《有机化学》课程PPT教学课件（Official PPT of Organic Chemistry 6th LG Wade by Prentice Hall，L. G. Wade, Jr.）Chapter 15 Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy

Organic Chemistry,6th 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 ©2006，Prentice Hall

Chapter 15 Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy Jo Blackburn Richland College, Dallas, TX Dallas County Community College District © 2006, Prentice Hall Organic Chemistry, 6th 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.Example: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. Example: 1,4- pentadiene. • Cumulated double bonds are on adjacent carbons. Example: 1,2-pentadiene. =>

Stabilities of Dienes Heat of hydrogenation for 1,4-pentadiene is -252 kJ/mol,about twice that of 1-pentene. ·△Hfor1-pentene is-126 kJ/mol and for trans-2-pentene is -116 kJ/mol,so expect -242 kJ/mol for trans-1,3-pentadiene. ·Actual△His-225kJ/mol,so the conjugated diene is more stable. Difference,-242 kJ(-225 kJ)=17 kJ/mol,is the resonance energy. => Chaper 15 3

Chaper 15 3 Stabilities of Dienes • Heat of hydrogenation for 1,4-pentadiene is –252 kJ/mol, about twice that of 1-pentene. • H for 1-pentene is -126 kJ/mol and for trans-2-pentene is -116 kJ/mol, so expect -242 kJ/mol for trans-1,3-pentadiene. • Actual H is -225 kJ/mol, so the conjugated diene is more stable. • Difference, -242 kJ (-225 kJ) = 17 kJ/mol, is the resonance energy. =>

Relative Stabilities cumulated terminal diene alkyne twice 1-pentene 1.2-pentadiene I-pentyne internal alkyne 292 2-pentyne more substituted isolated 291J dlene 入 isolated 275kJ 1.4-pentadiene diene 入 trans-1.4-hexadiene conjugated 252k diene 242k」 trans-1,3-pentadicne 225kJ alkane (pentane or hexane) Copyright 2005 Pearson Prentice Hall,Inc. => 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 1.34A Copyright2005 Pearson Prentice Hall,Inc

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. =>

Ethylene Pi MO's ·The combination of node two p orbitals must π* (antibonding)= give two molecular orbitals. destructive overlap energy of the isolated p orbitals on CI and C2 ● Constructive overlap is a bonding MO. energy (bonding)= 。Destructive overlap is an anti-bonding constructive overlap MO. => Copyright2005 Pearson Prentice Hall.Inc Chaper 15

Chaper 15 7 Ethylene Pi MO’s • The combination of two p orbitals must give two molecular orbitals. • Constructive overlap is a bonding MO. • Destructive overlap is an anti-bonding MO. =>

MO for 1,3-Butadiene ·Lowest energy, bonding bonding bonding ·All bonding interactions. ● Electrons are delocalized over four nuclei. Copyright2005 Peerson Prentce Hall,ine > Chaper 15 8

Chaper 15 8 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 bonding antibonding bonding interactions. 。1 antibonding interaction. ·A bonding MO. node Copyright2005 Pearson Prentice Hall.Inc. > Chaper 15 9

Chaper 15 9 2 MO for 1,3-Butadiene • 2 bonding interactions. • 1 antibonding interaction. • A bonding MO. =>

3*MO for 1,3-Butadiene antibonding antibonding ·Antibonding MO. bonding ·Empty at ground state. ·Two nodes. node node Copyright 2005 Pearson Prentice Hall.Inc Chaper 15 10

Chaper 15 10 3 * MO for 1,3-Butadiene • Antibonding MO. • Empty at ground state. • Two nodes. =>