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

The product of the reaction between \(\mathrm{C}_{2}\mathrm{H}_{4}\) and \(\mathrm{D}_{2}\) would be \(\mathrm{CH}_{2}\mathrm{D}-\mathrm{CH}_{2}\mathrm{D}\), based on the hydrogenation mechanism given in Section 12.7. The reaction can confirm the mechanism, as the formation of this product is consistent with the transfer of deuterium atoms to the double-bonded carbon atoms in the ethene molecule.

Step-by-step solution

Write down the mechanism

The reaction mechanism for the hydrogenation of ethene (\(\mathrm{C}_{2}\mathrm{H}_{4}\)) involves the formation of a pi complex between ethene and the hydrogen molecule, followed by the transfer of hydrogen atoms to the double-bonded carbon atoms, leading to the formation of ethane. In this case, we will consider the same mechanism with deuterium (\(\mathrm{D}_{2}\)) instead of hydrogen.

Apply the mechanism to the given reaction

Using the known mechanism, let's apply it to the reaction between \(\mathrm{C}_{2}\mathrm{H}_{4}\) and \(\mathrm{D}_{2}\). In the first step, a pi complex forms between the ethene molecule and the deuterium molecule. In the second step, the deuterium atoms will transfer to the ethene molecule.

Determine the product

According to the known mechanism, both deuterium atoms will transfer to the double-bonded carbon atoms in the ethene molecule, leading to the formation of \(\mathrm{CH}_{2}\mathrm{D}-\mathrm{CH}_{2}\mathrm{D}\) as the major product.

Discuss the mechanism confirmation

If the reaction between \(\mathrm{C}_{2}\mathrm{H}_{4}\) and \(\mathrm{D}_{2}\) yields a major product of \(\mathrm{CH}_{2}\mathrm{D}-\mathrm{CH}_{2}\mathrm{D}\), it confirms the mechanism for the hydrogenation of \(\mathrm{C}_{2}\mathrm{H}_{4}\). This is because the mechanism involves the transfer of deuterium (or hydrogen) atoms to the double-bonded carbon atoms, and the observed product formation is consistent with this mechanism.

Key concepts

Pi Complex

In the context of hydrogenation, a pi complex plays a significant role. It is formed when a molecule with a double bond, like ethene (\(\mathrm{C}_{2}\mathrm{H}_{4}\)), interacts with another molecule, such as hydrogen (\(\mathrm{H}_{2}\)) or deuterium (\(\mathrm{D}_{2}\)).

This interaction involves the electrons in the pi bond of ethene aligning with the particles of \(\mathrm{H}_{2}\) or \(\mathrm{D}_{2}\), creating a temporary complex.

This formation is crucial because it primes the ethene molecule for the subsequent steps in the reaction.

• The pi complex stabilizes the intermediate state.
• It facilitates electron density transfer from the double bond to the reacting molecule.

When deuterium is involved instead of regular hydrogen, the process is akin to hydrogenation but with key differences observed in isotopic substitution. This alters the properties slightly due to the heavier mass of deuterium.

Deuterium

Deuterium is a stable isotope of hydrogen, denoted as \(\mathrm{D}\), and contains one proton and one neutron in its nucleus. This makes it double the mass of a hydrogen atom, which only contains a single proton.

In reactions involving deuterium, such as the hydrogenation mechanism with ethene, deuterium behaves similarly to hydrogen in terms of forming and breaking bonds.

However, its added mass can influence the reaction rate and transition state energies slightly.

• Deuterium is used to track reactions due to its distinguishable mass.
• It helps confirm reaction pathways without changing the outcome significantly.

In the reaction with \(\mathrm{C}_{2}\mathrm{H}_{4}\), deuterium acts as a hydron donor. When added across the double bond, it forms \(\mathrm{CH}_{2} \mathrm{D}- \mathrm{CH}_{2} \mathrm{D}\), indicating that both deuterium atoms from \(\mathrm{D}_{2}\) successfully incorporated into the product.

Ethene

Ethene, also known as ethylene, is a simple alkene with the formula \(\mathrm{C}_{2}\mathrm{H}_{4}\). It has a double bond (\(\mathrm{C} = \mathrm{C}\)) making it highly reactive in chemical processes such as hydrogenation.

The pi bonds in ethene are responsible for its reactivity, allowing it to form pi complexes during reactions. These double bonds open the door for the incorporation of hydrogen or deuterium, converting \(\mathrm{C}_{2}\mathrm{H}_{4}\) into a single-bonded structure like ethane.

• Ethene serves as an important feedstock in chemical industries for producing polymers.
• It acts as a prototype for understanding the behavior of alkenes in addition reactions.

When ethene reacts in a hydrogenation mechanism, the \(\mathrm{D}_{2}\) interaction leads to the formation of a dibasic alkane, where each carbon atom in the original double bond bonds with deuterium.

Reaction Confirmation

Reaction confirmation refers to verifying the proposed mechanism using empirical evidence. In the case of hydrogenation involving deuterium, it is essential to ensure that the mechanism is consistent with the experimental results.

When ethene reacts with deuterium, the formation of \(\mathrm{CH}_{2}\mathrm{D}-\mathrm{CH}_{2}\mathrm{D}\) as the main product helps confirm that:

• The pi complex correctly forms between deuterium and ethene, allowing for proper hydrogenation.
• The deuterium atoms are being transferred correctly to the molecule’s double bonds.

Supporting the observed product aligns with the expected mechanism and rules out alternative reaction pathways. Additionally, isotopic labeling using deuterium offers an insightful perspective and highlights the actual position where atoms are incorporated during the reaction process.