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Chapter 11: Problem 77

Assuming that the mechanism for the hydrogenation of \(\mathrm{C}_{2} \mathrm{H}_{4}\) given in Section \(11-7\) 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 \(11-7 ?\)

Short Answer

Expert verified
Based on the given hydrogenation mechanism of C2H4, the predicted product of the reaction between C2H4 and D2 is \(CH_2D-CH_2D\). The reaction with D2 can be used to confirm the mechanism by comparing the predicted product with the actual product obtained experimentally. If the observed product matches the predicted \(CH_2D-CH_2D\), it supports the validity of the electrophilic addition mechanism given in Section 11-7.

Step by step solution

01

Understand the hydrogenation mechanism of C2H4

The hydrogenation mechanism of C2H4, as described in Section 11-7, is an electrophilic addition reaction that proceeds through a three-membered ring transition state. The mechanism can be briefly summarized as follows: 1. H2 molecule is adsorbed on the metal catalyst surface, splitting into two H atoms. 2. C2H4 molecule is adsorbed on the metal catalyst surface, polarizing the pi-bond into a sigma bond. 3. One H atom from the metal surface reacts with the C atom of C2H4 at the positively charged end. 4. Another H atom from the metal surface reacts with the other C atom of C2H4. The overall reaction is: C2H4 + H2 -> C2H6
02

Predict the reaction product between C2H4 and D2

Now, we need to apply this hydrogenation mechanism to the reaction between C2H4 and D2. Replacing H2 with D2 in the mechanism: 1. D2 molecule is adsorbed on the metal catalyst surface, splitting into two D atoms. 2. C2H4 molecule is adsorbed on the metal catalyst surface, polarizing the pi-bond into a sigma bond. 3. One D atom from the metal surface reacts with the C atom of C2H4 at the positively charged end. 4. Another D atom from the metal surface reacts with the other C atom of C2H4. The overall reaction is: C2H4 + D2 -> "\(CH_2D-CH_2D\)" So, based on the given mechanism, the product of the reaction between C2H4 and D2 is CH2D-CH2D.
03

Explain how the reaction with D2 can confirm the hydrogenation mechanism for C2H4

The reaction of C2H4 with D2 can be used to confirm the mechanism for the hydrogenation of C2H4 by comparing the predicted product with the actual product obtained from the reaction. If the experimentally observed product matches the predicted product (CH2D-CH2D), it supports the validity of the electrophilic addition mechanism given in Section 11-7. Moreover, if the CHD2-CH3 product was formed instead, it would suggest a different mechanism, like a 1,2-hydride (or deuteride) shift, indicating the need for reevaluating the proposed mechanism for the hydrogenation of C2H4.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Electrophilic Addition
In organic chemistry, **electrophilic addition** is a crucial reaction type that involves the addition of an electrophile to a nucleophile. This mechanism is central to understanding how molecules like ethylene (\( \mathrm{C}_2\mathrm{H}_4 \)) undergo transformations during reactions like hydrogenation.

During electrophilic addition, a pi bond (\( \pi \)-bond) in a molecule, which is weak and easily polarized, is broken to form stronger sigma bonds (\( \sigma \)-bonds).
  • The reaction between \( \mathrm{C}_2\mathrm{H}_4 \) and \( \mathrm{H}_2 \) involves the initial adsorption of \( \mathrm{H}_2 \) on a metal catalyst, where it splits into two hydrogen atoms.
  • Next, \( \mathrm{C}_2\mathrm{H}_4 \) is adsorbed, and the \( \pi \)-bond in ethylene is converted to a \( \sigma \)-bond as the hydrogen atoms add to the carbon atoms.
This mechanism proceeds through a **transition state** that helps facilitate the bond transformation. Understanding this step is key for comprehending how electrophilic addition reactions occur.
Transition State
The **transition state** is one of the central concepts in understanding reaction mechanisms, offering a glimpse into the fleeting, high-energy states molecules go through on their path from reactants to products.

In the hydrogenation of ethylene, it is described as a three-membered ring where both carbon atoms and the catalytic surface interact simultaneously with the hydrogen atoms adsorbed.
  • This state is transient and is never isolated but is crucial for lowering the activation energy required for the reaction.
  • It represents the peak of the energy barrier that must be overcome for the reaction to progress from reactants to products.
By analyzing such transition states, chemists gain deeper insights into the pathways of reactions and can manipulate conditions to favor particular outcomes, enhancing the efficiency and specificity of the reaction process.
Deuterium Labeling
**Deuterium labeling** is a valuable tool in organic chemistry for probing reaction mechanisms. By replacing \( \mathrm{H}_2 \) with \( \mathrm{D}_2 \) (deuterium, the isotope of hydrogen), researchers can track the pathway of atoms through a reaction, given the slight difference in mass and bond energy between hydrogen and deuterium.

In the context of the hydrogenation of ethylene, using \( \mathrm{D}_2 \) instead of \( \mathrm{H}_2 \) offers a unique way to test the proposed reaction mechanism.
  • If the process follows the described electrophilic addition, the product should be \( \mathrm{CH}_2\mathrm{D}-\mathrm{CH}_2\mathrm{D} \).
  • This can be verified by spectroscopic methods, where the position and integration of deuterium atoms provide evidence for the mechanism.
  • Observing a different product, such as \( \mathrm{CHD}_2-\mathrm{CH}_3 \), would suggest an alternative mechanism might be at play, like a rearrangement of atoms.
Deuterium labeling serves as a powerful experimental technique to confirm hypotheses about reaction pathways, offering insights beyond what conventional experiments might reveal.

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Most popular questions from this chapter

The activation energy for a reaction is changed from \(184 \space\mathrm{kJ} /\) mol to \(59.0 \space\mathrm{kJ} / \mathrm{mol}\) at \(600 .\) K by the introduction of a catalyst. If the uncatalyzed reaction takes about 2400 years to occur, about how long will the catalyzed reaction take? Assume the frequency factor \(A\) is constant, and assume the initial concentrations are the same.

Hydrogen peroxide and the iodide ion react in acidic solution as follows: $$\mathrm{H}_{2} \mathrm{O}_{2}(a q)+3 \mathrm{I}^{-}(a q)+2 \mathrm{H}^{+}(a q) \longrightarrow \mathrm{I}_{3}^{-}(a q)+2 \mathrm{H}_{2} \mathrm{O}(l)$$ The kinetics of this reaction were studied by following the decay of the concentration of \(\mathrm{H}_{2} \mathrm{O}_{2}\) and constructing plots of \(\ln \left[\mathrm{H}_{2} \mathrm{O}_{2}\right]\) versus time. All the plots were linear and all solutions had \(\left[\mathrm{H}_{2} \mathrm{O}_{2}\right]_{0}=8.0 \times 10^{-4} \mathrm{mol} / \mathrm{L} .\) The slopes of these straight lines depended on the initial concentrations of \(\mathrm{I}^{-}\) and \(\mathrm{H}^{+} .\) The results follow: The rate law for this reaction has the form $$\text { Rate }=\frac{-\Delta\left[\mathrm{H}_{2} \mathrm{O}_{2}\right]}{\Delta t}=\left(k_{1}+k_{2}\left[\mathrm{H}^{+}\right]\right)\left[\mathrm{I}^{-}\right]^{m}\left[\mathrm{H}_{2} \mathrm{O}_{2}\right]^{n}$$ a. Specify the order of this reaction with respect to \(\left[\mathrm{H}_{2} \mathrm{O}_{2}\right]\) and \(\left[\mathrm{I}^{-}\right]\) b. Calculate the values of the rate constants, \(k_{1}\) and \(k_{2}\) c. What reason could there be for the two-term dependence of the rate on \(\left[\mathrm{H}^{+}\right] ?\)

Cobra venom helps the snake secure food by binding to acetylcholine receptors on the diaphragm of a bite victim, leading to the loss of function of the diaphragm muscle tissue and eventually death. In order to develop more potent antivenins, scientists have studied what happens to the toxin once it has bound the acetylcholine receptors. They have found that the toxin is released from the receptor in a process that can be described by the rate law $$Rate \(=k[\text { acetylcholine receptor-toxin complex }]$$ If the activation energy of this reaction at \)37.0^{\circ} \mathrm{C}\( is \)26.2 \mathrm{kJ} /\( mol and \)A=0.850 \mathrm{s}^{-1},\( what is the rate of reaction if you have a 0.200-M solution of receptor-toxin complex at \)37.0^{\circ} \mathrm{C} ?$

Consider the following statements: "In general, the rate of a chemical reaction increases a bit at first because it takes a while for the reaction to get "warmed up.' After that, however, the rate of the reaction decreases because its rate is dependent on the concentrations of the reactants, and these are decreasing." Indicate everything that is correct in these statements, and indicate everything that is incorrect. Correct the incorrect statements and explain.

Describe at least two experiments you could perform to determine a rate law.
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