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Chapter 13: Problem 32

The heats of combustion for three compounds with the molecular formula \(\mathrm{C}_{5} \mathrm{H}_{8}\) are as follows: cyclopentene, \(\Delta H_{\text {comb }}=-1027\) kcal \(\mathrm{mol}^{-1} ; 1,4\) -pentadiene, \(\Delta H_{\text {comb }}=-1042\) kcal \(\mathrm{mol}^{-1}\); and 1-pentyne, \(\Delta H_{\text {comb }}=-1052\) kcal mol \(^{-1}\). Explain in terms of relative stability and bond strengths.

Short Answer

Expert verified
Cyclopentene is the most stable due to its less negative heat of combustion, followed by 1,4-pentadiene, with 1-pentyne being the least stable.

Step by step solution

01

- Understand the Problem

To find out which compound is most stable among cyclopentene, 1,4-pentadiene, and 1-pentyne, compare their heats of combustion. The heat of combustion provides information about the energy released during the combustion process. Lower heat of combustion indicates higher stability.
02

- Compare Heats of Combustion

Identify the heats of combustion for each compound:- Cyclopentene: \[ \text{螖H}_{\text {comb }} = -1027 \text{ kcal/mol} \]- 1,4-Pentadiene: \[ \text{螖H}_{\text {comb }} = -1042 \text{ kcal/mol} \]- 1-Pentyne: \[ \text{螖H}_{\text {comb }} = -1052 \text{ kcal/mol} \]The more negative the heat of combustion, the less stable the compound is.
03

- Determine Relative Stability

Compare the heats of combustion:- Cyclopentene: \[ -1027 \text{ kcal/mol} \] (least negative, most stable)- 1,4-Pentadiene: \[ -1042 \text{ kcal/mol} \] (intermediate stability)- 1-Pentyne: \[ -1052 \text{ kcal/mol} \] (most negative, least stable)Cyclopentene is the most stable, followed by 1,4-pentadiene, and 1-pentyne is the least stable.
04

- Analyze Bond Strengths

Consider the types of bonds in each compound and their relative strengths:- Cyclopentene: Contains a double bond in a five-membered ring leading to some degree of ring strain.- 1,4-Pentadiene: Contains two double bonds and has some degree of conjugation, which can stabilize the compound relative to 1-pentyne.- 1-Pentyne: Contains a triple bond, which is stronger but less stable overall compared to single or double bonds in terms of compound stability.
05

- Conclusion

The least negative heat of combustion (most stable compound) is cyclopentene. This can be attributed to its structure, with only one double bond and no conjugation. Although 1-pentyne has strong bonds, its overall structure is less stable due to the triple bond when compared to single and double-bonded compounds.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Heat of Combustion
The 'Heat of Combustion' refers to the amount of energy released when a substance undergoes a complete combustion reaction with oxygen. This measurement is significant because it gives insight into the energy content of a chemical compound and its stability. The more negative the heat of combustion value, the more energy is released, indicating that the compound has higher energy content and is less stable. Conversely, a less negative heat of combustion suggests a more stable compound. For example, Cyclopentene \( \Delta H_{\text{comb}} = -1027 \text{ kcal/mol} \) is more stable than 1-Pentyne \( \Delta H_{\text{comb}} = -1052 \text{ kcal/mol} \).
  • Cyclopentene: -1027 kcal/mol
  • 1,4-Pentadiene: -1042 kcal/mol
  • 1-Pentyne: -1052 kcal/mol
Molecular Stability
Molecular stability refers to how energy-efficient a molecule is in its current structure. A stable molecule has low internal energy and is less likely to react spontaneously. Stability is often evaluated by examining a molecule鈥檚 heat of combustion. For instance, Cyclopentene has the least negative heat of combustion (-1027 kcal/mol), making it the most stable among the three compounds. The reason for this stability lies in its structure, which includes only one double bond and a five-membered ring structure that contributes to its stability.
Bond Strengths
Bond strengths in a molecule play a crucial role in determining its stability. Stronger bonds usually mean more energy is required to break them, but this doesn't always correlate with greater stability in complex molecules. For example, 1-Pentyne contains a triple bond which is stronger than double and single bonds. However, this makes 1-Pentyne less stable compared to Cyclopentene and 1,4-Pentadiene. Understanding bond strengths can explain why certain molecules with strong individual bonds may still be less stable due to the overall structure and bond interactions.
Cyclopentene
Cyclopentene is a five-membered ring with one double bond. The structure of Cyclopentene contributes to its stability. Having only one double bond and very minimal ring strain makes it more stable compared to molecules with multiple double bonds or triple bonds. This is reflected in its heat of combustion at -1027 kcal/mol, making it the most stable among the compounds compared.
1,4-Pentadiene
1,4-Pentadiene is another compound with a molecular formula of \( \mathrm{C}_{5} \mathrm{H}_{8} \). This molecule has two double bonds located at the first and fourth positions, which introduces conjugation to the structure. Conjugation can stabilize the molecule to some degree because of the delocalization of electrons. However, due to the presence of two reactive double bonds, its overall heat of combustion is more negative (-1042 kcal/mol) compared to Cyclopentene, demonstrating its lesser stability.
1-Pentyne
1-Pentyne is characterized by a triple bond between the first and second carbon atoms in the chain. Although the triple bond is stronger, it makes the molecule less stable overall due to the high energy state of the triple bond and lack of electron delocalization. Consequently, 1-Pentyne has the most negative heat of combustion at -1052 kcal/mol, indicating it is the least stable compound among the ones compared.

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Most popular questions from this chapter

Propose reasonable syntheses of each of the following alkynes, using the principles of retrosynthetic analysis. Each alkyne functional group in your synthetic target should come from a separate molecule, which may be any two- carbon compound (e.g., ethyne, ethene, ethanal).

Propose a synthesis of trans-3-hexene starting with 1 -butyne.

Propose an efficient synthesis of 2,7 -dimethyl-4-octanone, using organic building blocks containing no more than four carbon atoms.

Compare the C2-C3 bonds in propane, propene, and propyne. Should they be any different with respect to either bond length or bond strength? If so, how should they vary?

Give the products of addition of one and two molecules of \(\mathrm{Cl}_{2}\) to 1 -butyne.
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