Question

7 multiple choice 2 points which of the following is a propagation step in the free radical chlorination of dichloromethane? ·chcl₂ + cl· → chcl₃ ·chcl₂ + ·chcl₂ → chcl₂chcl₂ ·chcl₂ + cl₂ → chcl₃ + cl· cl₂ + uv light → 2cl· ch₂cl₂ + cl· → chcl₃ + h·

Explanation:

Step1: Recall free - radical mechanism

Free - radical chlorination has initiation, propagation, and termination steps.

Step2: Analyze initiation

$Cl_2+UV\ light
ightarrow 2Cl\cdot$ is an initiation step as it generates free radicals from non - radical species.

Step3: Analyze termination

$\cdot CHCl_2+\cdot CHCl_2
ightarrow CHCl_2CHCl_2$ is a termination step as two radicals combine to form a non - radical.

Step4: Analyze propagation

In a propagation step, a radical reacts with a non - radical to form a new radical and a new non - radical. $\cdot CHCl_2+Cl_2
ightarrow CHCl_3 + Cl\cdot$ fits this as the $\cdot CHCl_2$ radical reacts with $Cl_2$ to form $CHCl_3$ and a new $Cl\cdot$ radical.

Step5: Analyze incorrect options

$\cdot CHCl_2 + Cl\cdot
ightarrow CHCl_3$ is incorrect as radicals don't combine like this in propagation. $CH_2Cl_2+Cl\cdot
ightarrow CHCl_3 + H\cdot$ is not a typical step in this reaction as the formation of $H\cdot$ is not common in this context.

Answer:

C. $\cdot CHCl_2+Cl_2
ightarrow CHCl_3 + Cl\cdot$