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Lecture Organic chemistry: Chapter 16 - L. G. Wade, Jr.

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Chapter 16 introduce to aromatic compounds. This chapter determine whether Hückel’s rule applies to a given structure, and predict whether the compound will be aromatic, antiaromatic, or nonaromatic; show how to construct the molecular orbitals of a conjugated cyclic system similar to benzene and cyclobutadiene;. | Chapter 16 Aromatic Compounds Organic Chemistry, 7th Edition L. G. Wade, Jr. Copyright © 2010 Pearson Education, Inc. Chapter 16 Discovery of Benzene Isolated in 1825 by Michael Faraday who determined C:H ratio to be 1:1. Synthesized in 1834 by Eilhard Mitscherlich who determined molecular formula to be C6H6. He named it benzin. Other related compounds with low C:H ratios had a pleasant smell, so they were classified as aromatic. Chapter 16 Kekulé Structure Proposed in 1866 by Friedrich Kekulé, shortly after multiple bonds were suggested. Failed to explain existence of only one isomer of 1,2-dichlorobenzene. C C C C C C H H H H H H Chapter 16 Resonance Structures of Benzene Benzene is actually a resonance hybrid between the two Kekulé structures. The C—C bond lengths in benzene are shorter than typical single-bond lengths, yet longer than typical double-bond lengths (bond order 1.5). Benzene's resonance can be represented by drawing a circle inside the . | Chapter 16 Aromatic Compounds Organic Chemistry, 7th Edition L. G. Wade, Jr. Copyright © 2010 Pearson Education, Inc. Chapter 16 Discovery of Benzene Isolated in 1825 by Michael Faraday who determined C:H ratio to be 1:1. Synthesized in 1834 by Eilhard Mitscherlich who determined molecular formula to be C6H6. He named it benzin. Other related compounds with low C:H ratios had a pleasant smell, so they were classified as aromatic. Chapter 16 Kekulé Structure Proposed in 1866 by Friedrich Kekulé, shortly after multiple bonds were suggested. Failed to explain existence of only one isomer of 1,2-dichlorobenzene. C C C C C C H H H H H H Chapter 16 Resonance Structures of Benzene Benzene is actually a resonance hybrid between the two Kekulé structures. The C—C bond lengths in benzene are shorter than typical single-bond lengths, yet longer than typical double-bond lengths (bond order 1.5). Benzene's resonance can be represented by drawing a circle inside the six-membered ring as a combined representation. Chapter 16 Structure of Benzene Each sp2 hybridized C in the ring has an unhybridized p orbital perpendicular to the ring which overlaps around the ring. The six pi electrons are delocalized over the six carbons. Chapter 16 Unusual Addition of Bromine to Benzene When bromine adds to benzene, a catalyst such as FeBr3 is needed. The reaction that occurs is the substitution of a hydrogen by bromine. Addition of Br2 to the double bond is not observed. Chapter 16 Resonance Energy Benzene does not have the predicted heat of hydrogenation of -359 kJ/mol. The observed heat of hydrogenation is -208 kJ/mol, a difference of 151 kJ. This difference between the predicted and the observed value is called the resonance energy. Chapter 16 Molar Heats of Hydrogenation Chapter 16 Annulenes Annulenes are hydrocarbons with alternating single and double bonds. Benzene is a six-membered annulene, so it can be named [6]-annulene.