Ch14slides - Chapter 14: Conjugated Dienes and Ultraviolet...

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1 Chapter 14: Conjugated Dienes and Ultraviolet Spectroscopy Diene: molecule with two double bonds Conjugated diene: alternating double and single bonds When the carbons of a conjugate diene all lie in the same plane, the π -molecular orbitals overlap. C=C double bonds C-C single bond Alkene Diene Conjugate Diene Conjugation : a series of overlapping p-orbitals alkenes conjugated to alkenes alkenes conjugated to carbonyls H O acrolein ( α , β -unsaturated aldehyde, enal) O cyclohexenone ( α , β -unsaturated ketone, enone) alkenes conjugated to non-bonding pairs of electrons O R N O H O Vitamin A (retinal) n poly-acetylene Arene butadiene
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2 Preparation of conjugated dienes (1,3-dienes) from alkenes: allylic bromination followed by dehydrohalogenation NBS, h ν CCl 4 Br (CH 3 ) 3 CO - K + Stability of conjugated double bonds: the double bonds of conjugated dienes are more stable than isolated double bonds. H 2 , Catalyst H 2 , Catalyst H 2 , Catalyst H 2 , Catalyst Δ H° (hydrogenation) -126 KJ/mol -253 KJ/mol ( 2 x 126 = 252) -110 KJ/mol (126 - 110 = 16) -236 KJ/mol ( 252 - 236 = 16) Table 14.1 (p. 525) π -molecular orbitals of an alkene
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3 π -molecular orbitals of butadiene 0 Nodes 3 bonding interactions 0 antibonding interactions BONDING MO 1 Nodes 2 bonding interactions 1 antibonding interactions BONDING MO 2 Nodes 1 bonding interactions 2 antibonding interactions ANTIBONDING MO 3 Nodes 0 bonding interactions 3 antibonding interactions ANTIBONDING MO ψ 2 is the Highest Occupied Molecular Orbital (HOMO) ψ 3 is the Lowest Unoccupied Molecular Orbital (LUMO) ψ 1 of Butadiene (no nodes, bonding MO) Bonding Interaction The four π -electrons in ψ 1 are delocalized over the four p-orbitals _ + _ + H 3 C CH 3 H 2 C CH 2 H 2 C CH CH 3 H 2 C CH CH CH 2 H 2 C CH CH CH 2 Table 14.2 (p. 528): Bond lengths in pm 154 133 149 148 134
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This note was uploaded on 03/11/2012 for the course CHEM 220A taught by Professor Sulikowski during the Fall '08 term at Vanderbilt.

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Ch14slides - Chapter 14: Conjugated Dienes and Ultraviolet...

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