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Dienes Chapter 8 Adapted from Turro & Breslow, Columbia University and Irene Lee, Case Western Reserve University 1
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Unsaturated hydrocarbons containing:
two double bonds: diene three double bonds: triene four double bonds: tetraene many double bonds: polyene
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Classification of Dienes
isolated diene conjugated diene cumulated diene C 2
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Nomenclature (2E,5E)-2,5-heptadiene (2E,4E)-2,4-heptadiene
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If there is a tie, the double bond gets the lowest number
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Relative Stabilities of Dienes
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Heats of Hydrogenation
1,3-pentadiene is kJ/mol more stable than 1,4-pentadiene, but some of this stabilization is because it also contains a more highly substituted double bond 252 kJ/mol 226 kJ/mol 4
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Heats of Hydrogenation
126 kJ/mol 115 kJ/mol 252 kJ/mol 226 kJ/mol 5
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Heats of Hydrogenation
126 kJ/mol 111 kJ/mol 126 kJ/mol 115 kJ/mol 252 kJ/mol 226 kJ/mol 5
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Heats of Hydrogenation
126 kJ/mol 111 kJ/mol when terminal double bond is conjugated with other double bond, its heat of hydrogenation is 15 kJ/mol less than when isolated 5
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Heats of Hydrogenation
126 kJ/mol 111 kJ/mol this extra 15 kJ/mol is known by several terms stabilization energy delocalization energy resonance energy 5
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Heats of Hydrogenation
Cumulated double bonds have relatively high heats of hydrogenation H2C C + CH2 2H2 CH3CH2CH3 DH° = -295 kJ + H2C CH2CH3 H2 CH3CH2CH3 DH° = -125 kJ 7
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Bonding in Conjugated Dienes
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Isolated diene 1,4-pentadiene 1,3-pentadiene Conjugated diene 10
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p bonds are independent of each other
Isolated diene p bonds are independent of each other 1,3-pentadiene Conjugated diene 10
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p bonds are independent of each other
Isolated diene p bonds are independent of each other p orbitals overlap to give extended p bond encompassing four carbons Conjugated diene 10
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less electron delocalization; less stable
Isolated diene less electron delocalization; less stable more electron delocalization; more stable Conjugated diene 10
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Conformations of Dienes
H H s-trans s-cis s prefix designates conformation around single bond s prefix is lower case (different from Cahn-Ingold-Prelog S which designates configuration and is upper case) 11
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Conformations of Dienes
H H s-trans s-cis s prefix designates conformation around single bond s prefix is lower case (different from Cahn-Ingold-Prelog S which designates configuration and is upper case) 11
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Conformations of Dienes
s-trans s-cis Both conformations allow electron delocalization via overlap of p orbitals to give extended p system 11
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s-trans is more stable than s-cis
Interconversion of conformations requires two p bonds to be at right angles to each other and prevents conjugation 12 kJ/mol 12
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16 kJ/mol 12 kJ/mol 15
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Bonding in Allenes 16
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A cumulated diene is less stable than an isolated diene
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cumulated dienes are less stable than isolated and conjugated dienes
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linear arrangement of carbons
Structure of Allene 118.4° 131 pm linear arrangement of carbons nonplanar geometry 19
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linear arrangement of carbons
Structure of Allene 118.4° 131 pm linear arrangement of carbons nonplanar geometry 19
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Bonding in Allene sp 2 sp sp 2 19
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Bonding in Allene 19
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Bonding in Allene 19
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Bonding in Allene 19
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Allenes of the type shown are chiral
Chiral Allenes Allenes of the type shown are chiral A X C B Y A π B; X π Y Have a stereogenic axis 25
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analogous to difference between:
Stereogenic Axis analogous to difference between: a screw with a right-hand thread and one with a left-hand thread a right-handed helix and a left-handed helix 26
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