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Free radical halogenation is a reaction that substitutes a chlorine or a bromine for a hydrogen on an alkane. This reaction is a photochemical one. That is, it occurs only when performed in the presence of uv light (abbreviated hv).

Typically, free radical reactions are described in three steps: initiation steps, propagation steps, and termination steps (described below). Note the use of a single headed arrow when describing the movement of a single electron.

Initiation Step:

The reaction begins with an initiation step, which is the separation of the halogen (X₂) into two radicals (atoms with a single unpaired electron) by the addition of uv light. This is called the initiation step because it initiates the reaction.

Propogation Steps:

The initiation step, or the formation of the chlorine radicals, is immediately followed by the propogation steps--steps directly involved in the formation of the product. As an example, isobutane (C₄H₁₀) will be used in the chlorination reaction. The first step is the abstraction of the hydrogen atom from the tertiary carbon (a tertiary carbon is a carbon that is attached to three other carbon atoms) Note that these are not protons (H+ ions) that are being abstracted, but actual hydrogen atoms since each hydrogen has one electron. This first propogation step forms the tertiary radical.

In the last step, the tertiary radical then reacts with another one of the chlorine molecules to form the product. Notice that another chlorine radical is regenerated, so this reaction can, in theory, go on forever as long as there are reagents. This is called a chain reaction.

A sidenote on free radical stabilities:

Hydrogens attached to more highly substituted carbons (ie. carbons with many carbons attached to them) are more reactive in free-radical halogenation reactions because the radical they form is stabilized by neighboring alkyl groups. These neighboring alkyl groups have the ability to donate some of their electron density to the electron-deficient radical carbon (a radical is short one electron of filling the atom's valence octet). Thus the hydrogen on the tertiary carbon here is abstracted in preference to the 9 other hydrogen atoms attached to a primary carbon (a carbon that is attached to only one other carbon atom) because it forms a more stable radical.

Here, the tertiary radical is stabilized by electron donation from neighboring alkyl groups.

Selectivity of free-radical halogenation

A point of note about free radical processes is that the intermediates are so highly reactive and short lived that usually you obtain a mixture of products, even though there is preference for forming more highly substituted free radical intermediates. In this example with isobutane, for instance, there would certainly be some abstraction of hydrogens attached to the primary carbons, leading to a different product than the above product (can you draw it out?).

Bromine reacts exactly the same way as chlorine; however, it is far more selective. If propane (CH₃CH₂CH₃), for example, was the substrate, 2-bromopropane would be the dominant product, and there would be only a small amount of 1-bromopropane. Free radical chlorination, though, would not be quite as selective, and there would be a greater amount of the chlorination of the primary carbon than in the bromination reaction.

Termination Steps:

Side reactions that can stop the chain reaction are called termination steps. These termination steps involve the destruction of the free-radical intermediates, typically by two of them coming together.

Other Halogens?

So why can't the other halogens such as fluorine or iodine be used? Iodine reacts endothermically (energetically uphill) and too slowly to be of much good in these free radical processes, while fluorine is at the other pole--it reacts too violently and too quickly to be selective, and can, if uncontrolled, even break carbon-carbon bonds. To understand why this is so, derive the DH's for the 4 reactions and compare them (you will find that flourination is highly exothermic, while iodonation is endothermic; chlorination and bromination, however, are right in the middle).

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