Question

1. Give an explanation for the following:

a. Cyclopentadiene dimerizes more readily than acyclic dienes.

b. Cyclopentadiene reacts more rapidly with maleic anhydride than with another cyclopentadiene molecule.

c. Commercial cyclopentadiene cannot be used as a conjugated diene in a Diels-Alder reaction.

d. The distillate of the fractional distillation is collected in ice.

e. Cyclopentadiene is added to a cool solution of maleic anhydride.

Answer 1

Organic chemistry

1. Explanations of the following,

a. Cyclopentadiene dimerizes more readily than the acyclic dienes. This is because the fixed cis-geometry of the dimerized product resulting from cyclopentadiene is entropically more favoured than the dimerized product resulting from the acyclic diene. The equilibrium constant for cyclopentadiene dimerization is very high and the molecule readily dimerized at room temperature.

b. Cyclopentadiene react more rapidly with maleic anhydride than with another cyclopenatdiene molecule. This is because in a diels-alder reaction the reaction works best when the diene is electron rich and the dienophile is electron deficient. Here cyclopentadiene is electron rich whereas maleic anhydride is electron deficient molecule. Thus reaction is maleic anhydride goes more rapidly than the reaction with another cyclopenatdiene.

c. commercial cyclopentadiene remains as a dimer at room temperature and thus it cannot be used as a diene in a diels-alder reaction. Prior to the reaction the diene is regenerated by a retro-diels alder reaction (cracking) by heating dimer to get monomer diene.

d. The distillate of the fractional distillation having the cyclopentadiene is collected in ice as the cyclopentadiene readily dimerizes and favors the dimer form at room temperature. So to avoid dimerization, the distillate is strictly collected over ice.

e. Cyclopentadiene is added to a cool solution of maleic anhydride as the overall diels-alder reaction of the two substrates is an exothermic reaction, that means heat is released during the reaction. So to avoid any accidents by the sudden release of a large amount of heat, the maleic anhydride is first cooled well and than slowly cyclopenatdiene is added to it.