Question

Following is an example of a type of reaction known as a Diels-Alder reaction (Chapter 20). CC1C=CCCC1 1,3 -Pentadiene Ethylene 3-Methylcyclohexene (a racemic mixture) The Diels-Alder reaction between a diene and an alkene is quite remarkable in that it is one of the few ways that chemists have to form two new carbon- carbon bonds in a single reaction. Given what you know about the relative strengths of carbon-carbon sigma and pi bonds, would you predict the Diels- Alder reaction to be exothermic or endothermic? Explain your reasoning.

Short answer

Answer: The Diels-Alder reaction is exothermic because the formation of two new carbon-carbon sigma bonds leads to lower overall energy for the new molecule, releasing energy to the surroundings.

Step-by-step solution

Understanding Diels-Alder Reaction

A Diels-Alder reaction is a cycloaddition reaction between a conjugated diene (a molecule with alternating double and single carbon-carbon bonds) and a dienophile (a molecule containing a double or triple bond that can react with the diene). In this reaction, two new carbon-carbon sigma bonds are formed, and the double bonds in the diene and the dienophile are converted to single bonds, creating a new cyclic compound.

Carbon-Carbon Sigma and Pi Bonds

A carbon-carbon sigma bond is a single covalent bond between two carbon atoms, formed by the overlap of two sp3 hybrid orbitals. A sigma bond has strong bonding due to the head-on overlap of orbitals and is stronger and more stable than a pi bond. A pi bond is a bond formed by the sideways overlap of two unhybridized p orbitals in a double or triple carbon-carbon bond. Pi bonds are weaker and less stable than sigma bonds because they have less effective orbital overlap.

Exothermic and Endothermic Reactions

An exothermic reaction is a chemical reaction that releases energy, usually in the form of heat, to its surroundings. These reactions have negative enthalpy change. In contrast, an endothermic reaction is a chemical reaction that absorbs energy from its surroundings. These reactions have positive enthalpy change.

Examine the Diels-Alder Reaction Bond Formation

In the given Diels-Alder reaction, two new carbon-carbon sigma bonds are formed, and the double bonds in the diene and the dienophile are converted into single bonds. This process results in the breaking of two pi bonds and the formation of two sigma bonds.

Determine Whether the Reaction is Exothermic or Endothermic

Since sigma bonds are stronger and more stable than pi bonds, the formation of two new sigma bonds leads to a lower overall energy for the new molecule. The difference in energy between the reactants (diene and dienophile) and the product is negative, leading to a release of energy to the surroundings, making this Diels-Alder reaction exothermic.