Free Radical Reactions
3 Factors That Stabilize Free Radicals
Last updated: September 28th, 2020 |
What Factors Affect Free-Radical Stability?
In the last post we introduced free radicals – neutral, electron-deficient chemical species with a partially filled orbital – and learned that they are highly reactive intermediates in organic chemistry.
In this post we’ll cover two of the most important concepts concerning these species: their geometry, and their stability. It’s this latter concept that we’ll see is particularly important for understanding many free-radical reactions in organic chemistry. [Spoiler: the factors that affect free radical stability are the same factors that stabilize carbocations [discussed previously here]
Table of Contents
- The Stability of Free Radicals Increases In The Order Methyl < Primary < Secondary < Tertiary
- Free Radicals Are Stabilized By Delocalization (“Resonance”)
- The Geometry Of Free Radicals Is That Of A “Shallow Pyramid” Which Allows For Overlap Of The Half-Filled p-Orbital With Adjacent Pi Bonds
- The Same Factors Which Stabilize Free Radicals Also Stabilize Carbocations
- (Advanced) References and Further Reading
Let’s talk a bit about stability first, and then circle back to their structure. Being electron deficient, you might already have a hunch regarding factors that might stabilize free radicals. Waaaay back, we talked about how a considerable portion of organic chemistry can be explained simply by understanding that: 1) opposite charges attract (and like charges repel), and 2) the stability of charges increases if it can be spread out over a greater volume. These still apply here!
Electron poor species are stabilized by neighboring atoms that can donate electron density. [“if you’re poor, it helps to have rich neighbors”]. The most common way to interpret “rich neighbors” here is the observation that increasing the number of alkyl groups on the carbon bearing the free radical increases its stability. Radical stability increases in the order methyl < primary < secondary < tertiary. [For a second, more conceptually complex example, see the bottom of the post]. **
Secondly, we have also learned that any factor which can lead to the electron deficient site being delocalized [spread out] over a larger area will also stabilize electron poor species. Previously, for example, we’ve seen that the positive charge of a carbocation is considerably stabilized when it is adjacent to a π bond.
That’s because the carbocation is sp2 hybridized and bears an empty p orbital, allowing for overlap with the adjacent p orbitals and therefore leading the positive charge to be delocalized over multiple carbon atoms, in a manner that is most easily grasped by drawing resonance structures.
Carbocations are flat – so it’s easy to see how the p orbital could be in line with adjacent p orbitals of a double bond. But what about the geometry of free radicals?
3. The Geometry of Free Radicals Is That Of A “Shallow Pyramid”, Which Allows For Overlap Of The Half-Filled p-Orbital With Adjacent Pi Bonds
If we draw out the electrons in a typical alkyl free radical, we see that there are three bonding pairs and a single unpaired electron, for a total of four occupied orbitals. By analogy to, say, amines, we might expect that the hybridization of the molecule to be sp3 and geometry of a free radical would be trigonal pyramidal. That’s actually a good approximation, except that the “pyramid” is a little shallower than it is for molecules which have a full lone pair. ** [see note below]
When the free radical is adjacent to a π bond, there’s a significant stabilization to be obtained if the p orbitals are all in line so they can overlap [“conjugation”] with each other. Overlap is increased (and the molecule’s energy lowered) if the “shallow pyramid” is flattened out. It’s a good approximation to think of a free radical adjacent to a π bond as being “sp2” hybridized.
So what does this all boil down to? The electron-deficient free radical can be delocalized over multiple carbons. Therefore, free radicals are stabilized by resonance.
If you read the article on the stabilization of carbocations, you might notice something: the same factors which stabilize free radicals are also the same factors which stabilize carbocations!
Quiz time: one of the most stable free radicals known is the triphenylmethyl radical, discovered by Moses Gomberg in 1900. In the absence of oxygen, this radical is indefinitely stable at room temperature. Can you identify the factors which might make this free radical particularly stable?
Next Post: What Factors Destabilize Free Radicals?
In addition to alkyl groups, free radicals are also stabilized by adjacent groups with lone pairs, such as oxygen and nitrogen. At first thought, oxygen might not seem like much of an electron donating group, since it’s quite electronegative. However, oxygen does have two lone pairs of electrons. How might these be involved?
The adjacent oxygen atom can donate electron density to the half-empty p orbital, which is a stabilizing interaction. The orbital picture looks like this.
Yes, there’s an electron in the antibonding orbital, but on the whole the interaction is stabilizing since bonding electrons outnumber antibonding electrons here.
** One note for advanced students – the “shallow pyramid” has a low barrier to inversion. This means that if a free radical is formed from an optically active chiral center, rapid racemization generally ensues.
- AN INSTANCE OF TRIVALENT CARBON: TRIPHENYLMETHYL.
Journal of the American Chemical Society 1900, 22 (11), 757-771
This is regarded as the famous paper that started the field of organic free-radical chemistry. Moses Gomberg (University of Michigan) was attempting to synthesize hexaphenylethane, and so tried reducing triphenylmethyl chloride (trityl chloride) with Zn metal. Instead, he got a yellow solution which gave a product incorporating oxygen when exposed to air. When he conducted this experiment under a CO2 blanket (remember, pure Ar was not readily available back then!), he found that the solution was indefinitely stable, and deduced that it was the triphenylmethyl radical, which reacted with oxygen when exposed to air, forming a peroxide. At the end of the paper, Gomberg states “This work will be continued and I wish to reserve the field for myself”. The ACS declared the University of Michigan as a national historical chemical landmark in 2000, celebrating the centenary of this discovery, and Prof. Melanie Sanford (U Michigan) currently holds the Moses Gomberg chair in Chemistry.
- Theoretical Interpretation of Carbon‐13 Hyperfine Interactions in Electron Spin Resonance Spectra
Martin Karplus and George K. Fraenkel
J. Chem. Phys. 1961, 35, 1312
The EPR (electron paramagnetic resonance) spectrum of the methyl radical leads to the conclusion that its structure could be either planar or a very shallow pyramid. Prof. Karplus is now at Harvard and received the Nobel Prize in Chemistry in 2013 for his contributions to theoretical chemistry.
- Structure of the tert-butyl radical
L. Bonazzola, N. Leray, and J. Roncin
Journal of the American Chemical Society 1977, 99 (25), 8348-8349
- Configuration of the tert-butyl radical
D. Griller, K. U. Ingold, P. J. Krusic, and H. Fischer
Journal of the American Chemical Society 1978, 100 (21), 6750-6752
The t-butyl radical has been studied extensively. Whereas experimental results have been interpreted in terms of both planar and slightly pyramidal structures, theoretical calculations favor a pyramidal structure.
- Reactions of Atoms and Free Radicals in Solution. I. A Study of the Substitution of Hydrogen on an Asymmetric Carbon Atom—The Chlorination of Primary Active Amyl Chloride
Herbert C. Brown, M. S. Kharasch, and T. H. Chao
Journal of the American Chemical Society 1940, 62 (12), 3435-3439
- N-Bromosuccinimide. III. Stereochemical Course of Benzylic Bromination
H. J. Dauben and Layton L. McCoy
Journal of the American Chemical Society 1959, 81 (20), 5404-5409
The above two papers demonstrate that radical reactions at chiral carbon centers leads to racemization, because of the easy inversion of the intermediate pyramidal radical.
- Ab Initio Calculations of the Relative Resonance Stabilization Energies of Allyl and Benzyl Radicals
David A. Hrovat and Weston Thatcher Borden
The Journal of Physical Chemistry 1994, 98 (41), 10460-10464
The stabilization energy of a vinyl group (in the allyl radical) and a phenyl group (in the benzyl radical) has been calculated to be 15.7 kcal/mol and 12.5 kcal/mol, respectively.
- Effects of adjacent acceptors and donors on the stabilities of carbon-centered radicals
G. Bordwell, Xianman Zhang, and Mikhail S. Alnajjar
Journal of the American Chemical Society 1992, 114 (20), 7623-7629
Table I in this paper contains stabilization energies of methyl radicals with various substituents (e.g. ·CH2X).