Why is diels alder exothermic

In fact, the opposite is true. Pure 4-hydroxybutyric acid is difficult to make and purify, and it tends to lactonize spontaneously and completely once formed. The intramolecular nature of lactonization explains the increase in rate, but does not account for lactone dominance at equilibrium. For this we must consider the free energy change in this lactonization:. Thus, quantitative lactonization of the 4-hydroxybutyric acid should be expected. With the exception of seven and eight-membered ring compounds, these lactones are not particularly strained, and are able to adopt Z-like ester conformations.

The rate of lactonization is small due to the lower probability of conformations in which the hydroxyl and carboxyl groups are near each other in space a negative entropy factor. If the concentration of hydroxy acid is high, dimerization and polymerization is favored.

At low concentrations of hydroxy acid, lactonization becomes competitive. Reactions which involve the formation of charged atoms and molecules are usually extremely endothermic in the gas phase, but may become spontaneous in certain solvents.

If ions are formed from a neutral compound, as when NaCl is dissolved in water, the oppositely charged cations and anions naturally attract each other, so formation of a dispersed homogeneous solution might appear to be energetically unfavorable.

To achieve charge separation of ions in solution, two solvent characteristics are particularly important. The first is the ability of solvent molecules to orient themselves between ions so as to attenuate the electrostatic force one ion exerts on the other. This characteristic is a function of the polarity of the solvent. When subject to the electric field of an ion, such polar molecules orient themselves to oppose the field, and in so doing they limit its reach. Solvents that have relatively acidic hydrogen atoms e.

Because their functional groups are made up of polar covalent bonds, protic solvents are often polar as well. A list of common protic and aprotic solvents is provided here. The dielectric constants provide a measure of solvent polarity. The second factor important in the stabilization of ions, which also resists their intimate recombination, is called solvation.

This refers to the ability of solvent molecules to stabilize ions by encasing them in a sheath of weakly bonded solvent molecules, thus somewhat dispersing the electrical charge. The water dipoles are drawn as red arrows, and partial charges are noted.

When water was added to the reaction mixture, the reaction was faster, with only a modest increase in the reaction temperature and 1,3,3a,4,5,7a-hexahydromethyloxoisobenzofurancarboxylic acid was obtained as a pure white solid. The demonstration may help students understand the role of solvents in organic reactions, why most reactions are carried out in solvents, as well as potential problems that may be encountered when developing solvent-free reactions.

This demonstration was originally developed as a laboratory exercise by Thomas Goodwin at Hendrix College. We thank anonymous reviewer who suggested adding this note.

We would like to thank Salvatore Lepore from the Department of Chemistry, Florida Atlantic University for suggesting this demonstration. One of the reviewers was able to obtain a sample for GC—MS analysis by swirling the solid residue with dichloromethane and filtering the resulting suspension through cotton. View Author Information. E-mail: [email protected].

Cite this: J. Article Views Altmetric -. The ratio of endo to exo products in this reaction is about four to one:. Note that in the endo product above, the anhydride is on the underside of the new six-membered ring, whereas in the exo , it points away. This is indeed less sterically hindered. Furthermore, given enough heat, the Diels-Alder product can revert back to starting materials. Another related explanation is in the footnotes. Our last post showed the mechanism of the Diels-Alder reaction through the orbital interactions of the highest occupied molecular orbital HOMO of the diene and the lowest unoccupied molecular orbital LUMO of the dienophile.

When the orbital interactions are drawn out in the two different transition states, we see something like this. Note that in the endo transition state the electron-withdrawing groups point toward the two carbons that will eventually comprise the C 2 -C 3 alkene, whereas in the exo transition state, the electron-withdrawing groups point away from the C 2 -C 3 alkene:.

Again, note how the end o transition state has the electron withdrawing group positioned over the diene:. This interaction stabilizes the endo transition state to an extent that compensates for the slightly greater steric hindrance. Under typical reaction conditions at relatively low temperature, the product distribution reflects the difference in energy between the exo and endo transition states — which is not necessarily the same thing as the difference in energy between the products!

If heated sufficiently, Diels-Alder products can revert to their starting materials, and an equilibrium between the reactants and products can be established.

Under these conditions, the product distribution will reflect the difference in energy between the exo and endo products which tends to favor the exo. In fairness, there is some debate as to whether secondary orbital effects actually exist. The secondary orbital interaction is marked with a red dashed line, below left. Although, it should be noted, cyclopentadiene can be kept in the freezer indefinitely.

In the retro Diels-Alder, a single molecule the Diels-Alder product reverts to two starting molecules the diene and dienophile , which represents a large net increase in entropy. Note 3. The difference in rate of gives a difference in activation energy of 3. Great post! I think you drew the endo product for the furan maleic anhydride reaction by accident.

On the fix list! EDIT: fixed. Thank you! Great post. Is there something special about cyclopentadiene that causes it to have this propensity? A quick Google search shows that DA reactions have been done with conjugated linoleic acids and reactive dienophiles like N-methyl maleimide: DOI: Thanks James. I was more referring to linoleic undergoing DA reactions with itself. Diels Alder reactions in nature are well known, but usually intramolecular where there is higher effective concentration of reactants.

Never thought chemistry would make me laugh. That was a good one with the cyclopentadiene offer! Keep the great work. Thank you for the clear explanation! I was wondering could an exo product react with cyclopentadiene to form some three ring structure. In other words, could the thermodynamically stable exo product act as a dienophie with a diene that formed it creating another diels alder product of three rings?

To get the reaction to work, you would likely have to heat it, and then you might run into problems with the retro-Diels-Alder occurring. So if one were to treat that exo diels alder product with an electron-poor diene, another diels-alder reaction could occur. Your email address will not be published.