Ch. 9 - Alkenes II: Oxidation and Reduction

Mullins - Organic Chemistry: A Learner Centered Approach 1st Edition

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Mullins 1st Edition

Ch. 9 - Alkenes II: Oxidation and Reduction

Problem 50b

Chapter 8, Problem 50b

One way to think about concerted reactions is to imagine them as being stepwise reactions where, besides the slowest step, all others have infinitesimally small activation energies. Considering the hypothetical stepwise mechanism and actual concerted mechanism of epoxide formation, show what a reaction coordinate diagram might look like for each possibility.
(b) Concerted , actual mechanism (butterfly transition state:

Verified step by step guidance

Step 1: Understand the concept of a concerted reaction. A concerted reaction occurs in a single step where bonds are broken and formed simultaneously, without any intermediates. The transition state represents the highest energy point along the reaction coordinate.

Step 2: Visualize the reaction coordinate diagram for a concerted mechanism. The x-axis represents the reaction coordinate (progress of the reaction), and the y-axis represents the potential energy. For a concerted reaction, there is only one peak corresponding to the transition state.

Step 3: Consider the 'butterfly transition state' for epoxide formation. This term refers to the geometry of the transition state, where the molecule adopts a structure resembling a butterfly as bonds are simultaneously broken and formed. This geometry minimizes energy and facilitates the reaction.

Step 4: Sketch the reaction coordinate diagram. Start with the reactants at a lower energy level, followed by a single peak (the transition state) at the highest energy point, and then the products at a lower energy level than the reactants. Label the transition state as the 'butterfly transition state.'

Step 5: Compare this concerted mechanism to a hypothetical stepwise mechanism. In a stepwise mechanism, there would be multiple peaks and valleys on the reaction coordinate diagram, representing intermediates and transition states. However, for the concerted mechanism, there is only one peak, as the reaction occurs in a single step.

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Key Concepts

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

Concerted Reactions

Concerted reactions occur in a single step where bonds are formed and broken simultaneously, leading to the formation of products without any intermediates. This mechanism is characterized by a transition state that represents the highest energy point along the reaction pathway. An example is the formation of an epoxide from an alkene, where the reaction proceeds through a cyclic transition state.

Reaction Coordinate Diagram

A reaction coordinate diagram is a graphical representation that illustrates the energy changes during a chemical reaction. The x-axis typically represents the progress of the reaction, while the y-axis shows the energy of the system. Key features include the reactants, products, transition states, and intermediates, allowing for visual comparison of different mechanisms, such as concerted versus stepwise.

Butterfly Transition State

The butterfly transition state is a specific type of transition state observed in certain concerted reactions, characterized by a unique arrangement of atoms that resembles a butterfly's wings. This configuration allows for simultaneous bond formation and breaking, facilitating the transformation of reactants into products. Understanding this transition state is crucial for visualizing the energy landscape of reactions like epoxide formation.

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Intermediates vs. Transition States

Related Practice

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.

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Textbook Question

One way to think about concerted reactions is to imagine them as being stepwise reactions where, besides the slowest step, all others have infinitesimally small activation energies. Considering the hypothetical stepwise mechanism and actual concerted mechanism of epoxide formation, show what a reaction coordinate diagram might look like for each possibility.

(a) Stepwise, hypothetical mechanism:

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Textbook Question

Bromination of a highly electron-rich alkene such as 2-methoxybut-2-ene has been shown to produce approximately equal mixtures of the trans- and cis-dibromide. Suggest an explanation for this observation.

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Textbook Question

In light of your answer to Assessment 9.47, predict the product of the following reactions we have seen previously where an alcohol is substituted for water.

(c)

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Textbook Question

Bromination of buta-1,3-diene with a single equivalent of Br₂ can give either of two products. (a) Which of these products (A or B) would you predict to be more stable? Justify your answer.

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Textbook Question

In light of your answer to Assessment 9.47, predict the product of the following reactions we have seen previously where an alcohol is substituted for water.

(b)

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