The isodesmic reaction series shown below has been used to compare the stabilities of the cyclopolyene ketenes. The total energies (HF/6-31G*) are given in hartrees. Calculate the stabilization found for each cyclopolyene ketene for n=1–3. Account for the differences in stabilization and compare the results of these exocyclic ketenes to the corresponding cyclopolyenones (Problem 8.7).
The completely conjugated cyclic polyenones have attracted considerable interest. Consider the following aspects of their properties:
a. The relative basicity of carbonyl oxygens can be measured by studying the strength of hydrogen bonding with a hydrogen donor such as phenol. The Keq for 1:1 complexation of the following substituted cyclenones was determined. What conclusions do you draw from these data?
b. There have been extensive physical and chemical studies on cyclopropenone, cyclopentadienone, and cycloheptatrienone (tropone). The results of these studies can be briefly summarized as follows: (a) cyclopropenone appears to be stabilized by 20±5 kcal/mol, relative to localized model structures; (b) cyclopentadienone is a kinetically unstable molecule; (c) tropone is estimated to be stabilized by less than 10 kcal/mol, relative to localized models. It is nonplanar and rather reactive. Rationalize these results in terms of MO concepts.
The Correct Answer and Explanation is :
For the stabilization of each cyclopolyene ketene for ( n = 1-3 ), the total energy of the compounds (HF/6-31G*) is typically compared. To calculate the stabilization for these cyclopolyene ketenes, we can compare their total energy to that of the corresponding localized structures. If the calculated total energy of the cyclic ketene is lower than that of the corresponding model structure, the difference represents the stabilization energy.
To properly calculate the stabilization, the total energy difference between the cyclopolyene ketene and the localized model structure for each case must be obtained. However, I do not have access to the exact numerical values in the images you’ve provided, but I can explain the general approach and interpretation of stabilization.
The stabilization for each cyclopolyene ketene comes from the delocalization of the π-electrons over the cyclic structure, which makes the molecule more stable compared to a hypothetical localized structure. For ( n = 1-3 ), stabilization arises due to resonance and electron delocalization. As the number of atoms in the ring increases (from cyclopropenone to cyclopentadienone to cycloheptatrienone), the extent of stabilization and the type of bonding changes.
Next, comparing the stabilization of these ketenes with cyclopolyenones (as described in Problem 8.7), cyclopolyenones are also stabilized through delocalization, though the extent of stabilization can differ due to the nature of the substituents and the presence of the carbonyl group.
For part a, regarding the hydrogen bonding data and the basicity of carbonyl oxygens in the substituted cyclenones, we need to analyze the Keq for 1:1 complexation with phenol. The relative basicity can be assessed by examining the equilibrium constants. A higher ( K_{\text{eq}} ) indicates a stronger interaction with the hydrogen donor, suggesting a more basic oxygen. This can give insight into how the electronic structure of the cyclenones (including substituent effects) influences their basicity.
For part b, rationalizing the stability of cyclopropenone, cyclopentadienone, and cycloheptatrienone (tropone) with MO concepts:
- Cyclopropenone is stabilized by 20±5 kcal/mol relative to localized models. The stabilization arises due to the delocalization of π-electrons over the three-membered ring. This delocalization helps to stabilize the molecule by lowering the overall energy.
- Cyclopentadienone is kinetically unstable because the conjugated π-system in the 5-membered ring does not favor planarity, making the molecule reactive.
- Tropone (cycloheptatrienone) is stabilized by less than 10 kcal/mol and is nonplanar. The less pronounced stabilization indicates that the electron delocalization in this 7-membered ring is not as strong as in cyclopropenone, and the nonplanar geometry makes the molecule more reactive.
In summary, the molecular orbital theory explains the stabilization of these cyclic ketenes through delocalization of π-electrons, but the extent of stabilization is influenced by factors such as ring size and planarity.
To help you visualize the structural differences between these compounds, I’ll generate an image showing the molecular structures of cyclopropenone, cyclopentadienone, and tropone.
Here is the image showing the molecular structures of cyclopropenone, cyclopentadienone, and tropone. This should help visualize the differences between these compounds and their conjugated systems.
Let me know if you need further clarification!
