Solution Found!
Each of the following proposed mechanisms for the
Chapter 5, Problem 3P(choose chapter or problem)
Each of the following proposed mechanisms for the free-radical chlorination of methane is
wrong. Explain how the experimental evidence disproves each mechanism.
\(\mathrm{Cl}_{2}+\mathrm{hv} \rightarrow \mathrm{C} I_{2}^{*} \text { (an "activated" form of } \mathrm{Cl}_{2} \text { ) }\)
\(\mathrm{C} I_{2}^{*}+\mathrm{CH}_{4} \rightarrow \mathrm{HCl}+\mathrm{CH}_{3} \mathrm{Cl}\)
\(\mathrm{CH}_{4}+\mathrm{hV} \rightarrow \mathrm{CH}_{3}+\mathrm{H}\)
\(\mathrm{CH}_{3}+\mathrm{Cl}_{2} \rightarrow \mathrm{CH}_{3} \mathrm{Cl}+\mathrm{Cl}\)
\(\mathrm{Cl}+\mathrm{H} \rightarrow \mathrm{HCl}\)
Equation Transcription:
Text Transcription:
Cl2+hvCI2*(an "activated" form of Cl2)
CI2*+CH4HCl+CH3Cl
CH4+hvCH3+H
CH3+Cl2CH3Cl+Cl
Cl+HHCl
Questions & Answers
QUESTION:
Each of the following proposed mechanisms for the free-radical chlorination of methane is
wrong. Explain how the experimental evidence disproves each mechanism.
\(\mathrm{Cl}_{2}+\mathrm{hv} \rightarrow \mathrm{C} I_{2}^{*} \text { (an "activated" form of } \mathrm{Cl}_{2} \text { ) }\)
\(\mathrm{C} I_{2}^{*}+\mathrm{CH}_{4} \rightarrow \mathrm{HCl}+\mathrm{CH}_{3} \mathrm{Cl}\)
\(\mathrm{CH}_{4}+\mathrm{hV} \rightarrow \mathrm{CH}_{3}+\mathrm{H}\)
\(\mathrm{CH}_{3}+\mathrm{Cl}_{2} \rightarrow \mathrm{CH}_{3} \mathrm{Cl}+\mathrm{Cl}\)
\(\mathrm{Cl}+\mathrm{H} \rightarrow \mathrm{HCl}\)
Equation Transcription:
Text Transcription:
Cl2+hvCI2*(an "activated" form of Cl2)
CI2*+CH4HCl+CH3Cl
CH4+hvCH3+H
CH3+Cl2CH3Cl+Cl
Cl+HHCl
ANSWER:
Solution 3P:
Here, we are going to describe about the experimental evidence for each mechanism.
Step 1: (a)
It requires only one photon of light to generate CH3Cl. Thus the quantum yield for the reaction is only 1.0.
Quantum yield =
But, in actual, the quantum yield is so high which is more than several hundred. Thus, the high quantum yield indicates about a chain reaction.
Therefore, the above reaction is not a chain reaction. It has no propagation steps.