Quick Overview of Aromatic Compounds

Quick Overview of Aromatic Compounds - Aromatic Compounds...

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Aromatic Compounds and Aromaticity Solomons 6 th Edition Chapter 14 p 614 – 654 Chapter 15 p 655 – 703 (Reactions) You will by now be familiar with the structure of benzene C 6 H 6 or Discovered in 1825 by Michael Faraday (RI). Molecular formula deduced by Mitscherlich in 1834. The fragrant odour of benzene and its derivatives led them to being classed as “aromatic”. This classification now has a chemical meaning – “aromaticity” is associated with a special stability resulting from structure. Elucidation of the structure posed a problem – the molecular formula C 6 H 6 indicated a highly unsaturated compound (double and/or triple bonds) but benzene does not show this behaviour. Kekulé (1865) conceived a cyclic structure, C C C C C C H H H H H H but this would imply alternating single and double bonds (C-C = 1.47Å, C=C = 1.34Å). Kekulé suggested that two forms of benzene were in rapid equilibrium: Later spectroscopic evidence showed all bond lengths to be equal and intermediate between single and double bond lengths (1.39 Å). It was also found that benzene was a flat (planar) molecule. We now look at benzene using two different possible approaches to try to describe its stability.
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A. VALENCE BOND APPRAOCH Resonance hybrid, 2 canonical forms not nb: -Try to represent both single and double bond character of each bond. Remember with resonance structures, neither of the extremes actually exists – the structure is somewhere in between. Further, all bond angles in benzene are 120º (revise 12.5), π electrons are delocalised . Resonance theory states that if more than one resonance form can be drawn for a molecule, then the actual structure is somewhere in between them. Furthermore, the actual energy of the molecule is lower than might be expected for any of the contributing structures. If a molecule has equivalent resonance structures it is much more stable than either canonical would be – hence the extra stability of benzene (called resonance energy ). B. MOLECULAR ORBITAL REPRESENTATION OF BENZENE (MO THEORY) The bond angles of 120 ° in benzene suggests that C atoms are sp 2 hybridised. An alternative representation therefore starts with a planar framework and considers overlap of the p orbitals ( π electrons). x6 unused p orbital delocalised π system sp 2 hybridised C ( Simple MO rules ) Mix n x p atomic orbitals n π molecular orbitals! Unicorn Dragon Rhinoceros Resonance structrures Real structure (hybrid)
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Remember ethene? (p 26) 3 Higher energy π * In benzene there are 6 x overlapping p orbitals 6 π MO’s 3 Lower energy π The exact calculation of their position (shown below) is beyond our discussion. Each MO can accommodate 2 electrons, so for benzene we see all electrons are paired
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This note was uploaded on 02/10/2011 for the course CHEM 2420 taught by Professor Staff during the Spring '08 term at Univeristy of Wyoming- Laramie.

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Quick Overview of Aromatic Compounds - Aromatic Compounds...

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