Free Radical Reactions
By James Ashenhurst
Free Radical Initiation: Why Is “Light” Or “Heat” Required?
Last updated: April 12th, 2019
If you come across just a few free-radical reactions, you should notice a familiar pattern. Every free-radical reaction that you’ll encounter is accompanied by either “heat” or “light”.
In fact, this is one of the most important clues to knowing you’re dealing with a free radical reaction!
So why is that?
Bond Breaking Requires An Input Of Energy
Free radicals are created when a bond undergoes homolytic cleavage – that is, the bond breaks such that each atom receives the same number of electrons.
It’s important to recognize that breaking a bond requires an input of energy to the molecule.* The energy required for bond cleavage commonly comes from two sources – you guessed it – heat, or light.
Take chlorine, for example. The heating of chlorine gas (actually – more commonly, of chlorine dissolved in a solvent) or exposure of chlorine gas to visible light results in significant cleavage of the Cl-Cl bond to deliver two free radicals, as shown below.
[Why light? recall from general chemistry, E =hγ ; there is a relationship between the frequency of photons of electromagnetic radiation (“light”) and their energy. Photons that collide with molecules impart energy to them; this can be sufficient to break bonds if sufficient conditions are met – see below for more]
The First Step In Free Radical Chain Reactions Is “Initiation”
In free radical reactions, this first step – homolytic cleavage of a bond to yield free radicals – is referred to as “initiation”. In an “initiation step, the number of free radicals is always increased”.
You might ask: once a bond breaks into two radicals, what’s to stop it from re-combining? Good point! In fact – and this is often neglected in textbooks – it’s more proper to think of bond-breaking and bond-forming as being in equilibrium with each other. As we’ll go into details in the next post, only a very small concentration of free radicals is required for a reaction to take place.
This first step – initiation – is often quite slow. In fact, free radical reactions are often observed to have an induction period. That is, after all the reagents are mixed together, there is often a variable period of time where no reaction is observed, followed by a sudden – sometimes explosive! – acceleration of the rate.
In the case of chlorination of alkanes, as we’ll see, the generation of an unstable chlorine radical is followed by subsequent removal of a hydrogen atom from an alkane followed by chlorination of the carbon free radical, in a chain reaction.
In the next post we’ll talk about the three stages of a free radical reaction – initiation, propagation, and termination.
For more specific information about what is actually happening when heat or light interacts with a molecule and how this leads to fragmentation, read on below the fold.
Next Post: Initiation, Propagation, Termination
So what’s going on?
Fully understanding the importance of heat or light (i.e. energy put into the system) requires thinking about molecular orbitals. Recall that when two atoms come together to form a single (“sigma”) bond, there are two ways by which their orbitals can overlap. Constructive orbital overlap – where both orbitals have the same “sign” – results in an orbital in the space between the two atoms. Two electrons held between two positively charged nuclei results in an overall lowering of the energy of the system due to attraction between the opposite charges. This is referred to as “bonding“, and the overall energy of stabilization is referred to as the bond dissociation energy.
There is also an alternative means of orbital overlap, between two orbitals of opposite sign. This results in destructive interference, and therefore zero electron density in the space between the two atoms; the electrons are instead localized to the space away from the other atom. The result is that two positively charged nuclei are held tightly together in space without any negatively charged electrons to “glue” them together —> this is unstable, and referred to as “antibonding” (σ*). Even though putting electrons in the antibonding orbital results in instability, it’s the only orbital an electron can possibly be promoted to in our simple example. ** [Note 2 on antibonding below]
Remember that energy levels in molecular orbitals work like staircases, not ramps. Imagine yourself on a staircase: the step you are standing on is the highest occupied step, and the next step up is the lowest unoccupied step. In molecules, we refer to the highest occupied molecular orbital [HOMO] which is the sigma orbital in the leftmost part of the diagram, and the lowest unoccupied molecular orbital [LUMO] which is the sigma* orbital.
When thermal energy (“heat”) is imparted to a molecule in a quantity roughly equal to the energy gap between the HOMO and LUMO, an electron can be promoted from the HOMO to the LUMO, resulting in the situation shown on the right. Here, one electron is in the bonding orbital and the other is in an antibonding orbital. No longer do we have net stabilization from the two chlorine atoms being bonded relative to the two chlorine atoms being separate [in fact, it is even more unstable due to the repulsion of the two electrons] – therefore, the most favorable course of action is for the bond to break.
Likewise, light can also act in place of heat. The energy gap between HOMO and LUMO is some value ΔE. When the frequency of light that shines upon the molecule such that E = hγ, an electron will likewise be promoted from the HOMO to the LUMO, and bond cleavage can occur.
[* Note -A common misconception that breaking bonds releases energy. This misconception probably arises because ATP is the body’s source of energy and energy is released when it hydrolyzes to ADP. However the fact that energy is released by this is because the P-O bonds in ATP are weaker than the P-O bonds being formed by hydrolysis. It’s this “trading” of a less stable P-O bond for a more stable P-O bond that releases energy.]
** Students often have a hard time understanding antibonding. I recall once being asked, “why does antibonding exist?”. I will offer a non-technical analogy as means of explanation.
Imagine two people that have never met and are unaware of each others existence. Now imagine those two people meeting, getting interested in each other, falling in love, and finally getting married and living in the same house. We’ve gone from indifference (zero energy, as reference) to love (a lowering of the overall energy of the system). The couple is attracted to each other.
Now, imagine one partner being unfaithful or partaking in some type of betrayal, and the other partner finds out. Now we have two people living in close quarters who have a strong animus to each other. This is hatred – a much more unstable situation than it was before they knew of each other’s existence. This leads to immediate separation (fragmentation) to the point where they are far apart again.
Bonding = love
Non-bonding = indifference
Antibonding = hatred
Does that make sense as an analogy?