Chapter 15 - Conjugated systems, orbital symmetry, and ultraviolet spectroscopy. In this chapter, we consider the unique properties of conjugated systems, the theoretical reasons for this extra stability, and some of the characteristic reactions of molecules containing conjugated double bonds. We also study ultraviolet spectroscopy, a tool for determining the structures of conjugated systems. | Chapter 15 Copyright © 2010 Pearson Education, Inc. Organic Chemistry, 7th Edition L. G. Wade, Jr. Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy Chapter 15 Conjugated Systems Conjugated double bonds are separated by one single bond. Isolated double bonds are separated by two or more single bonds. Conjugated double bonds are more stable than isolated ones. Chapter 15 Heat of Hydrogenation of Conjugated Bonds For conjugated double bonds, the heat of hydrogenation is less than the sum for the individual double bonds. The more stable the compound, the less heat released during hydrogenation, Conjugated double bonds have extra stability. Chapter 15 Relative Stabilities twice 1-pentene more substituted Chapter 15 Structure of 1,3-Butadiene Single bond is shorter than Å. Electrons are delocalized over molecule. There is a small amount of overlap across the central C—C bond, giving it a partial double bond character. Chapter 15 | Chapter 15 Copyright © 2010 Pearson Education, Inc. Organic Chemistry, 7th Edition L. G. Wade, Jr. Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy Chapter 15 Conjugated Systems Conjugated double bonds are separated by one single bond. Isolated double bonds are separated by two or more single bonds. Conjugated double bonds are more stable than isolated ones. Chapter 15 Heat of Hydrogenation of Conjugated Bonds For conjugated double bonds, the heat of hydrogenation is less than the sum for the individual double bonds. The more stable the compound, the less heat released during hydrogenation, Conjugated double bonds have extra stability. Chapter 15 Relative Stabilities twice 1-pentene more substituted Chapter 15 Structure of 1,3-Butadiene Single bond is shorter than Å. Electrons are delocalized over molecule. There is a small amount of overlap across the central C—C bond, giving it a partial double bond character. Chapter 15 Molecular Orbitals (MOs) Pi molecular orbitals are the sideways overlap of p orbitals. p orbitals have two lobes. Plus (+) and minus (-) indicate the opposite phases of the wave function, not electrical charges. 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. Chapter 15 Ethylene Pi MOs The combination of two p orbitals must give two molecular orbitals. Constructive overlap is a bonding MO. Destructive overlap is an antibonding MO. Chapter 15 1 MO for 1,3-Butadiene Lowest energy. All bonding interactions. Electrons are delocalized over four nuclei. Chapter 15 2 MO for 1,3-Butadiene Two bonding interactions. One antibonding interaction. A bonding MO. Higher energy than 1 MO and not as strong. Chapter 15 3* MO for 1,3-Butadiene Antibonding MO. Empty at ground state. Two nodes. Vacant in the ground state. Chapter 15 4* MO for .
