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Free Radical Halogenation

Which step is...

Question

Which step is chain propagation step in the following mechanism?
$C l_{2} h v - \to C l^{∙} + C l^{∙}$
$C l^{∙} + C H_{4} \to^{∙} C H_{3} + H C l$
$C l^{∙} + C l^{∙} \to C l_{2}$
$^{∙} C H_{3} +^{∙} C l \to C H_{3} C l$

A

(i)

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B

(ii)

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C

(iii)

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D

(iv)

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Solution

The correct option is B (ii)
Elementary steps in which the number of free radicals consumed is equal to the number of free radicals generated are called chain propagation steps.
(i): Free radicals consumed - 0; Free radicals generated - 2
(ii): Free radicals consumed - 1; Free radicals generated - 1
(iii): Free radicals consumed - 2; Free radicals generated - 0
(iv): Free radicals consumed - 2; Free radicals generated - 0
Thus, $(i i)$ is a chain propagation step.

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Q. Select the chain propagation steps in the free radical chlorination of methane.

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Q. Consider two alternative mechanism for the chlorination of methane. Which step is crucial in deciding which mechanism is not likely to be observed?
Mechanism I Mechanism II

A ${C l}_{2} ⟶ {2 C l}^{∙}$	A ${C l}_{2} ⟶ {2 C l}^{∙}$
B ${C l}^{∙} + {C H}_{4} ⟶ H C l +^{∙} {C H}_{3}$	D ${C l}^{∙} + {C H}_{4} ⟶ {C H}_{3} C l + H^{∙}$
C $∙ C H_{3} + {C l}_{2} ⟶ {C H}_{3} C l + {C l}^{∙}$	E $H^{∙} + {C l}_{2} ⟶ H C l + {C l}^{∙}$

Q. Consider two alternative mechanisms for the chlorination of methane, Which step is crucial in deciding which mechanism is not likely to be observed?
Mechanism I:

A . C l_{2} \to 2 C l^{*}

B . C l^{*} + C H_{4} \to H C l +^{*} C H_{3}

C . ˙ C H_{3} + C l_{2} \to C H_{3} C l + C l^{*}

D . C l_{2} \to 2 C l^{*}

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Q. The cholorination of methane may proceed according to one of the two alternative mechanisms (only propagation step are shown). Which statement about these alternative reaction is correct?
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C l^{*} + C H_{4} \to H C l + H_{3} C^{*}

H_{3} C^{*} + C l_{2} \to H_{3} C C l + C l^{*}

C l^{*} + C H_{4} \to H_{3} C C l + H^{*}

H^{*} + C l_{2} \to H C l + C l^{*}

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(1)

C l_{2} \to 2 C l^{\cdot}

C l^{\cdot} + C H_{4} \to C H_{3} C 1 + H^{\cdot}

C 1^{\cdot} + C H_{4} \to C H_{3}^{\cdot} + H C I

H^{\cdot} + C l_{2} \to H C l + C l^{\cdot}

C H_{3}^{\cdot} + C l_{2} \to C H_{3} C I + C l^{\cdot}

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