Organic Chemistry,7th Edition L.G.Wade,Jr. Chapter 15 Conjugated Systems,Orbital Symmetry, and Ultraviolet Spectroscopy Copyright 2010 Pearson Education,Inc
Chapter 15 Copyright © 2010 Pearson Education, Inc. Organic Chemistry, 7th Edition L. G. Wade, Jr. Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy
Conjugated Systems C-CH 3 uc-c H conjugated double bonds 1,3-pentadiene (more stable than isolated double bonds) HH、 H C-C C=C >C= C=C CH, CH2 isolated double bonds 1,4-pentadiene Copyright2010 Pearson Prentice Hall,Inc 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 2
Chapter 15 2 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
Heat of Hydrogenation of Conjugated Bonds I-pentene 2-pentene predicted Predicted: -126kJ -116kJ -242kJ(-57.7kcal) 2H2 actual value -225kJ(-53.7kcal) trans-1,3-pentadiene more stable by 17 kJ (4.0 kcal) Copyright 2010 Pearson Prentice Hall,Inc. 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 3
Chapter 15 3 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
Relative Stabilities cumulated terminal diene alkyne C- twice 1-pentene 1.2-pentadiene I-pentyne internal alkyne 292kJ 2-pentyne more substituted (69.8 kcal) isolated 291kJ diene (69.5 keal) isolated 275kJ 1.4-pentadiene diene (65.8kcl trans-1.4-hexadiene conjugated 252kJ diene (60.2 keal) 242kJ trans-1,3-pentadiene (57.7 kcal) 225kJ (53.7kcal) alkane (pentane or hexane) Copyright 2010 Pearson Prentice Hall,Inc. Chapter 15 4
Chapter 15 4 Relative Stabilities twice 1-pentene more substituted
Structure of 1,3-Butadiene small amount of overlap partial double bond 1.34A H H H H 1.48A 1.34A H Copyright10 Pearson Prentice Hall,inc. a Single bond is shorter than 1.54 A. 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 5
Chapter 15 5 Structure of 1,3-Butadiene ▪ Single bond is shorter than 1.54 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