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Chapter 20: Problem 44

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. (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

Expert verified
Question: Compare the stabilities of cyclopentadienone and cycloheptatrienone and explain the observed differences in reactivity. Answer: The stability difference between cyclopentadienone and cycloheptatrienone can be attributed to their resonance structures. Cyclopentadienone resonance structures involve a positive charge on the electron-withdrawing oxygen atom, making them less stable. In contrast, the resonance structures of cycloheptatrienone have a negative charge on the oxygen atom, reducing its electron-withdrawing effect and making the structures more stable. This leads to a more stable cycloheptatrienone overall and explains the observed differences in reactivity.

Step by step solution

01

Understanding the Diels-Alder reaction

The Diels-Alder reaction is a cycloaddition reaction involving a conjugated diene and a dienophile (usually an alkene or alkyne). The reaction results in a new six-membered cyclic compound, where two new carbon-carbon sigma bonds and one new pi bond are formed.
02

Drawing the structural formula of the Diels-Alder adduct for cyclopentadienone

Cyclopentadienone, as a molecule with alternating single and double bonds, can be considered as a conjugated diene with a carbonyl group. In the Diels-Alder reaction, the carbonyl group acts as the dienophile. After the reaction, a six-membered ring is formed, and the carbonyl group is still attached to the ring. The structural formula of the Diels-Alder adduct can be drawn as follows: ![Diels-Alder adduct of cyclopentadienone](https://i.imgur.com/2RgMvDE.png)
03

Comparing the resonance structures of cyclopentadienone and cycloheptatrienone

To analyze the stability difference between cyclopentadienone and cycloheptatrienone, we should investigate their resonance structures. Draw the possible resonance structures for both cyclopentadienone and cycloheptatrienone, keeping in mind that the more stable resonance structures contribute more to the overall stability: Cyclopentadienone resonance structures: ![Cyclopentadienone resonance structures](https://i.imgur.com/Q9xffbT.png) Cycloheptatrienone resonance structures: ![Cycloheptatrienone resonance structures](https://i.imgur.com/iv2j8lG.png)
04

Explaining the stability difference between cyclopentadienone and cycloheptatrienone

In the case of cyclopentadienone, the resonance structures involve a positive charge on the oxygen atom, which is an electron-withdrawing atom. This makes the resonance structures less stable. In contrast, the resonance structures of cycloheptatrienone involve a negative charge on the oxygen atom, effectively reducing the electron-withdrawing effect of the carbonyl group. As a result, the resonance structures of cycloheptatrienone are more stable, leading to a more stable compound overall.

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Most popular questions from this chapter

Predict the structure of the major product formed by 1,2 -addition of HCl to 2-methyl1,3 -butadiene (isoprene).

What diene and dienophile might you use to prepare the following racemic DielsAlder adduct?

Show how to distinguish between 1,3 -cyclohexadiene and 1,4 -cyclohexadiene by ultraviolet spectroscopy.

Estimate the stabilization gained as a result of conjugation when 1,4 -pentadiene is converted to trans-1,3-pentadiene. Note that the answer is not as simple as comparing the heats of hydrogenation of 1,4 -pentadiene and trans-1,3-pentadiene. Although the double bonds are moved from unconjugated to conjugated, the degree of substitution of one of the double bonds is also changed, in this case from a monosubstituted double bond to a trans disubstituted double bond. To answer this question, you must separate the effect that is the result of conjugation from that caused by a change in the degree of substitution.

Predict the product(s) formed by addition of one mole of \(\mathrm{Br}_{2}\) to 2,4 -hexadiene.
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