Organic Chemistry, 5th Edition nENEs L.G. Wade jr Chapter 15 Conjugated Systems Orbital Symmetry, and Ultraviolet Spectroscopy Jo blackburn Richland college, dallas TX Dallas County Community College District c 2003. Prentice hall
Chapter 15 Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy Jo Blackburn Richland College, Dallas, TX Dallas County Community College District © 2003, Prentice Hall Organic Chemistry, 5th Edition L. G. Wade, Jr
H H H Definitions Conjugated double bonds are separated by one single bond. EXample: 1, 3-pentadiene Isolated double bonds are separated by two or more single bonds. 1, 4-pentadiene Cumulated double bonds are on adjacent carbons. EXample: 1, 2-pentadiene > Chaper 15
Chaper 15 2 Definitions • Conjugated double bonds are separated by one single bond. Example: 1,3-pentadiene. • Isolated double bonds are separated by two or more single bonds. 1,4-pentadiene. • Cumulated double bonds are on adjacent carbons. Example: 1,2-pentadiene. =>
H Resonance Energy wT H Heat of hydrogenation for trans-1, 3 pentadiene is less than expected AH for 1-pentene is 30.0 kcal/mol and for trans-2-pentene is 27.4 kcal/mol, so expect 57. 4 kcal for trans-1, 3-pentadiene Actual AH is 53.7 kcal, so the conjugated diene is more stable Difference, (574-53.7)3.7 kcal/mol, is the esonance energy > Chaper 15
Chaper 15 3 Resonance Energy • Heat of hydrogenation for trans-1,3- pentadiene is less than expected. • H for 1-pentene is 30.0 kcal/mol and for trans-2-pentene is 27.4 kcal/mol, so expect 57.4 kcal for trans-1,3-pentadiene. • Actual H is 53.7 kcal, so the conjugated diene is more stable. • Difference, (57.4 – 53.7) 3.7 kcal/mol, is the resonance energy. =>
H H H Relative Stabilities cumulated terminal diene alkyne 1,2-pentadiene 1-pentyne nternal alkyne twice I-pentene 2-pentyne kcal isolated 69.5 diene more substituted isolated 1. 4-pentadiene 65.8 diene trans-14-hexadiene conjugated 60.2 diene kcal 57.4 kcal trans-1, 3-pentadiene kcal alkane(pentane or hexane > Chaper 15
Chaper 15 4 Relative Stabilities twice 1-pentene more substituted =>
Structure of 1, 3-Butadiene EsS Most stable conformation is planar Single bond is shorter than 1. A Electrons are delocalized over molecule small amount of overlap partial double bond H H 1.34A H 分—Cm H H H 1.48A1.34A H H Chaper 15
Chaper 15 5 Structure of 1,3-Butadiene • Most stable conformation is planar. • Single bond is shorter than 1.54 Å. • Electrons are delocalized over molecule. =>
Constructing H H H Molecular orbitals Pi molecular orbitals are the sideways overlap of p orbitals p orbitals have 2 lobes. Plus(+)and minus C indicate the opposite phases of the wave unction, not electrical charge When lobes overlap constructively, ( and +,or-and -)a bonding Mo is formed When and -lobes overlap, waves cancel out and a node forms; antibonding Mo.=> Chaper 15
Chaper 15 6 Constructing Molecular Orbitals • Pi molecular orbitals are the sideways overlap of p orbitals. • p orbitals have 2 lobes. Plus (+) and minus (-) indicate the opposite phases of the wave function, not electrical charge. • When lobes overlap constructively, (+ and +, or - and -) a bonding MO is formed. • When + and - lobes overlap, waves cancel out and a node forms; antibonding MO. =>
丌1 MO for1,3- Butadiene≈a H Lowest energy All bonding interactions Electrons are delocalized over four nuclei > Chaper 15
Chaper 15 7 1 MO for 1,3-Butadiene • Lowest energy. • All bonding interactions. • Electrons are delocalized over four nuclei. =>
H H T2 MO for 1, 3-Butadiene 2 bonding interactions 1 antibonding interaction 兀 a bonding mo > Chaper 15
Chaper 15 8 2 MO for 1,3-Butadiene • 2 bonding interactions • 1 antibonding interaction • A bonding MO =>
I* MO for 1, 3-Butadiene -_u H Antibonding mo Empty at ground兀* state Two nodes => Chaper 15
Chaper 15 9 3 * MO for 1,3-Butadiene • Antibonding MO • Empty at ground state • Two nodes =>
H H H MO for 1.3-Butadiene All antibonding interactions Highest energy Vacant at ground state > Chaper 15 10
Chaper 15 10 4 * MO for 1,3-Butadiene • All antibonding interactions. • Highest energy. • Vacant at ground state. =>