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.
Imagine cyclopentadiene is added to a solution containing both methyl vinyl
ketone and maleic anhydride at extremely low temperatures. Which would be
the major product?
A) The exo adduct of cyclopentadiene and maleic anhydride
B) The endo adduct of cyclopentadiene and methyl vinyl ketone
C) The exo adduct of cyclopentadiene and methyl vinyl ketone
D) The endo adduct of cyclopentadiene and maleic anhydride
Correct answer: D. The endo adduct of cyclopentadiene and
maleic anhydride would be the major product. This can be determined directly
from the data provided in Table 2 in the passage. Conceptually, kinetic control
of a reaction dominates at lower temperatures; the product that has the lowest
activation energy is favored because this element of reaction is constrained at low
temperatures. The endo adduct is often the kinetic product of Diels-Alder reaction
due to favorable secondary orbital interactions, yielding lower activation energy.
From the passage, one can realize that at low temperatures, the endo adducts
are favored (higher endo/exo ratios), moreso for the cyclopentadiene + maleic
anhydride reaction
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