Question

Assuming that the mechanism for the hydrogenation of \(\mathrm{C}_{2} \mathrm{H}_{4}\) given in Section \(12.7\) is correct, would you predict that the product of the reaction of \(\mathrm{C}_{2} \mathrm{H}_{4}\) with \(\mathrm{D}_{2}\) would be \(\mathrm{CH}_{2} \mathrm{D}-\mathrm{CH}_{2} \mathrm{D}\) or \(\mathrm{CHD}_{2}-\mathrm{CH}_{3} ?\) How could the reaction of \(\mathrm{C}_{2} \mathrm{H}_{4}\) with \(\mathrm{D}_{2}\) be used to confirm the mechanism for the hydrogenation of \(\mathrm{C}_{2} \mathrm{H}_{4}\) given in Section \(12.7 ?\)

Short answer

Based on the mechanism for the hydrogenation of \(\mathrm{C}_{2}\mathrm{H}_{4}\) given in Section \(12.7\), the predicted product for the reaction of \(\mathrm{C}_{2}\mathrm{H}_{4}\) with \(\mathrm{D}_{2}\) would be \(\mathrm{CH}_{2}\mathrm{D}-\mathrm{CH}_{2}\mathrm{D}\). The reaction supports the mechanism as it demonstrates the catalytic role of the metal surface, the dissociation of diatomic deuterium on the metal surface, and each carbon in the alkene reacting with a deuterium atom, resulting in the formation of \(\mathrm{CH}_{2}\mathrm{D}-\mathrm{CH}_{2}\mathrm{D}\).

Step-by-step solution

Understand the hydrogenation mechanism

First, we need to understand the mechanism for the hydrogenation of \(\mathrm{C}_{2}\mathrm{H}_{4}\). According to Section \(12.7\), the mechanism involves the following steps: 1. The alkene (\(\mathrm{C}_{2}\mathrm{H}_{4}\)) coordinates to the metal surface as the metal acts as a catalyst. 2. A hydrogen molecule (\(\mathrm{H}_{2}\)) dissociates into two hydrogen atoms on the metal surface. 3. A hydrogen atom reacts with one of the carbons of the \(\mathrm{C}_{2}\mathrm{H}_{4}\), and another hydrogen atom reacts with the other carbon. This step results in the alkane product.

Determine the reaction mechanism with D₂

To predict the product of the reaction of \(\mathrm{C}_{2}\mathrm{H}_{4}\) with \(\mathrm{D}_{2}\), we can replace the hydrogen atoms with deuterium atoms (\(\mathrm{D}\)) in the mechanism: 1. The alkene (\(\mathrm{C}_{2}\mathrm{H}_{4}\)) coordinates to the metal surface. 2. A deuterium molecule (\(\mathrm{D}_{2}\)) dissociates into two deuterium atoms on the metal surface. 3. A deuterium atom reacts with one of the carbons of the \(\mathrm{C}_{2}\mathrm{H}_{4}\), and another deuterium atom reacts with the other carbon.

Determine the product of the reaction of C₂H₄ with D₂

Based on the mechanism described in Step 2, a deuterium atom will react with each carbon in the \(\mathrm{C}_{2}\mathrm{H}_{4}\) molecule. This would lead to the formation of \(\mathrm{CH}_{2}\mathrm{D}-\mathrm{CH}_{2}\mathrm{D}\) as the product of the reaction.

Explain how the reaction confirms the mechanism

The observed product of the reaction between \(\mathrm{C}_{2}\mathrm{H}_{4}\) and \(\mathrm{D}_{2}\) confirms the proposed mechanism for the hydrogenation of \(\mathrm{C}_{2}\mathrm{H}_{4}\), as it demonstrates that: 1. The metal surface is catalyzing the reaction, 2. The diatomic deuterium undergoes dissociation on the metal surface, and 3. A deuterium atom reacts with each carbon in the alkene, resulting in the formation of \(\mathrm{CH}_{2}\mathrm{D}-\mathrm{CH}_{2}\mathrm{D}\). If an alternative product such as \(\mathrm{CHD}_{2}-\mathrm{CH}_{3}\) was formed, it would suggest a different reaction mechanism not consistent with the one proposed in Section \(12.7\).

Key concepts

Catalysis

Catalysis plays a pivotal role in chemical reactions, particularly in the hydrogenation of alkenes. In the case of \( \mathrm{C}_{2}\mathrm{H}_{4} \), a metal surface acts as a catalyst to facilitate the reaction. Catalysts are substances that increase the rate of a chemical reaction without being consumed by the reaction itself.This is achieved by providing an alternative reaction pathway with a lower activation energy. The metal catalyst helps the dissociation of the hydrogen or deuterium (\( \mathrm{H}_{2} \) or \( \mathrm{D}_{2} \)) on its surface. Once dissociated, the individual hydrogen or deuterium atoms become more reactive, readily bonding with the alkene.
By catalyzing these reactions on the surface, metal catalysts help ensure that each reactant molecule effectively encounters the necessary conditions to form the desired product, enhancing both efficiency and specificity.

Deuterium labeling

Deuterium labeling involves the substitution of hydrogen atoms in a molecule with deuterium. Deuterium is a stable isotope of hydrogen and can provide valuable insights into the reaction mechanisms through these substitutions.In the exercise, \( \mathrm{C}_{2}\mathrm{H}_{4} \) reacts with \( \mathrm{D}_{2} \), and the deuterium labeling technique helps test the hydrogenation mechanism described. By using \( \mathrm{D}_{2} \) instead of \( \mathrm{H}_{2} \), we can observe the formation of products like \( \mathrm{CH}_{2}\mathrm{D}-\mathrm{CH}_{2}\mathrm{D} \).
This labeling method allows chemists to verify if the proposed mechanism involves straightforward hydrogenation or any alternative paths. If the deuterium atoms readily attach to the carbons in the alkene in the expected fashion, it confirms that deuterium effectively mirrors the pathway hydrogen would follow. Hence, this evidence strongly supports the proposed mechanism of hydrogenation.

Reaction Mechanism

The reaction mechanism is central to understanding how \( \mathrm{C}_{2}\mathrm{H}_{4} \) interacts with \( \mathrm{D}_{2} \) in the presence of a metal catalyst. This stepwise process involves several stages, starting with the coordination of the alkene molecule to the metal surface.

• The initial step is the adsorption of the alkene.
• The next involves the dissociation of the diatomic deuterium (\( \mathrm{D}_{2} \)) into two independent atoms on the surface.
• These deuterium atoms then interact with the carbons in the alkene, facilitating their conversion into an alkane.

Such detailed mechanisms allow chemists to predict the outcomes of these reactions accurately. If the product formed is \( \mathrm{CH}_{2}\mathrm{D}-\mathrm{CH}_{2}\mathrm{D} \), it validates these mechanistic steps.Additionally, any deviation in the expected product, such as the formation of \( \mathrm{CHD}_{2}-\mathrm{CH}_{3} \), implies that alternate reaction pathways may occur. Understanding this can shed light on the broader behaviors of similar chemical reactions.

Metal Surface Interaction

In hydrogenation, the interaction between molecules and the metal surface is vital. It defines the behavior and outcomes of the catalyzed reaction. For the \( \mathrm{C}_{2}\mathrm{H}_{4} \) and \( \mathrm{D}_{2} \) reaction, the metal surface serves as a host that supports and facilitates the transformation of reactants to products.The metal surface provides an environment where dissociative adsorption can occur efficiently. Here, both the alkene and the hydrogen/deuterium molecule engage with the metal. These interactions allow molecules to be temporarily held in place, encouraging chemical bonds to form or break more effectively.
Ultimately, this surface interaction is not only integral to the reaction taking place but also critical in ensuring that reactions proceed efficiently and yield the correct product as anticipated, such as \( \mathrm{CH}_{2}\mathrm{D}-\mathrm{CH}_{2}\mathrm{D} \). Understanding this interaction is crucial for designing effective catalysts and improving industrial chemical processes.