Alkanes are notoriously unreactive as they are typically inert to most chemical reactions, they are used as solvents, lubricants and fuels. However, they do react with radicals. So, naturally, we need to discuss what a radical is and how radicals react. We also need to clarify some definitions. So, let's get started!
Radicals
A radical is an unpaired electron. So, for a carbon with a radical this would mean that we have a carbon molecule with 7 electrons. Since radical containing carbons lack an octet, they are electron deficient. What about hybridization and where the radical is located on, for example, methane (CH4)? Take a look below.
Now that we know a little bit about what a radical is, what about radical stability? Radicals are a type of reactive intermediate so they don't hang around very long. However, some radicals are easier to make than others. What makes a radical more or less stable? Before we can talk about stability, we need a few more definitions. We need to classify a carbon atom by the number of other carbon atoms attached to it. So, a methyl carbon is a carbon atom with only hydrogens attached to it. A primary carbon is a carbon with only one other carbon attached to it, a secondary carbon has two other carbons attached, a tertiary carbon has three other carbons attached, and a quaternary carbon has four other carbons.
Now that we know this, we can say that a general trend of radical stability is that tertiary is more stable than secondary, which is more stable than primary. This is an important facet to consider. If something is more stable, then it is easier to form. This concept plays an important role in how radicals are formed and the rate at which they are formed. We will come back to this a little bit later but for now, have a look at the following reaction coordinate diagram.
Radical chlorination of methane
Now that we have discussed what a radical is and their general stability, we can look at an example reaction. Every radical reaction will have the same three steps; initiation, propagation, and termination. Let's discuss each step independently and then we will put the entire process together.
The first step is the initiation step. Here, we generate a radical usually by light or heat. Bond dissociation enthalpies help us figure what happens here. The the bond dissociation enthalpy (BDE, can also be called bond dissociation energies) is the amount of energy required to break a bond homolytically (one electron going to each atom that is part of the bond. Remember that radicals are high energy intermediates, so that means they are highly reactive species. We want to break the bond with the lowest energy first and use that reactive species to break another, higher energy species. Looking at the BDE for a C-H bond in methane, we find that it requires 439 KJ/mol energy to break. Whereas when we look at the Cl-Cl bond, it only requires 240 KJ/mol to break. So, we want to break the Cl-Cl bond first, then use that reactive radical to break higher energy bonds.
The second step is a propagation step. Radical reactions are a type of chain reaction, meaning that when we generate a radical, using light for example, one photon can initiate the process and the entire reaction continues from there. Importantly, each propagation step should produce a new radical species. The propagation steps continue until one or more of the reagents are used up.
The termination step is when two radicals combine and we end up with a product that is not a radical. Termination events stop the chain reaction. There are many possible termination events that can happen during the reaction. Have a look at the figure below.
That's your introduction to radical chlorination of methane. The next post will discuss selectivity when using substrates other than methane and halogenation with bromine as well. Thanks for reading!
Comments