Lecture #4

Lecture #4 - Chapter 14 Aromatic Compounds One classication...

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Chapter 14 Aromatic Compounds One classifcation oF organic compounds is as: Aliphatic vs. Aromatic Aliphatic compounds were originally classifed as “Fat-like” materials. Now classifed as alkanes, alkenes, alkynes, and their derivatives. Aromatic compounds were originally classifed as having a Fragrant odor and having a very high hydrogen to carbon ratio. Now, aromatic compounds are exemplifed by benzene and known to have specifc electronic structures and reactivity.
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The Special Stability of Benzene Benzene does not show the chemical reactivity of an alkene. The aromatic ring structure shows an unusual stability, a resistance to change. rapid hydroxylation no reaction rapid addition no reaction rapid addition no reaction hydrogenation 25 o C, 20 psi very slow hydrogenation >100 o C, 1500 psi Comparative Chemical Reactivity of Cyclohexene and Benzene Reagents cold, dilute KMnO 4 Br 2 /CCl 4 HI H 2 /Ni
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Aromatic Substitution Reactions The chemical reactivity of benzene is characterized by substitution rather than addition , which preserves the aromatic ring . X This pattern of reactivity suggests the aromatic ring is unusually stable .
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A Quantitative Measure of Stability: Heats of Hydrogenation of Benzene and Model Cycloalkenes The heats of hydrogenation of cyclohexene, 1,3-cyclohexadiene, and benzene are compared in the table below. The expected values for ! H o are based on the observed ! H o for cyclohexene. H 2 H 2 + H 2 ! H o H 2 (kcal/mol) observed expected Heats of Hydrogenation -28.6 ----- + 2H 2 Pt Pt + 3H 2 Pt -55.4 -57.2 As a conjugated diene, 1,3-cyclohexadiene shows resonance stabilization of 57.2-55.4 = 1.8 kcal/mol. -49.8 -85.8 The observed and expected heats of hydrogenation for benzene show a remarkable extra stabilization of 85.8 - 49.8 = 36.0 kcal/mol.
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The Resonance Energy of Benzene The special stabilization of benzene ( 36.0 kcal/mol ) revealed by comparing the observed heat of hydrogenation of benzene ( 49.8 kcal/mol ) with the hypothetical compound 1,3,5-cyclohexatriene ( 85.8 kcal/mol ) is called resonance energy . The resonance stabilization in 1,3-cyclohexadiene and benzene obtained from the heats of hydrogenation are shown in the potential energy diagram below. Potential Energy + H 2 -28.6 + 2H 2 -55.4 1.8 (Resonance Energy) + 3H 2 + 3H 2 -49.8 36.0 (Resonance Energy)
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The Kekule' Structure for Benzene C C C C C C H H H H H H or Kekule' structure for benzene, C 6 H 6
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Resonance Theory Description of Benzene According to Resonance Theory , the two equivalent Kekule' structures of benzene are resonance structures that contribute to the hybrid .The hybrid is 36.0 kcal/mol more stable than the Kekule' structures. Kekule' structures hybrid Resonance Energy The hexagon with the circle inside is widely used to represent the hybrid of benzene.
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Bond Equivalency in Benzene Spectroscopic studies indicate that benzene is planar, with the geometry of a regular hexagon, and carbon-carbon bond lengths of 1.39 A. o
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This note was uploaded on 01/23/2011 for the course CHEM 322BL taught by Professor Singer during the Spring '07 term at USC.

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Lecture #4 - Chapter 14 Aromatic Compounds One classication...

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