Question

Question: Each of the following proposed mechanisms for the free-radical chlorination of methane is wrong. Explain how the experimental evidence disproves each mechanism.

(a)

$$\begin{gathered} {\text{CI}}_2{\text{+hv→CI}}_2{\text{(anactivatedformofCI}}_2\text{)} \\ {\text{CI}}_2{\text{+CH}}_4{\text{→HCl+CH}}_3\text{Cl} \\ \left(b\right) \\ C{H}_4+hv\to C{H}_3+H \\ C{H}_3+{\text{Cl}}_2\text{→}C{H}_3\text{Cl+Cl} \\ Cl+H\to HCl \end{gathered}$$

Short answer

Answer

(a)The mechanism of the reaction requires that one photon of light (hv) must be added for each CH₃Cl . The high quantum suggests it to be a chain reaction, but due to the absence of propagation steps, it cannot be a chain reaction.

(b)The energy of light required to break a H-CH₃ bond is 435KJ/mol . Again, the energy needed to start the reaction is 242KJ/mol , which is much less than the energy needed to break the bond.

Step-by-step solution

Free radicals

An atom or group of atoms containing odd or unpaired electrons is known as the free radical. The unpaired electron is represented by a single unpaired dot in the formula. Free radicals are electrically neutral. They are highly reactive species formed by homolytic fission of a covalent bond.

Steps involved in a free radical chain reaction

In a free-radical chain reaction, free radicals are generally created in the initiation steps. A free radical and a reactant is combined to yield a product and another free radical in the propagation steps. Lastly, the number of free radicals generally decreasesin the termination steps.

Explanation

(a) The mechanism of the reaction requires that one photon of light (hv) must be added for each CH₃Cl . The high quantum suggests it to be a chain reaction, but due to the absence of propagation steps, it cannot be a chain reaction.

(b) The energy of light required to break H-CH₃ a bond is .435KJ/mol Again, the energy needed to start the reaction is242KJ/mol , which is much less than the energy needed to break the bond.