Chapter 15
Dienes, Resonance, and Aromaticity
Classification of Dienes
• Dienes: compounds containing
two double bonds
• Cumulenes: compounds in which
one carbon participates in two
double bonds
• Conjugated Dienes: compounds
that contain two double bonds
that are separated by a single
bond
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Heats of Formation
15.1 Structure and Stability of Dienes
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Why Are Conjugated Dienes More Stable?
1) Different hybridization in bonds
– s bonds with more s-character are stronger
– These bonding differences account for 5 - 6 kJ mol-1
of the stabilization
15.1 Structure and Stability of Dienes
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2) Stabilization through the p system
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MO Picture of 1,3-Butadiene
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Delocalization
• The p1 MO shows that the electron density is
spread across the entire molecule
• The electrons are said to be delocalized
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Conformations of Dienes
15.1 Structure and Stability of Dienes
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Conformations of Dienes
• Newman projections:
15.1 Structure and Stability of Dienes
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The Diels-Alder Reaction
• Discovered by Otto Diels and Kurt Alder in
1928 (shared Nobel Prize in chemistry 1950)
• Involves a diene and a dienophile (alkene or
alkyne)
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The Diels-Alder Reaction
• Cycloaddition reaction: An addition reaction
that results in formation of a ring
• 1,4-addition reaction or a conjugate addition
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• Pericyclic reaction: concerted reaction that
involves the cyclic flow of electrons
15.3 The Diels-Alder Reaction
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Problems
• Draw the products you get from the following
Diels-Alder reactions
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• Cyclic dienes can lead to multicyclic compounds
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Effect of Diene Conformation
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Effect of Diene Conformation
15.3 The Diels-Alder Reaction
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15.3 The Diels-Alder Reaction
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Transition State of the Diels-Alder Reaction
Stereochemistry of the Diels-Alder Reaction
• The stereochemistry of the dienophile is
maintained during a DA rxn
• The concerted transition state results in synaddition of the diene to the dienophile
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Stereochemistry of the Diels-Alder Reaction
• Syn-addition is also revealed with substituents
at the terminal ends of the diene
15.3 The Diels-Alder Reaction
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Endo and Exo Products
• Endo product: The alkene substituents are cis
to the outer diene substituents (Ro)
• Exo product: The alkene substituents are trans
to the outer diene substituents (Ro)
• The endo product generally forms faster
• Endo vs exo products are also observed when
cyclic dienes are used
Problems
• Predict the products for the following
reactions:
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