Question

The slowest chain propagation step of the following reaction is

$CH_4 + SO_2Cl_2 \xrightarrow{R_2O_2} CH_3Cl + SO_2 + HCl$

• A. $RO^\bullet + CH_4 \longrightarrow ROH + ^\bullet CH_3$
• B. $^\bullet CH_3 + SO_2Cl_2 \longrightarrow CH_3Cl + ^\bullet SO_2Cl$
• C. $^\bullet SO_2Cl \longrightarrow SO_2 + ^\bullet Cl$
• D. $Cl^\bullet + CH_4 \longrightarrow HCl + ^\bullet CH_3$

Answer 1

Answer: (D)

D

Answer 2

The reaction proceeds via a free radical chain mechanism. The main propagation steps are $Cl^\bullet + CH_4 \longrightarrow HCl + ^\bullet CH_3$, $^\bullet CH_3 + SO_2Cl_2 \longrightarrow CH_3Cl + ^\bullet SO_2Cl$, and $^\bullet SO_2Cl \longrightarrow SO_2 + ^\bullet Cl$. Among these steps, the hydrogen abstraction from methane by the chlorine radical (step D) is typically the step with the highest activation energy and thus the slowest step in the propagation sequence for the radical chlorination of methane.