2_Aromaticity

2_Aromaticity - Chem 267 Basic Organic Chemistry 2 Prof....

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Prof. Dr. Xiangdong Fang University of Waterloo Chem 267 Basic Organic Chemistry 2 1
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2 Aromaticity 1) The fragrant scent of benzene and its derivatives led them to being classed as “ aromatic ”; 2) Aromaticity is associated with a special stability resulting from benzene structure; H 2 ,N i H 2 ,N i H 2 ,N i H = -28.6 kcal/mol H calc = 2 x (-28.6) = -57.2 kcal/mol H obs = -55.4 kcal/mol Difference = 1.8 kcal/mol H calc = 3 x (-28.6) = -85.5 kcal/mol H obs = -49.8 kcal/mol Difference = 36 kcal/mol benzene cyclohexene 1,3-cyclohexadiene Benzene is unusually stable and not very reactive
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3 Benzene is NOT “1,3,5-cyclohexatriene”! 1,3,5-cyclohexatriene double bond: 1.34A o single bond: 1.54 A o
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4 MO Rules for Benzene 1) Six p atomic orbitals overlap to form six molecular orbitals; 2) 3 MOs’ are bonding, 3 antibonding; 3) The lowest-energy MO will have all bonding interaction, no node; 4) As the energy of MO increases, the number of nodes increases; 5) System symmetric, so there are two pairs of degenerate orbitals; 6) Six electrons fill three bonding orbitals; all bonding orbitals are filled (closed shell), and thus gives an extremely stable arrangement ( aromatic stabilization )
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5 Molecular Orbital Diagrams of Benzene 0 node 1 node 2 nodes 3 nodes
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Criteria for Aromaticity ( Hückle’s Rule ) 1) Must be cyclic ; 2) Must be planar ; 3) Each of the ring atom must have a p orbital , and these p orbitals must be
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This note was uploaded on 07/07/2010 for the course CHEM 267 taught by Professor Fang during the Winter '09 term at Waterloo.

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2_Aromaticity - Chem 267 Basic Organic Chemistry 2 Prof....

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