Chapter 15 Dienes, Resonance, and Aromaticity

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Presentation transcript:

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

Heats of Formation 15.1 Structure and Stability of Dienes

Why Are Conjugated Dienes More Stable? 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

Stabilization through the p system

MO Picture of 1,3-Butadiene

Delocalization The p1 MO shows that the electron density is spread across the entire molecule The electrons are said to be delocalized

Conformations of Dienes 15.1 Structure and Stability of Dienes

Conformations of Dienes Newman projections: 15.1 Structure and Stability of Dienes

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)

The Diels-Alder Reaction Cycloaddition reaction: An addition reaction that results in formation of a ring 1,4-addition reaction or a conjugate addition

Pericyclic reaction: concerted reaction that involves the cyclic flow of electrons 15.3 The Diels-Alder Reaction

Problems Draw the products you get from the following Diels-Alder reactions

Cyclic dienes can lead to multicyclic compounds

Effect of Diene Conformation

Effect of Diene Conformation 15.3 The Diels-Alder Reaction

15.3 The Diels-Alder Reaction

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 syn-addition of the diene to the dienophile

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

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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