Day 49 MCAT Practice Question

Passage 8: Diels-Alder Reaction

In an intriguing study, researchers conducted a study on the Diels-Alder reaction

between cyclopentadiene and maleic anhydride. The Diels-Alder reaction is a

pericyclic reaction that forms a six-membered ring via a concerted mechanism. A

series of experiments was performed, with a focus on reaction rate, the influence

of temperature, and the equilibrium between major and minor products under

different conditions.



To begin, cyclopentadiene and maleic anhydride were reacted in toluene solvent

at increasing temperatures, and samples were taken at different time points to

determine reaction rates. Data was tabulated in Table 1:

Cyclopentadiene + Maleic Anhydride → Norsolene Diimide (Reaction 1)



Product distribution between endo and exo adducts was also analyzed. An endo

adduct of the Diels-Alder reaction is one in which the newly added substituents are

positioned on the same side of the ring as the electron-withdrawing group on the

dienophile. This results in a more sterically hindered, but stabilizing, interaction via

secondary orbital overlap. The exo adduct is just the opposite: the substituents are

placed on opposite sides of the ring in a fashion that is less sterically hindered but

also lacks secondary orbital interactions.



Temperature and solvent polarity were varied and this product distribution was

measured. An alternative reaction between cyclopentadiene and methyl vinyl

ketone (as opposed to maleic anhydride) was observed under the same conditions.

These results are shown in Table 2:

The presence of chemical species that were not consumed during the course

of reaction (AlCl3 and NaOMe) were also examined for their effects on product

distribution

While the general trends conformed to theoretical expectations, anomalies were

noted. At higher concentrations of AlCl3, the expected dominance of the exoadduct did not materialize as strongly as anticipated. Furthermore, the presence of

NaOMe skewed toward the endo adduct even at higher temperatures, and it was

theorized that pH effects were responsible for this deviation.

Given the role of temperature in derived reaction rates provided in the

passage, which of the following thermodynamic assumptions is definitely true?

A) The reaction is endothermic because the rate constant increases with

temperature

B) The reaction is exothermic because the rate constant increases with

temperature

C) The activation energy of the reaction is positive

D) The reaction reaches equilibrium faster at higher temperatures