Question

All attempts to synthesize cyclopentadienone yield only a Diels-Alder adduct. Cycloheptatrienone, however, has been prepared by several methods and is stable. Hint: Consider important resonance contributing structures. O=C1C=CC=C1 O=c1cccccc1 Cyclopentadienone Cycloheptatrienone (a) Draw a structural formula for the Diels-Alder adduct formed by cyclopentadienone. (b) How do you account for the marked difference in stability of these two ketones?

Short answer

Answer: The difference in stability between cyclopentadienone and cycloheptatrienone can be attributed to their resonance structures. Cyclopentadienone has no contributing resonance structures, while cycloheptatrienone has multiple resonance contributing structures, making it much more stable and less reactive than cyclopentadienone.

Step-by-step solution

Part (a): Drawing the Diels-Alder Adduct

The Diels-Alder reaction is a cycloaddition reaction between a conjugated diene and a dienophile. In this case, cyclopentadienone acts as both the diene and the dienophile. Draw the structure of cyclopentadienone and react it with itself in the following manner: the double bond of the dienophile section reacts with the double bond on one end of the diene section, creating a new 6-membered ring. The resulting Diels-Alder adduct would be bicyclo[2.2.2]oct-5-en-2-one. Here's the structural formula: O / __C__ | | __C C__ / | | C==C | C==C | | C___C==O

Part (b): Accounting for Stability Difference

The marked difference in stability between cyclopentadienone and cycloheptatrienone can be attributed to their resonance structures. Resonance structures are alternative ways of representing a molecule that distribute electrons across the molecule more uniformly. A molecule with more resonance structures experiences an overall stabilization effect. Cyclopentadienone has no contributing resonance structures, as its carbonyl group cannot be conjugated with its double bonds without breaking the carbonyl double bond (only the Diels-Alder adduct is formed). On the other hand, cycloheptatrienone can be readily stabilized by resonance, considering its conjugated system of 7 carbon atoms (including the carbonyl group). In this case, the carbonyl group can easily share its electron pairs and participate in the delocalization of electrons throughout the molecule. Because the cycloheptatrienone has multiple resonance contributing structures, its overall stability is much higher than that of the cyclopentadienone, making it less reactive and allowing it to be synthesized by various methods.