Ch 15 - Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy

Ch 15 - Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy

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15 Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy Double bonds can interact with each other if they are separated by just one single bond. Such interacting double bonds are said to be conjugated. Double bonds with two or more single bonds separating them have little interaction and are called isolated double bonds. For example, 1,3-pentadiene has conjugated double bonds, and 1,4-pentadiene has isolated double bonds. conjugated double bonds (more stable than isolated do u ble bonds) --"""'c=c/' --" ""'C=C/' /' / --... C H2 isolated double bonds Because of the interaction between the double bonds, systems containing conju- gated double bonds tend to be more stable than similar systems with isolated double bonds. 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 spec- troscopy, a tool for determining the structures of conjugated systems. In Chapter 7, we used heats of hydrogenation to compare the relative stabilities of alkenes. For example, the heats of hydrogenation of 1 -pentene and trans-2-pentene show that the disubstituted double bond in trans-2-pentene is 10 kJ/mol (2.5 kcal/mol) more stable than the monosubstituted double bond in 1-pentene. t).HO = -]26 kJ (- 30. 1 kcal) J -pentene tlW = - 1 16 kJ (-27.6 kcaJ) trans-2-pentene 1 5-1 Introduction 15-2 Stabilities of Dienes 663
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