1. What is the limiting reagent in the reaction between 1,3-cyclopentadiene and
ID: 857229 • Letter: 1
Question
1. What is the limiting reagent in the reaction between 1,3-cyclopentadiene and maleic anhydride, and why do you
think this reagent rather than the other one was made limiting? Show your calculations.
2. Why does 1,3-cyclopentadiene react more rapidly with maleic anhydride than with another molecule of itself?
3. Why does the solution of maleic anhydride (in ethyl acetate-petroleum ether) need to be homogeneous when
1,3- cyclopentadiene is added to it?
4. Why is the solution containing the desired product to be cooled slowly?
Explanation / Answer
1. What is the limiting reagent in the reaction between 1,3-cyclopentadiene and maleic anhydride, and why do you think this reagent rather than the other one was made limiting? Show your calculations.
(cyclopentadiene: 0.1 mL Maleic Anhydride 0.1g Ethyl Acetate 0.4 mL Petroleum Ether 0.4 mL Product: 0.79 g exp melting point 135-145 C lit melting point 165 C………..taken from comments section as it’s not given in the question itself)
Cyclopentadiene: Molecular weight: 66.10 g/mol, density: 0.79 g/ml
Maleic anhydride: Molecular weight: 98.06 g/mol
Amount of cyclopentadiene used: 0.1 mL equals 0.1 x 0.79 = 0.079 g = 0.079/66.10 = 0.001195 moles
Amount of maleic anhydride used: 0.1 g, = 0.1/98.06 = 0.001019 moles
According to the stoichiometry of reaction 1 mole of cyclopentadiene with 1 mole of maleic anhydride affords 1 mole of Diels Alder product, cis-Norbornene-5,6-endo-dicarboxylic anhydride. Since the number of moles of maleic anhydride used are less than that of cyclopentadiene the former is the limiting reagent in this reaction.
2. Why does 1,3-cyclopentadiene react more rapidly with maleic anhydride than with another molecule of itself?
Cyclopentadiene is a good diene for the Diels-Alder reaction because of its locked s-cis conformation. Also, being in a ring, the cyclopentadiene is locked into the s-cis conformation that facilitates the Diels-Alder Reaction. Maleic anhydride functions well as a good dienophile because of its electron-withdrawing groups and relatively small size. Electron-withdrawing groups (the acyl groups) remove electron density from the dienophile’s double bond, which enhance its reactivity with electron-rich dienes.
At room temperature 1,3-cyclopentadiene reacts itself to form a dimer. But in presence of maleic anhydride it readily forms the desired product cis-Norbornene-5,6-endo-dicarboxylic anhydride as maleic anhydride is a better dienophile than cyclopentadiene itself. Further the product formed is more thermodynamically more stable than the dimer of cyclopentadiene.
3. Why does the solution of maleic anhydride (in ethyl acetate-petroleum ether) need to be homogeneous when 1,3- cyclopentadiene is added to it?
Maleic anhydride is a solid compound at room temperature with m.p. 52.60o C. For a reaction to occur smoothly and completely we should make sure it is completely dissolved before adding cyclopentadiene to it. Otherwise the reaction will be incomplete and self coupled product, dimer of cyclopentadiene may be predominant than the desired product.
4. Why is the solution containing the desired product to be cooled slowly?
Slow cooling allows for the formation of nice crystals of the product. Otherwise the rapid cooling of the product may precipitate it in a random fashion.
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