Chlorination of Methane

 

HALOGENETION OF ALKANE

RADICAL HALOGENATION

Alkanes are the simplest of all organic compounds and undergo very few reactions. One of these reactions is halogenation, or the substitution of single hydrogen on the alkane for a single halogen (Cl₂ or Br₂) to form a haloalkane.

Mechanism

The reaction proceeds through the radical chain mechanism which is characterized by three steps: initiation, propagation, and termination. Initiation requires an input of energy but after that the reaction is self-sustaining.

Step 1: Initiation

During the initiation step free, radicals are created when ultraviolet light or heat causes the X-X halogen bond to undergo homolytic cleavage to create two halogen free radicals. It is important to note that this step is not energetically favorable and cannot occur without some external energy input. After this step, the reaction can occur continuously (as long as reactants provide) without more energy input.

Step 2: Propagation

The next two steps in the mechanism are called propagation steps. A chlorine radical abstracts hydrogen atoms from methane in the first propagation step. This gives hydrochloric acid (HCl, the inorganic product of this reaction) and the methyl radical. In the second propagation step, the methyl radical reacts with more of the chlorine starting material (Cl₂). One of the chlorine atoms becomes a radical and the other combines with the methyl radical to form the alkyl halide product.

Step 3: Termination

In the three termination steps of this mechanism, radicals produced in the mechanism undergo radical coupling to form a sigma bond. These are called termination steps because a free radical is not produced as a product, which prevents the reaction from continuing. Combining the two types of radicals produced can be combined from three possible products. Two chlorine radicals and a couple form more halogen reactants (Cl₂). Chlorine radical and a methyl radical can couple to form more products (CH₃Cl). And finally, two methyl radicals can couple to form a side product of ethane (CH₃CH₃).

This reaction is a poor synthetic method due to the formation of polyhalogenated side products. The desired product occurs when one of the hydrogen atoms in the methane has been replaced by a chlorine atom. However, the reaction doesn't stop there, and all the hydrogens in the methane can in turn be replaced by chlorine atoms to produce a mixture of chloromethane, dichloromethane, trichloromethane, and tetrachloromethane.

Energetics

ΔH = (Energy put into reaction) – (Energy given off from reaction)

If more energy is put into a reaction than is given off, the ΔH is positive, the reaction is endothermic and not energetically favorable. If more energy is given off in the reaction than was put in, the ΔH is negative, the reaction is said to be exothermic and is considered favorable. The figure below illustrates the difference between endothermic and exothermic reactions.

ΔH can also be calculated using bond dissociation energies (ΔH°):

ΔH=∑ΔH∘ of bonds broken−∑ΔH∘ of bonds formed

Let’s look at our specific example of the chlorination of methane to determine if it is endothermic or exothermic:

Since, the ΔH for the chlorination of methane is negative, the reaction is exothermic. Energetically this reaction is favorable.

R [in R-H]	Methyl	Ethyl	i-Propyl	t-Butyl
Bond Dissociation Energy [kcal/mol]	103	98	95	93

REFERENCE

1. https://chem.libretexts.org
2. Organic Chemistry [Part 1] by I.L Finar