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How do you account for the formation of ethane during chlorination of methane ? |
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Answer» Solution :Halogenation is supposed to proceed via free radical chain mechanism involving three steps namely (a) Initiation, (b) Propagation and (c) Termination as given below : (a) Initiation (Step-I) : The reaction is initiated by homolysis of chlorine molecule in the presence of light or HEAT. The Cl-Cl bond is WEAKER than the C-C and C-H bond and hence, is easiest to break. `underset("Dichlorine")(Cl-Cl)underset("Homolysis")overset(hv)rarr underset("Chlorine free radicals")(overset(.)(Cl)+overset(.)(Cl))` ....(i) (b) Propagation (Step-II) : Chlorine free radical attacks the methane molecule and takes the reaction in the forward direction by breaking the C-H bond to generate methyl free radical with the formation of H-Cl. (i) `underset("Methane")(CH_(4))+underset("radicals")underset("free")underset("Chlorine")(overset(.)(Cl))overset(hv)rarr underset("radicals")underset("free")underset("Methyl")(overset(.)(Cl))+H-Cl` ...(ii) (ii) The methyl radical thus obtained attacks the second molecule of chlorine to form `CH_(3)-Cl` with the liberation of ANOTHER chlorine free radical by homolysis of chlorine molecule. `underset("radicals")underset("free")underset("Methyl")(overset(.)(CH_(3)))+underset("of chlorine")underset("Moelcule")(Cl-Cl)overset(hv)rarr underset("Chloromethane")(H_(3)C)- underset("radicals")underset("free")underset("Chlorine")(Cl+overset(.)(Cl))`...(ii) The chlorine and methy free radicals generated above repeat steps (a) and (b) respectively and thereby setup a chain of reactions. The propagation steps (a) and (b) are thse which directly give PRINCIPAL products, but manyother propagation steps are possible and may occur. Two such steps given below explain how more highly halogenated products are formed. `CH_(3)Cl + overset(.)(Cl) rarr overset(.)(C)H_(2)Cl+HCl` `overset(.)(C)H_(2)Cl + (Cl)-Cl rarr overset(.)(C)H_(2)Cl_(2)+overset(.)(Cl)` (c) Termination (Step -II) : The reaction stops after some time due to consumption of reactants and /or due to the following side reactions : The possible chain terminating steps are : (i) `overset(.)(Cl) + overset(.)(Cl)rarrCl + Cl` (ii) `H_(3)overset(.)(C)+ overset(.)(C)H_(3) rarr H_(3)C-CH_(3)` (Ethane) (iii) `H_(3)overset(.)(C)+ overset(.)(Cl) rarr CH_(3)-Cl` Through in (c), `CH_(3)-Cl`, the one of the product is formed but free radicals are consumed and the chain is terminated. Chlorination of methane ethane is obtained because according to step-III (b) ethane is obtained when joining of methyl free radical. |
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