Question

Draw a reaction coordinate diagram for the solvolysis of \(2,2,2\) -triphenylethyl chloride in acetic acid. Pay special attention to the phenonium-ion intermediate. What would be the difference in this diagram if the phenonium ion were a transition state instead of an intermediate?

Short answer

In the reaction coordinate diagram for the solvolysis of 2,2,2-triphenylethyl chloride in acetic acid, the phenonium ion intermediate is represented by an energy level above the reactants. The diagram has two energy barriers connecting the reactants, the phenonium ion intermediate, and the products. If the phenonium ion were a transition state instead of an intermediate, there would be no separate energy level for it, and there would be only one energy barrier between reactants and products. The transition state energy level would be at the highest energy point of the energy barrier.

Step-by-step solution

Understand the mechanism for the solvolysis of 2,2,2-triphenylethyl chloride in acetic acid

First, we need to understand the mechanism for the solvolysis of 2,2,2-triphenylethyl chloride. In this process, the chloride anion leaves, generating a phenonium ion intermediate. The solvent (acetic acid) then attacks the phenonium ion and is deprotonated, leading to the formation of a product.

Draw the reactants, intermediates, and products of the reaction

Draw the structure for 2,2,2-triphenylethyl chloride, the phenonium ion intermediate, and the final product resulting from the solvolysis in acetic acid. #for the Phenonium ion intermediate, \[ C_{c_1} = charged carbon (c_1) \] #for the Product, \[ C_{c_1}OH = product \]

Draw the reaction coordinate diagram

To draw the reaction coordinate diagram, we will plot the energy levels of the reactants, intermediates, and products along the reaction coordinate. Label the axes as "Energy" on the vertical axis, and "Reaction Coordinate" on the horizontal axis. 1. Plot the energy level of the reactants (2,2,2-triphenylethyl chloride) on the left side of the diagram. 2. Plot the energy level of the phenonium ion intermediate above the reactants. 3. Indicate an energy barrier (activation energy) for the conversion from reactants to the phenonium ion intermediate. 4. Plot the energy level of the products after the solvent has attacked and deprotonated the phenonium ion. 5. Indicate an energy barrier (activation energy) for the conversion from the intermediate to the products.

Compare the diagram with and without a transition state

Now, we will discuss the difference in the diagram if the phenonium ion were a transition state instead of an intermediate. In this case, we would not have an energy level for an intermediate between the reactants and the products. There would be a single energy barrier between the reactants and the products, representing the activation energy for the reaction. The transition state energy level would be at the highest energy point of the energy barrier.

Key concepts

Solvolysis Mechanism

Solvolysis is a chemical reaction in which a solute is dissolved in a solvent, resulting in the breaking of a chemical bond in the solute. Specifically, it's when a solvent causes the cleavage of a bond. In our case, we look at the solvolysis of 2,2,2-triphenylethyl chloride in acetic acid. The process generally unfolds in a few clear steps:

• The chloride anion detaches from the triphenylethyl group, generating a positively charged intermediate.
• The intermediate created is known as a phenonium ion. It plays a central role in this mechanism.
• Acetic acid, our solvent, attacks the phenonium ion, stabilizing it and leading the reaction towards product formation.
• Finally, deprotonation occurs, resulting in the final product.

This mechanism highlights the importance of the solvent, here acetic acid, in facilitating the reaction by engaging with the intermediate phenonium ion.

Phenonium Ion

The phenonium ion is a crucial intermediate in certain solvolysis reactions, including the one we're examining. This intermediate is a type of carbocation, which is a positively charged carbon atom, formed when the chloride leaves the triphenylethyl chloride molecule. The formation involves the delocalization of electrons across phenyl rings, lending stability to what might otherwise be a precariously reactive species.
Under typical conditions, the phenonium ion is not very stable; however, its unique structure allows for various interactions that can stabilize it for some time. It is this stabilization that gives rise to specific pathways in chemical reactions, dictating how the remainder of the reaction will proceed.
This ion's existence is evidenced by the appearance of a distinct dip or plateau in the reaction coordinate diagram, marking it as an energetically distinct state.

Transition State

In a chemical reaction, the transition state represents a fleeting, high-energy configuration of atoms at the peak of an energy barrier. Consider it a 'snapshot' of atoms on the brink of rearranging into products. When we draw a reaction coordinate diagram, the transition state's energy level is at its maximum along the reaction path.
It's significant to note the difference between intermediates, like the phenonium ion, and transition states. Intermediates are stable enough to have measurable lifetimes, while transition states occur so quickly that they are almost impossible to isolate or study directly in detail.

• In the context of this reaction, if the phenonium ion were conceptualized as only a transition state and not an intermediate, our diagram would reflect a seamless transition between reactants and products with only a single apex representing the energy barrier.
• This distinction reflects on how electrons reorganize themselves in the transformation from reactants to products.

Energy Barrier

An energy barrier, or activation energy, is the minimum amount of energy required to initiate a chemical reaction. It reflects the resistance to the transformation of reactants into products. This concept is visually represented in reaction coordinate diagrams, where it manifests as the 'hump' between reactants and products.

• In our studied solvolysis reaction, there are typically two primary energy barriers. The first is the transition to the phenonium ion from the reactant, 2,2,2-triphenylethyl chloride.
• The second barrier is crossed as this ion transforms into the final product.

Lowering the height of these barriers through catalysts or specific solvents can speed up the reaction, influencing reaction rates effectively. Thus, the energy barrier plays a significant role in determining both the feasibility and speed of a chemical reaction.

Acetic Acid Solvent

Acetic acid plays a dual role in our solvolysis reaction as both a solvent and a reactant. As the solvent, it provides the medium for the reaction to occur. The polar nature of acetic acid stabilizes ions like the phenonium ion, facilitating their formation.
Furthermore, acetic acid directly involves in the reaction's chemistry. It attacks the phenonium ion, contributing a nucleophile which is key to forming the final product. This extends the understanding of a solvent beyond simply 'dissolving' into playing an active role in shaping the course of the reaction.
By participating in the reaction, acetic acid transitions from facilitating the environment to partaking directly in the molecular transformations. This illustrates its essential role not just as a backdrop, but as a critical participant in the solvolysis mechanism at play.