Question

Draw energy versus reaction progress diagrams for the concerted and two-step mechanisms for the addition of \(\mathrm{BH}_{3}\) to alkenes.

Short answer

Concerted: Single peak energy diagram. Two-step: Two-peak diagram with an intermediate.

Step-by-step solution

Understand Reaction Mechanisms

Review the two types of mechanisms for the addition of \( \mathrm{BH}_{3} \) to alkenes: concerted mechanism and two-step mechanism.

Draw Energy Diagram for Concerted Mechanism

For a concerted mechanism, draw a single peak energy diagram with reaction progress on the x-axis and energy on the y-axis. Mark the reactants at the beginning, the transition state at the peak, and the products at the end. This indicates that the reaction occurs in one step without intermediates.

Draw Energy Diagram for Two-Step Mechanism

For a two-step mechanism, draw a diagram with two peaks, representing two transition states. The reactants start at the beginning, the first transition state at the first peak, an intermediate at a local minimum between peaks, the second transition state at the second peak, and the final products at the end.

Label Each Part of the Diagrams

Label the reactants, intermediates, transition states, and products clearly on both diagrams. For the concerted mechanism, only one transition state is labeled. For the two-step mechanism, there are two transition states and one intermediate labeled.

Key concepts

headline of the respective core concept

A concerted mechanism refers to a type of reaction where all the bond-breaking and bond-making processes happen simultaneously in a single step. In the context of adding \( \mathrm{BH}_{3} \) to an alkene, this means both reactants transform directly into products without any intermediates forming during the process.
During the concerted reaction, the energy path shows just one transition state. The energy diagram is uncomplicated: it starts with the energy level of the reactants, rises to a peak representing the transition state, and then descends to the energy level of the products.
Key points to note in a concerted mechanism:

• Only one transition state in the energy diagram
• No intermediates are formed
• The reaction occurs in a single step

Understanding this mechanism is crucial because it means we are dealing with a high energy state only once throughout the reaction.

headline of the respective core concept

The two-step mechanism involves the formation of an intermediate, meaning the reaction occurs in stages. When adding \( \mathrm{BH}_{3} \) to an alkene by this mechanism, the process is broken into two distinct steps.
The corresponding energy diagram has two transition states and one intermediate. It begins at the reactants' energy level, increases to the first peak for the first transition state, dips to a local minimum for the intermediate, and rises again for the second transition state before finally descending to the products' energy level.
Important details about the two-step mechanism include:

• Two peak transition states in the energy diagram
• An intermediate stage where the energy momentarily decreases
• The reaction proceeds in multiple steps

This pathway suggests that the reaction complexity is higher but may sometimes be more favorable under certain conditions.

headline of the respective core concept

An energy diagram is a graphical representation that shows the change in energy during a chemical reaction with respect to the progress of the reaction. The x-axis typically represents the reaction progress, while the y-axis represents the energy levels of the molecules involved.
For a concerted mechanism, the energy diagram is relatively simple with one peak representing a single transition state.
In the case of a two-step mechanism, the diagram is more complex with two peaks symbolizing two different transition states and an intermediate between them.
Key aspects to remember when looking at energy diagrams:

• Transition states are shown as peaks
• Intermediates are shown as local minima between peaks
• The height of the peaks correlates to the activation energy required for each step

Energy diagrams are useful tools because they visually represent the energy changes during the reaction and help identify the number of steps and intermediates involved.