Question

Which step is the chain propagation step in the following mechanism?
A.$C{l_2}\xrightarrow{{hv}}C{l^ \bullet } + C{l^ \bullet }$
B.$C{l^ \bullet } + C{H_4}{ \to ^ \bullet }C{H_3} + HCl$
C.$C{l^ \bullet } + C{l^ \bullet } \to C{l_2}$
D.\[^ \bullet C{H_3} + C{l^ \bullet } \to C{H_3}Cl\]

Answer 1

Hint: The steps involved in a reaction from which the reaction continues or proceeds forward are known as propagation steps. If the produced radicals are equal to the consumed radicals in number then it is a chain propagation step and if the number of produced radicals is less than consumed one then this reaction will not proceed for a longer time.

Complete step by step answer:
Propagation steps basically involve hydrogen abstraction or addition of radicals at the place of double bonds. To make the reaction long lasting the radical formation must go on. The reaction stops or terminates when two radical species come together and form a stable non-radical adduct/molecule.
That’s why we have to find out which step of the reaction is producing radicals equal to the number of consumed radicals.
$C{l_2}\xrightarrow{{hv}}C{l^ \bullet } + C{l^ \bullet }$
In this reaction the number of free radicals consumed is zero and the number of free radicals generated are $2$ .
The number of consumed radicals is not equal to the number of produced radicals.
So this is not a chain propagation step.
$C{l^ \bullet } + C{H_4}{ \to ^ \bullet }C{H_3} + HCl$
In this reaction the number of free radicals consumed is $1$ and the number of free radicals generated is $1$ .
The number of consumed radicals is equal to the number of produced radicals.
$ \Rightarrow $It is a chain propagation step of the given mechanism.
$C{l^ \bullet } + C{l^ \bullet } \to C{l_2}$
In this reaction the number of free radicals consumed are two and the number of free radicals generated is zero.
The number of consumed radicals is not equal to the number of produced radicals.
So this is not a chain propagation step.
Similarly in
\[^ \bullet C{H_3} + C{l^ \bullet } \to C{H_3}Cl\]
The number of consumed radicals i.e. $2$ is not equal to the number of produced radicals i.e. ${\text{zero}}$ that’s why it is not a chain propagation step.
Hence option (B) is correct.

Note:
Basically the concept of chain propagation step is defined for elementary reactions (the reactions in which one or more species react and form direct products in a single reaction). These reactions have only two types of products: radical and non-radical, no intermediate is formed.