Chapter 16 - Wade(1)

Chapter 16 - Wade(1) - Introduction to General and Organic...

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Chapter 16 Aromatic Compounds Dr. David P. Brown Department of Chemistry Introduction to General and Organic Chemistry III
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Chapter 16 2 Introduction 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 C 6 H 6 . Other related compounds with low C:H ratios had a pleasant smell , so they were classified as aromatic . =>
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Chapter 16 3 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 =>
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Chapter 16 4 Resonance Structure Each sp 2 hybridized C in the ring has an unhybridized p orbital perpendicular to the ring which overlaps around the ring.
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Chapter 16 5 Unusual Reactions • Alkene + KMnO 4 diol (addition) Benzene + KMnO 4 no reaction . Alkene + Br 2 /CCl 4 dibromide (addition) Benzene + Br 2 /CCl 4 no reaction .
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Chapter 16 6 Unusual Stability Hydrogenation of just one double bond in benzene is endo thermic ! =>
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Chapter 16 7 Annulenes All cyclic conjugated hydrocarbons were proposed to be aromatic. However, cyclobutadiene is so reactive that it dimerizes before it can be isolated. And cyclooctatetraene adds Br 2 readily. Must look at MO’s to explain aromaticity.
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Chapter 16 8 MO Rules for Benzene Six overlapping p orbitals must form six molecular orbitals . Three will be bonding, three antibonding.
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Chapter 16 - Wade(1) - Introduction to General and Organic...

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