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.
A similar Diels-Alder reaction between cyclopentadiene and a different
dienophile, ethyl acrylate, was performed under identical conditions. Given
data presented in the passage and considering electronic effects and steric
hindrance of the substituents on ethyl acrylate, predict how the reaction rate
at 25°C for cyclopentadiene and ethyl acrylate would compare to the reaction
rate of cyclopentadiene and maleic anhydride at 25°C.
A) It would be lower due to increased steric hindrance
B) It would be higher due to decreased steric hindrance and less
electron-withdrawing effect
C) It would be similar due to similar electronic effects and steric hindrance
D) This comparison is unpredictable without more data
Correct answer: B. Ethyl acrylate is less sterically hindered than
maleic anhydride, whose anhydride group is quite bulky. This reduced steric
hindrance generally promotes a faster reaction. Additionally, maleic anhydride has
two strong electron withdrawing groups (carbonyl moieties). This stabilizes the
transition state of the reaction, but it also decreases the electron density of the
dienophile, essentially making it less reactive toward the diene (a fundamental
component of the Diels-Alder reaction). Ethyl acrylate’s single ester group is
a weaker electron-withdrawing group than maleic anhydride’s carbonyl group,
resulting in a higher electron density and faster reaction with cylcopentadiene
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