Free Radical Halogenation
In this section, we’ll learn about free radical halogenation, a reaction that adds a halogen to an alkane. This is our first mechanism, or method by which a molecule reacts with another. Mechanisms explains how electrons move to go from starting molecule(s) to product(s). We use arrows to clearly indicate the movement of electrons throughout this transformation from reactants to products.
The mechanism for radical halogenation shows how a halogen is added to an alkane. As the name suggests, we must go through a radical intermediate in the reaction.
Radicals are atoms or molecules that have an unpaired electron, so they are therefore lacking a complete octet. Radicals are rigorously looking to combine with another atom or molecule to find an electron to pair with and complete its octet. As a result, radicals are very unreactive and unstable.
As a general rule of thumb, tertiary radicals are more stable than secondary radicals, which are more stable than primary radicals.
Free Radical Halogenation Mechanism
The overall reaction for free radical halogenation (note that specifically radical chlorination is shown) is:
But how is this reaction completed? The free radical halogenation mechanism occurs in a set of three steps.
- Initiation Step
- Propagation Step
- Termination Step
The first step of free radical halogenation is called the initiation step. In this step, an energy input is required, often in the form of heat or light (called hυ):
The heat/ light provides the energy to break the two chlorine atoms apart. Specifically, this bond cleavage is homolytic, meaning the bond breaks evenly giving 1 electron to each atom. This creates two highly reactive radical species. The main goal of initiation in free radical halogenation is to create the radicals needed to begin the reaction.
Also, note there is a new arrow used in this free radical halogenation mechanism. It is called a fish-hook arrow (because it looks exactly like a fish hook), and it has it only has one side of the typical arrow head. This arrow is often commonly called a single-barbed arrow, and it indicates the movement of just a single electron, rather than the pair of electrons that a normal, double-barbed arrow uses.
The second step of free radical halogenation is called propagation, and it is the most important step of the reaction. It occurs in 2 distinct steps.
Propagation halogenates the alkane producing the desired product of the reaction. Using the mechanism shown above, a chlorine radical is also produced, which can then react with another substrate molecule (in this reaction the substrate is propane) to go through another round of the reaction. As a result, this reaction is self-propagating.
The third step of free radical halogenation ends the reaction as all the radicals become quenched:
The termination step occurs between two radicals produced throughout the course of the reaction. This will cause the reaction to stop because the reaction cannot continue without the presence of radicals.
In summary, the desired product is formed by the propagation step, while the initiation and the termination steps create and quench the radicals respectively.
While this entire reaction was shown with chlorine, all the halogens except for iodine (fluorine, chlorine, and bromine) can undergo this same reaction.
Where exactly does the halogen add?
Say you’re asked to give the product of radical chlorination with the following alkane:
There a few different carbons to which the chlorine could add. How do you know where the chlorine goes? As it turns out, the different halogens have different selectivities for adding to primary, secondary, and tertiary carbons.
If a radical chlorination reaction occurred, we’d expect the products to be ~10% primary chlorination, ~40% secondary chlorination, and ~50% tertiary chlorination.
If this same reaction were to occur with fluorine, each product would be formed in about the same amounts. If it were to happen with bromine, there would almost entirely tertiary bromination products.
Note the iodine does not undergo free radical halogenation as it is not an energetically favorable reaction.