《有机化学》课程PPT教学课件（Organic Chemistry，5th Edition，L. G. Wade, Jr.，Prentice Hall）Chapter 15 Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy

Organic Chemistry,5th Edition L.G.Wade,Jr. Chapter 15 Conjugated Systems, Orbital Symmetry,and Ultraviolet Spectroscopy Jo Blackburn Richland College,Dallas,TX Dallas County Community College District ©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

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. 二> Chaper 15 2

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. =>

Resonance Energy Heat of hydrogenation for trans-1,3- pentadiene is less than expected. ·△Hfor1-pentene is30.0kcal/mol and for trans-2-pentene is 27.4 kcal/mol,so expect 57.4 kcal for trans-1,3-pentadiene. ·Actual△His53.7kcal,so the conjugated diene is more stable. Difference,(57.4-53.7)3.7 kcal/mol,is the resonance energy. => Chaper 15 3

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. =>

Relative Stabilities cumulated terminal diene alkyne =C- 入 1,2-pentadiene 1I-pentyne internal alkyne twice 1-pentene 69.8 2-pentyne kcal isolated 69.5 diene more substituted kcal isolated 65.8 1,4-pentadiene diene kcal trans-1,4-hexadiene conjugated 60.2 diene keal 57.4 trans-1,3-pentadiene kcal 53.7 kcal alkane (pentane or hexane) Chaper 15 4

Chaper 15 4 Relative Stabilities twice 1-pentene more substituted =>

Structure of 1,3-Butadiene Most stable conformation is planar. Single bond is shorter than 1.54 A. Electrons are delocalized over molecule. small amount of overlap partial double bond 1.34A H H 1.48A 11.34A H Chaper 15 5

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 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.= Chaper 15 6

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. =>

MO for 1,3-Butadiene ·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. =>

2 MO for 1,3-Butadiene 。2 bonding interactions 。1 antibonding interaction ·A bonding MO => Chaper 15 8

Chaper 15 8 2 MO for 1,3-Butadiene • 2 bonding interactions • 1 antibonding interaction • A bonding MO =>

3*MO for 1,3-Butadiene ·Antibonding MO ·Empty at ground π state ·Two nodes => Chaper 15 9

Chaper 15 9 3 * MO for 1,3-Butadiene • Antibonding MO • Empty at ground state • Two nodes =>

4*MO for 1,3-Butadiene ·All antibonding interactions. 刘 ·Highest energy. ·Vacant at ground state. 二> Chaper 15 10

Chaper 15 10 4 * MO for 1,3-Butadiene • All antibonding interactions. • Highest energy. • Vacant at ground state. =>