Textbook Question
Conjugated dienes, molecules containing two alkenes separated by one single bond, are discussed in detail in Chapter 21.
(b) How might you account for this difference in stability?
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9. Alkenes and Alkynes
Alkene Stability
Problem 51
Textbook Question
Consider the Cope rearrangement, a reaction we describe in Chapter 20.
(a) Using the knowledge we have gained here in Chapter 9, suggest a one-step, concerted mechanism that explains the formation of B from A.
(b) Which side of the reaction would you expect to be favored? Justify your answer.
(c) Which product, A or B, would you expect to be hydrogenated with the more exothermic heat of hydrogenation?
Verified step by step guidance1
Step 1: Analyze the Cope rearrangement mechanism. The Cope rearrangement is a [3,3]-sigmatropic reaction, which involves the concerted movement of electrons. In this case, the reaction starts with compound A, where the π-electrons in the double bonds and the σ-electrons in the single bonds rearrange to form compound B. Draw the curved arrows to show the electron movement.
Step 2: Consider the thermodynamic stability of A and B to determine which side of the reaction is favored. Compound B introduces a stereogenic center and a conjugated diene system, which increases stability due to conjugation. Compare the stability of the conjugated diene in B versus the isolated double bonds in A.
Step 3: Evaluate the heat of hydrogenation for A and B. Hydrogenation is more exothermic for less stable compounds. Since A has isolated double bonds and lacks conjugation, it is less stable than B. Therefore, A would release more heat upon hydrogenation compared to B.
Step 4: Justify the favorability of the reaction direction. The formation of B is favored due to the increased stability from conjugation and the stereogenic center. This aligns with the principle that reactions tend to favor the formation of more stable products.
Step 5: Summarize the mechanism and thermodynamic considerations. The Cope rearrangement proceeds via a concerted mechanism, favoring the formation of B due to its conjugated diene system. A would exhibit a more exothermic heat of hydrogenation due to its lower stability.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Cope Rearrangement
The Cope rearrangement is a thermal rearrangement of 1,5-hexadienes that involves a concerted mechanism, resulting in the migration of substituents across a double bond. This reaction is characterized by the formation of a new sigma bond and the breaking of an existing sigma bond, all occurring in a single step without intermediates. Understanding this rearrangement is crucial for predicting the products formed from the starting materials in the question.
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Definition of Cope Rearrangement
Concerted Mechanism
A concerted mechanism refers to a reaction pathway where bond breaking and bond forming occur simultaneously in a single step, without the formation of intermediates. This type of mechanism is essential in understanding the Cope rearrangement, as it allows for the prediction of product formation based on the alignment and overlap of orbitals during the transition state. Recognizing the concerted nature of the reaction helps in visualizing how the reactants transform into products.
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Heat of Hydrogenation
The heat of hydrogenation is the amount of energy released when a compound is hydrogenated, which can be used to assess the stability of alkenes. More stable alkenes have lower heats of hydrogenation because they are less reactive and require less energy to convert to alkanes. In the context of the question, comparing the heats of hydrogenation for products A and B will help determine which product is more stable and thus favored in the reaction.
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