH H HHH HH H
Dr. Wolf's CHM 201 & However, this proposal suggested isomers of the kind shown were possible. Yet, none were ever found. Kekulé Formulation of Benzene HXX HH H HXX HH H
Dr. Wolf's CHM 201 & Structural studies of benzene do not support the Kekulé 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.
Dr. Wolf's CHM 201 & pm All C—C bond distances = 140 pm
Dr. Wolf's CHM 201 & pm 146 pm 134 pm 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.
Dr. Wolf's CHM 201 & A Resonance Picture of Bonding in Benzene
Dr. Wolf's CHM 201 & Instead of Kekulé's suggestion of a rapid equilibrium between two structures: HHH HH H HHH HH H Kekulé Formulation of Benzene
Dr. Wolf's CHM 201 & 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 HHH HH H HHH HH H
Dr. Wolf's CHM 201 & Circle-in-a-ring notation stands for resonance description of benzene (hybrid of two Kekulé structures) Resonance Formulation of Benzene
Dr. Wolf's CHM 201 & 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
Dr. Wolf's CHM 201 & heat of hydrogenation: compare experimental value with "expected" value for hypothetical "cyclohexatriene" H°= – 208 kJ Thermochemical Measures of Stability + 3H 2 Pt
Dr. Wolf's CHM 201 & kJ/mol 231 kJ/mol 208 kJ/mol 360 kJ/mol 3 x cyclohexene Figure 11.2 (p 404)
Dr. Wolf's CHM 201 & kJ/mol 360 kJ/mol 3 x cyclohexene Figure 11.2 (p 404) "expected" heat of hydrogenation of benzene is 3 x heat of hydrogenation of cyclohexene
Dr. Wolf's CHM 201 & kJ/mol 360 kJ/mol 3 x cyclohexene 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 is the resonance energy of benzene
Dr. Wolf's CHM 201 & hydrogenation of 1,3- cyclohexadiene (2H 2 ) gives off more heat than hydrogenation of benzene (3H 2 )! 231 kJ/mol 208 kJ/mol Figure 11.2 (p 404)
Dr. Wolf's CHM 201 & heat of hydrogenation = 208 kJ/mol heat of hydrogenation = 337 kJ/mol 3H 2 Pt Pt Cyclic conjugation versus noncyclic conjugation
Dr. Wolf's CHM 201 & 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 Resonance Energy of Benzene
Dr. Wolf's CHM 201 & An Orbital Hybridization View of Bonding in Benzene
Dr. Wolf's CHM 201 & Orbital Hybridization Model of Bonding in Benzene Figure 11.3 Planar ring of 6 sp 2 hybridized carbons
Dr. Wolf's CHM 201 & Orbital Hybridization Model of Bonding in Benzene Figure 11.3 Each carbon contributes a p orbital Six p orbitals overlap to give cyclic system; six electrons delocalized throughout system
Dr. Wolf's CHM 201 & Orbital Hybridization Model of Bonding in Benzene Figure 11.3 High electron density above and below plane of ring
Dr. Wolf's CHM 201 & The Molecular Orbitals of Benzene
Dr. Wolf's CHM 201 & Energy Bonding orbitals Antibonding orbitals Benzene MOs 6 p AOs combine to give 6 MOs 3 MOs are bonding; 3 are antibonding
Dr. Wolf's CHM 201 & Energy Bonding orbitals Antibonding orbitals Benzene MOs All bonding MOs are filled No electrons in antibonding orbitals
Dr. Wolf's CHM 201 & The Three Bonding MOs of Benzene
Dr. Wolf's CHM 201 & Substituted Derivatives of Benzene and Their Nomenclature
Dr. Wolf's CHM 201 & ) Benzene is considered as the parent and comes last in the name. General Points
Dr. Wolf's CHM 201 & ExamplesExamples Bromobenzene tert-Butylbenzene Nitrobenzene NO 2 C(CH 3 ) 3 Br
Dr. Wolf's CHM 201 & ) 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
Dr. Wolf's CHM 201 & bromo-1-chloro-4-fluorobenzene ExampleExample Br Cl F
Dr. Wolf's CHM 201 & 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-)
Dr. Wolf's CHM 201 & ExamplesExamples o-ethylnitrobenzene NO 2 CH 2 CH 3 ClCl m-dichlorobenzene (1-ethyl-2-nitrobenzene)(1,3-dichlorobenzene)
Dr. Wolf's CHM 201 & Certain monosubstituted derivatives of benzene have unique names Benzene Derivatives
Dr. Wolf's CHM 201 & Benzaldehyde Benzene Derivatives CHO
Dr. Wolf's CHM 201 & Benzoic acid Benzene Derivatives COHO
Dr. Wolf's CHM 201 & Styrene Benzene Derivatives CH 2 CH
Dr. Wolf's CHM 201 & Toluene Benzene Derivatives CH 3
Dr. Wolf's CHM 201 & Acetophenone Benzene Derivatives CCH 3 O
Dr. Wolf's CHM 201 & Phenol Benzene Derivatives OH
Dr. Wolf's CHM 201 & Anisole Benzene Derivatives OCH 3
Dr. Wolf's CHM 201 & Aniline Benzene Derivatives NH 2
Dr. Wolf's CHM 201 & Benzene derivative names can be used as parent OCH 3 NO 2 OCH 3 Anisole p-Nitroanisole or 4-Nitroanisole
Dr. Wolf's CHM 201 & Easily confused names phenylphenolbenzyl OH CH 2 —
Dr. Wolf's CHM 201 & Polycyclic Aromatic Hydrocarbons
Dr. Wolf's CHM 201 & resonance energy = 255 kJ/mol most stable Lewis structure; both rings correspond to Kekulé benzene NaphthaleneNaphthalene
Dr. Wolf's CHM 201 & Anthracene Phenanthrene resonance energy: 347 kJ/mol 381 kJ/mol Anthracene and Phenanthrene
Dr. Wolf's CHM 201 & Physical Properties of Arenes
Dr. Wolf's CHM 201 & Resemble other hydrocarbons nonpolar insoluble in water less dense than water Physical Properties
Dr. Wolf's CHM 201 & Reactions of Arenes: A Preview 1. Some reactions involve the ring. 2. In other reactions the ring is a substituent.
Dr. Wolf's CHM 201 & a) Reduction Catalytic hydrogenation (Section 11.4) Birch reduction (Section 11.11) b) Electrophilic aromatic substitution (Chapter 12) c) Nucleophilic aromatic substitution (Chapter 23) 1. Reactions involving the ring 2. The ring as a substituent (Sections )
Dr. Wolf's CHM 201 & catalytic hydrogenation (Section 11.4) Birch reduction (Section 11.11) HH HH HH HHH HH H HH Reduction of Benzene Rings HH H HHHH HHH H H