Rules of the Day 4-15 - the pi electrons in low energy...

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Rules of the Day 4-15-09 1. Featured Golden Rule of Chemistry: 7. Pi electrons prefer to be delocalized over as many adjacent sp 2 hybridized atoms (or sp hybridized atoms in some cases) as possible, and aromaticity is the most stable form of pi electron delocalization. Pi electrons cannot delocalize onto or through sp 3 hybridized atoms since an sp 3 atom has no 2p orbital available. 2. According to the Huckel definition of aromaticity, monocyclic molecules will be aromatic if they are 1) flat , 2) all the ring atoms are sp2 hybridized (sp in rare cases) and 3) there are 4n + 2 pi electrons , where n = 0,1,2,3,4,5,6. ...... 3. The 36 kcal/mol extra stability (unreactivity) of aromatic species derives from putting all
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Unformatted text preview: the pi electrons in low energy molecular orbitals that extend over the sp2 hybridized ring atoms. These molecular orbitals involve overlap (in phase and out of phase) of the 2p orbitals on the sp2 hybridized ring atoms. 4. Aromaticity stabilizes ions and atoms in molecules exist in hybridization states that maximize aromatocit y . The lone pair on a ring atom will be in a 2p orbital if that leads to aromaticity . This explains why pyridine is much more basic than pyrrole. You will need to understand this before the next test....
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This note was uploaded on 09/22/2010 for the course CH 53200 taught by Professor Bocknack during the Spring '10 term at University of Texas at Austin.

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