Problem 45 - Chemistry The Molecular Science

Question

Assume that each gas-phase reaction occurs via a single bimolecular step. For which reaction would you expect the steric factor to be more important? Why? $$ \begin{aligned} \mathrm{H}_{2} \mathrm{C} &=\mathrm{CH}_{2}+\mathrm{H}_{2} \longrightarrow \mathrm{H}_{3} \mathrm{C}-\mathrm{CH}_{3} \text { or } \\ \left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}=\mathrm{CH}_{2}+\mathrm{HBr} \longrightarrow &\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CBr}-\mathrm{CH}_{3} \end{aligned} $$

Step-by-Step Solution

Answer

The steric factor is more important in the isobutylene and hydrogen bromide reaction due to increased steric hindrance.

Step 1: Understanding Reaction Participants

The first reaction involves ethylene \( \mathrm{H}_{2}\mathrm{C} = \mathrm{CH}_{2} \) and hydrogen \( \mathrm{H}_{2} \) combining to form ethane \( \mathrm{H}_{3}\mathrm{C} - \mathrm{CH}_{3} \).The second reaction involves isobutylene \((\mathrm{CH}_{3})_{2}\mathrm{C} = \mathrm{CH}_{2} \)and hydrogen bromide \(\mathrm{HBr}\) to form \((\mathrm{CH}_{3})_{2}\mathrm{CBr} - \mathrm{CH}_{3}\).The different substituents and structural bulkiness affect how easily the reacting molecules can come into contact and bind.

Step 2: Defining the Steric Factor

In chemical kinetics, the steric factor \( P \) is a measure of the probability that molecules will collide with the correct orientation to react. A lower steric factor indicates that the spatial alignment is crucial for the molecules to interact effectively.

Step 3: Analyzing Reaction Complexity

For the first reaction, ethylene and hydrogen are both small, simple molecules with few structural constraints, leading to a relatively high probability of effective collision regardless of orientation. In contrast, the second reaction involves isobutylene, which is bulkier, facing more steric hindrance due to the larger number of atoms and bonds involved. These factors make spatial orientation more critical for proper colliding.

Step 4: Identifying the Reaction with Greater Steric Importance

Based on the complexity analysis, steric effects, which hinder efficient collisions, are more significant in reactions involving larger, bulkier molecules. The steric factor would thus be more crucial for the reaction involving isobutylene and hydrogen bromide \(((\mathrm{CH}_{3})_{2}\mathrm{C} = \mathrm{CH}_{2} + \mathrm{HBr})\).

Step 5: Conclusion

The steric factor plays a more critical role in the second reaction due to the increased steric hindrance from the bulky isobutylene molecule, as opposed to the simpler ethylene in the first reaction.

Key Concepts

Bimolecular Reactions

Bimolecular reactions are a fundamental category of chemical processes where two molecules, the reactants, come together to form new molecules, known as products. These reactions involve the collision and interaction between two distinct particle species, resulting in the transformation into different chemical entities.
A classic example of this is a reaction between hydrogen gas (\( \mathrm{H}_2 \)) and ethylene (\( \mathrm{H}_2\mathrm{C} = \mathrm{CH}_2 \)), which produces ethane (\( \mathrm{H}_3\mathrm{C} - \mathrm{CH}_3 \)).
However, bimolecular reactions are not just about atoms bumping into each other. The orientation and energy of these molecules upon collision greatly influence the outcome.
Key aspects of bimolecular reactions include:

• The reaction rate depends on the concentration of both reactants. Higher concentrations typically lead to more frequent collisions, hence increasing the reaction rate.
• Spatial arrangement is critical as effective reactions require specific orientations to break and form bonds properly.

Through these interactions, we can better understand the dynamics of reactions, allowing us to predict conditions under which the transformation is most efficient.

Chemical Kinetics

Chemical kinetics is the branch of chemistry that deals with understanding the speed or rate of chemical reactions and how various factors influence this rate. It helps in deciphering reaction pathways and determining the reaction mechanism.
When analyzing reactions, particularly bimolecular ones, chemical kinetics provides important insights:
One important factor is the reaction rate, which informs us how quickly a reaction progresses. This rate is affected by parameters like temperature, pressure, and the physical state of reactants.

• Temperature: Increasing temperature generally speeds up reactions because it provides molecules with more energy to collide effectively.
• Concentration: A higher concentration of reactants leads to more frequent effective collisions, speeding up the reaction.

In the context of steric factors, kinetics can help explain why certain reactions proceed more slowly. For example, when bulky molecules such as isobutylene are involved, there is less freedom for the molecules to align properly, increasing the time it takes for them to react.

Molecular Collision Theory

Molecular Collision Theory is a theoretical framework used to describe how and why molecules react. It posits that chemical reactions occur when reactant molecules effectively collide with sufficient energy and proper orientation.
The theory outlines a few key assumptions:

• Molecules must collide to react; however, not every collision results in a reaction.
• Effective collisions that lead to a reaction must occur with adequate energy to overcome the activation energy barrier.

The steric factor, a key aspect of Collision Theory, reflects how spatial orientation affects reaction success. In our discussed reactions, for instance, ethylene and hydrogen have less stringent orientation needs due to their smaller size and simple structure.
Compared to isobutylene reactions, where the bulkier molecular structure requires a much more precise alignment to ensure a successful reaction, leading to a greater importance of the steric factor.