Consider the feasibility of a free-radical chain mechanism for hydrogenation of ethylene in the vapor state at \(25^{\circ}\) by the following propagation steps: $$ \begin{aligned} &\mathrm{CH}_{3}-\mathrm{CH}_{2}+\mathrm{H}_{2} \rightarrow \mathrm{CH}_{3}-\mathrm{CH}_{3}+\mathrm{H} \\ &\mathrm{CH}_{2}=\mathrm{CH}_{2}+\mathrm{H} \cdot \rightarrow \mathrm{CH}_{3} \mathrm{CH}_{2} \end{aligned} $$

Short Answer

Expert verified
In conclusion, the proposed free-radical chain mechanism for hydrogenation of ethylene in the vapor state at \(25^{\circ}\) might not be feasible due to several factors. These factors include the high reactivity of free radical species (H·) leading to competing side reactions, high energy barriers, and low selectivity towards the desired ethane product (CH3-CH3).

Step by step solution

01

Understanding the Hydrogenation of Ethylene

In this process, ethylene (or ethene), which is a double-bonded carbon compound (CH2=CH2), undergoes hydrogenation to form ethane (CH3-CH3). This means a hydrogen molecule (H2) is added across the double bond of the ethylene molecules, breaking the bond and then forming new single bonds with the carbons.
02

Identify the Free Radical

A free radical is a highly reactive molecule with an unpaired electron. In this case, the hydrogen atom (H·) is acting as a free radical due to its unpaired electron.
03

Examine the Propagation Steps

The given propagation steps for this hydrogenation mechanism involve the following reactions: 1. An ethyl radical CH3-CH2 reacts with a hydrogen (H2) molecule, forming ethane (CH3-CH3) and releasing a new free-radical hydrogen atom (H·). 2. The free-radical hydrogen atom (H·) reacts with ethylene (CH2=CH2) to form the ethyl radical (CH3-CH2).
04

Determine the Feasibility of the Mechanism

For this mechanism to be feasible, the reactions should be thermodynamically favorable, which means the overall Gibbs free energy change of the process should be negative. Additionally, the reaction rates should be satisfactory and the selectivity (i.e., the prevalence of ethane as a product over other possible by-products) should be high. However, hydrogenation of alkenes using atomic hydrogen is known to be slow and not very feasible due to energy barriers, competing side reactions, and low solubility of gaseous hydrogen in liquids. Radicals like H· are also likely to react with the starting materials, intermediates, or products, leading to the formation of other compounds. Furthermore, the hydrogen free radical is highly reactive, and it can react with other compounds in the vapor state, potentially leading to the formation of side products, which would decrease the selectivity towards the desired ethane product. In conclusion, although it is theoretically possible for a free-radical chain mechanism to occur, the proposed hydrogenation of ethylene in the vapor state at 25°C involving this mechanism might not be feasible. Reasons include the high reactivity of free radical species leading to competing side reactions, high energy barriers, and low selectivity towards the desired end product.

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