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Ch. 7 - Structure and Synthesis of Alkenes; Elimination
Wade - Organic Chemistry 9th Edition
Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook
All textbooksWade 9th EditionCh. 7 - Structure and Synthesis of Alkenes; EliminationProblem 48
Chapter 7, Problem 48

A double bond in a six-membered ring is usually more stable in an endocyclic position than in an exocyclic position. Hydrogenation data on two pairs of compounds follow. One pair suggests that the energy difference between endocyclic and exocyclic double bonds is about 9 kJ/mol. The other pair suggests an energy difference of about 5 kJ/mol. Which number do you trust as being more representative of the actual energy difference? Explain your answer.

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1
Step 1: Analyze the heats of hydrogenation data provided for the two pairs of compounds. The first pair shows an energy difference of 5 kJ/mol (116 kJ/mol for exocyclic and 111 kJ/mol for endocyclic). The second pair shows an energy difference of 9 kJ/mol (110 kJ/mol for exocyclic and 105 kJ/mol for endocyclic).
Step 2: Understand the concept of stability in double bonds. Endocyclic double bonds are generally more stable than exocyclic double bonds due to better conjugation and less steric strain. This stability is reflected in lower heats of hydrogenation for endocyclic double bonds.
Step 3: Evaluate the reliability of the data. The heats of hydrogenation for the second pair (difference of 9 kJ/mol) are closer in magnitude and consistent with the expected trend of endocyclic double bonds being more stable. The first pair (difference of 5 kJ/mol) shows a smaller difference, which might be influenced by additional factors such as substituent effects or experimental error.
Step 4: Consider the structural differences between the compounds in each pair. The second pair has simpler structures with fewer substituents, which reduces the likelihood of confounding factors affecting the heats of hydrogenation. This makes the second pair's data more representative of the intrinsic stability difference between endocyclic and exocyclic double bonds.
Step 5: Conclude that the energy difference of 9 kJ/mol is more representative of the actual stability difference between endocyclic and exocyclic double bonds, as it aligns better with theoretical expectations and is supported by cleaner experimental data.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Endocyclic vs. Exocyclic Double Bonds

Endocyclic double bonds are located within a ring structure, while exocyclic double bonds are outside the ring. The stability of these bonds can differ significantly due to steric and electronic factors. Generally, endocyclic double bonds are more stable because they benefit from the ring's structural constraints, which can lead to less steric hindrance and more favorable orbital overlap.
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Heats of Hydrogenation

Heats of hydrogenation refer to the energy released when a double bond is converted to a single bond through the addition of hydrogen. This value is indicative of the stability of the double bond; lower heats of hydrogenation suggest greater stability. By comparing the heats of hydrogenation for different compounds, one can infer the relative stabilities of their double bonds, aiding in the assessment of endocyclic versus exocyclic configurations.
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Energy Difference Interpretation

The energy difference between endocyclic and exocyclic double bonds can be assessed through experimental data, such as heats of hydrogenation. In this case, two pairs of compounds suggest different energy differences (9 kJ/mol and 5 kJ/mol). The more representative value may depend on factors like the specific compounds studied and their structural features, which can influence the stability of the double bonds and the accuracy of the heat measurements.
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Related Practice
Textbook Question

Predict the products of E1 elimination of the following compounds. Label the major products.

(a)

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Textbook Question

For each alkene, indicate the direction of the dipole moment. For each pair, determine which compound has the larger dipole moment.

c. cis-1,2-dibromo-1,2-dichloroethene or cis-1,2-dichloroethene

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Textbook Question

Predict the products formed by sodium hydroxide-promoted dehydrohalogenation of the following compounds. In each case, predict which will be the major product.

a. 1-bromobutane

b. 2-chlorobutane

c. 3-bromopentane

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Textbook Question

The energy difference between cis- and trans-but-2-ene is about 4 kJ/mol; however, the trans isomer of 4,4-dimethylpent-2-ene is nearly 16 kJ/mol more stable than the cis isomer. Explain this large difference.

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Textbook Question

For each alkene, indicate the direction of the dipole moment. For each pair, determine which compound has the larger dipole moment

b. cis-1,2-dibromoethene or trans-2,3-dibromobut-2-ene

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Textbook Question

Predict the products of E1 elimination of the following compounds. Label the major products.

(c)

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