Problem 11 Propose a mechanism for the foll... [FREE SOLUTION]

Chapter 20: Problem 11

Propose a mechanism for the following Cope rearrangement.

Short Answer

Expert verified

Question: Describe the mechanism of the Cope rearrangement in a 1,5-diene system. Answer: The mechanism of the Cope rearrangement in a 1,5-diene system involves a [3,3]-sigmatropic shift with a concerted movement of electron pairs. This leads to the formation of a new double bond between the second and sixth atom, while breaking the original double bonds between the first and second atom, and between the fourth and fifth atom. Simultaneously, new single bonds are formed between the first and fourth atom, and between the fifth and sixth atom. The transition state during this rearrangement is stabilized by the delocalization of electron pairs over the six atoms in the 1,5-diene system. The final product has a new double bond between the second and sixth atom, and new single bonds between the first and fourth atom, and between the fifth and sixth atom.

Step by step solution

Identify the 1,5-diene system

First, identify the 1,5-diene system in the given molecule. The 1,5-diene system consists of a chain of six atoms, with double bonds at the first and fifth positions.

Formation of the new double bond

In the Cope rearrangement, the [3,3]-sigmatropic shift involves movement of electron pairs in a concerted way to form a new double bond. The new double bond will be formed between the second and sixth atom of the 1,5-diene system.

Breaking of the old double bonds

As the new double bond is formed, the old double bonds (between the first and second atom, and between the fourth and fifth atom) will break. The electron pairs from these double bonds will shift to form new single bonds between the first and fourth atom, and between the fifth and sixth atom.

Stabilization of the transition state

During the Cope rearrangement, the transition state is stabilized by the delocalization of electron pairs over the six atoms in the 1,5-diene system. This delocalization allows for the simultaneous breaking and forming of the bonds without the need for any intermediates.

Final product

After the completion of the Cope rearrangement, the final product formed has a new double bond between the second and sixth atom, and new single bonds between the first and fourth atom, and between the fifth and sixth atom. This constitutes the overall reaction mechanism of the Cope rearrangement.

Unlock Step-by-Step Solutions & Ace Your Exams!

Full Textbook Solutions
Get detailed explanations and key concepts
Unlimited Al creation
Al flashcards, explanations, exams and more...
Ads-free access
To over 500 millions flashcards
Money-back guarantee
We refund you if you fail your exam.

Over 30 million students worldwide already upgrade their learning with 91影视!

Key Concepts

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

Sigmatropic rearrangement

In organic chemistry, a sigmatropic rearrangement is a fascinating type of reaction. It involves the shift of a sigma bond adjacent to one or more pi systems. The sigma bond movement results in the migration of atoms or groups from one position to another within a molecule. The Cope rearrangement is a classic example of a [3,3]-sigmatropic rearrangement. Here, the rearrangement leads to the formation of new bonds and possibly changes in molecular connectivity. Cope rearrangements occur through a concerted mechanism, meaning all bond-making and bond-breaking occur simultaneously. No intermediates are involved. This results in a very smooth and efficient reaction. The transition state plays a key role in determining the success of the rearrangement, as it is the high-energy state the molecule must pass through during the reaction.

1,5-diene system

A 1,5-diene system is a common structural feature in organic molecules that undergo Cope rearrangements. In this system, two double bonds are separated by three single-bonded carbon atoms. This specific arrangement of double bonds is crucial for the rearrangement to proceed.

The double bonds are the key players in the electron shifts that occur during the rearrangement.
In the Cope rearrangement, a new sigma bond forms between the second and sixth atoms of the original diene system.

The presence of a 1,5-diene system allows for the unique electronic interactions required during a sigmatropic rearrangement. These electron interactions enable the molecule to achieve a transition state that can effectively transform into a new molecular arrangement. The transformation is facilitated by the inherent flexibility within the 1,5-diene, allowing the molecule to reach an energetically favorable state during the reaction.

Transition state stabilization

A critical aspect of any sigmatropic rearrangement, including the Cope rearrangement, is the stabilization of the transition state. During the rearrangement, the molecule passes through this high-energy state. The transition state is characterized by partially formed and broken bonds.
Stabilization here means there is electron delocalization across the carbon framework of the 1,5-diene system. This ensures that despite the high energy, the structure is retained long enough to proceed to the next stage of the reaction.

This delocalization reduces the energy barrier of the reaction, making the rearrangement more feasible.
It allows for an almost seamless transformation from reactants to products.

Essentially, the better the transition state is stabilized, the more smoothly the Cope rearrangement can proceed. Efficient stabilization results in reactions that proceed with high specificity and low energy requirement, essential characteristics of favorable chemical transformations.

91影视

Propose a mechanism for the following Cope rearrangement.

Short Answer

Step by step solution

Identify the 1,5-diene system

Formation of the new double bond

Breaking of the old double bonds

Stabilization of the transition state

Final product

Key Concepts

Sigmatropic rearrangement

1,5-diene system

Transition state stabilization

One App. One Place for Learning.

Most popular questions from this chapter

Recommended explanations on Chemistry Textbooks

Chemistry Branches

Physical Chemistry

Organic Chemistry

Making Measurements

Chemical Analysis

Nuclear Chemistry

Study anywhere. Anytime. Across all devices.