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Chapter11 - Organic Chemistry CHE 275 Chapter 11 Arenes and...

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Organic Chemistry CHE 275 Chapter 11 Arenes and Aromaticity
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Benzene Toluene Naphthalene Examples of Aromatic Hydrocarbons H H H H H H CH 3 H H H H H H H H H H H H H
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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 Kekulé proposes structure of benzene.
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Kekulé proposed a cyclic structure for C 6 H 6 with alternating single and double bonds. Kekulé Formulation of Benzene H H H H H H
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Later, Kekulé revised his proposal by suggesting a rapid equilibrium between two equivalent structures. Kekulé Formulation of Benzene H H H H H H H H H H H H
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However, this proposal suggested isomers of the kind shown were possible. Yet, none were ever found. H X X H H H H X X H H H Kekulé Formulation of Benzene
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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. Benzene has the shape of a regular hexagon. Structure of Benzene
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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. 140 pm 140 pm 140 pm 140 pm 140 pm 140 pm
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Instead of Kekulé's suggestion of a rapid equilibrium between two structures: H H H H H H H H H H H H Kekulé Formulation of Benzene
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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
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Circle-in-a-ring notation stands for resonance description of benzene (hybrid of two Kekulé structures) Resonance Formulation of Benzene
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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
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heat of hydrogenation: compare experimental value with "expected" value for hypothetical "cyclohexatriene" Δ H°= – 208 kJ Thermochemical Measures of Stability + 3H 2 Pt
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231 kJ/mol 120 kJ/mol 360 kJ/mol 3 x cyclohexene 208 kJ/mol
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120 kJ/mol 360 kJ/mol 3 x cyclohexene "expected" heat of hydrogenation of benzene is 3 x heat of hydrogenation of cyclohexene
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208 kJ/mol 360 kJ/mol 3 x cyclohexene 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
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hydrogenation of 1,3- cyclohexadiene (2H 2 ) gives off more heat than hydrogenation of benzene (3H 2 )!
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Chapter11 - Organic Chemistry CHE 275 Chapter 11 Arenes and...

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