Radical halogenation in the lab

The chlorination of an alkane provides a simple example of a free radical chain reaction.  In the initiation phase, a chlorine molecule undergoes homolytic cleavage after absorbing energy from light:

The chlorine radical then abstracts a hydrogen, leading to an alkyl radical (step 2), which reacts with a second chlorine molecule (step 3) to form the chloroalkane product plus chlorine radical, which then returns to repeat step 2.

Likely chain termination steps are the condensation of two alkyl radical intermediates or condensation of an alkane radical with a chlorine radical.

Alkane halogenation reactions exhibit a degree of regiospecificity: if 2-methylbutane is subjected to a limiting amount of chlorine, for example, chlorination takes place fastest at the tertiary carbon.

This is because the tertiary radical intermediate is more stable than the secondary radical intermediate that results from abstraction of the proton on carbon 3, and of course both are more stable than a primary radical intermediate.   Recall that the Hammond postulate tells us that a lower-energy intermediate implies a lower-energy transition state, and thus a faster reaction.

Unfortunately, chloroalkanes will readily undergo further chlorination resulting in polychlorinated products, so this is not generally a terribly useful reaction from a synthetic standpoint.

Alkanes can be brominated by a similar reaction.   The regiochemical trends are the same as for chlorination, but significantly more pronounced (in other words, bromination is more regioselective).  This is because hydrogen abstraction by bromine radical is much less exergonic than by chorine radical – and this in turn means that the transition state for abstraction by bromine resembles the resulting intermediate more closely than the transition state for abstraction by chlorine resembles its intermediate.

Another way of saying the same thing is that the bromination transition state has more radical character than the chlorination transition state.  Trends in radical stability thus have a greater influence on the speed of hydrogen abstraction.

The reaction proceeds through the radical chain mechanism. The radical chain mechanism is characterized by three steps: initiation, propagation and termination.  Initiation requires an input of energy but after that the reaction is self-sustaining.  The first propagation step uses up one of the products from initiation, and the second propagation step makes another one, thus the cycle can continue until indefinitely.

Step 1: Initiation

Initiation breaks the bond between the chlorine molecule (Cl₂). For this step to occur energy must be put in, this step is not energetically favorable. After this step, the reaction can occur continuously (as long as reactants provide) without input of more energy.  It is important to note that this part of the mechanism cannot occur without some external energy input, through light or heat.

Step 2: Propagation

The next two steps in the mechanism are called propagation steps. In the first propagation step, a chlorine radical combines with a hydrogen on the methane. This gives hydrochloric acid (HCl, the inorganic product of this reaction) and the methyl radical. In the second propagation step more of the chlorine starting material (Cl₂) is used, one of the chlorine atoms becomes a radical and the other combines with the methyl radical.

The first propagation step is endothermic, meaning it takes in heat (requires 2 kcal/mol) and is not energetically favorable. In contrast the second propagation step is exothermic, releasing 27 kcal/mol. Since the second propagation step is so exothermic, it occurs very quickly. The second propagation step uses up a product from the first propagation step (the methyl radical) and following Le Chatelier’s principle, when the product of the first step is removed the equilibrium is shifted towards it’s products. This principle is what governs the unfavorable first propagation step’s occurance.

Step 3: Termination

In the termination steps, all the remaining radicals combine (in all possible manners) to form more product (CH₃Cl), more reactant (Cl₂) and even combinations of the two methyl radicals to form a side product of ethane (CH₃CH₃).

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