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Chlorination of methane

Methane is chlorinated by HOMOLYTIC cleavage of the C-H bonds and the action of chlorine and temperature or light.

Animation of hydrogen abstraction from methane by a chlorine atom

The chlorination of methane is favorable (exothermic) but the mixture of CH4 and Cl2 can stand for days without reacting, provided it is neither heated nor illuminated. The reaction is thus not spontaneous. Why?

Energy balance of the chlorination of methane: bonds of 105 and 58 kcal/mol break and bonds of 85 and 103 form; ΔH° = −25 kcal/mol
Radical mechanism of the chlorination of methane: initiation, propagation and termination

A reaction mechanism is a logical sequence of steps that allows us to understand which bonds break, which ones form and at what precise moment of the whole process.

Energy profile of the chlorination of methane with its transition states and reactive intermediates

In a reaction sequence, there exist TRANSITION STATES, where bonds are either breaking or forming, and REACTIVE INTERMEDIATES, which are transient species, of relatively high energy but local minima in the general reaction scheme.

Orbitals in hydrogen abstraction: transition state (Ea = 4 kcal/mol) and sp2 + 2p methyl radical as reactive intermediate

As a reaction progresses, the reactants get closer and, if an effective collision occurs, the bond breaking and/or forming takes place. This is not a favorable process because, at short enough distances, molecules repel each other. The potential energy increases until a state of maximum energy is reached, the so-called TRANSITION STATE, in short TS. The energy needed for the reactants to reach the TS is the ACTIVATION ENERGY, which is why many exothermic reactions do not take place spontaneously. Once the TS is overcome, the process can directly lead to the final products or, more commonly, to a REACTIVE INTERMEDIATE (in short RI) that, through a new TS, leads to the final products or, in more complex mechanisms, to a new RI, then to a new TS and so on and so forth. A reaction mechanism is the sequential account of all the TS's and RI's through which the reaction is expected to take place. The most important carbon RI's in organic reactions are the carbocations (R3C+), the free radicals (R3C·), the carbenes (R2C:) and the carbanions (R3C−).