Why direct iodenation of methane is not possible
Answers
Iodine reacts with methane reversibly. When iodine reacts with methane, it gives methyl iodide and hydrogen iodide which is a strong reducing agent and it can convert to iodomethane back to methane.
C H subscript 4 space end subscript space plus space space I subscript 2 space space rightwards arrow over leftwards arrow with blank on top space space C H subscript 3 I space space space space plus space space space space H I
To overcome this difficulty iodination is carried out in presence of strong oxidizing agents like HIO3 which oxidize HI formed during the reaction.
H I O subscript 3 space space space plus space space 5 H I space space space rightwards arrow with blank on top space 3 H subscript 2 O space space space plus space space I subscript 2
In the chain initiation of methane iodination, the activation energy is even lower than it is in fluorination. Therefore, one could assume that methane iodination runs more rapidly than fluorination. However, this is not the case! The complete chain propagation (+54 kJ/mol), and, in particular, the first reaction (+142 kJ/mol), is very endothermic. As a result, the radical iodination of methane does not take place.