Free Radical Reactions
Bond Dissociation Energies = Homolytic Cleavage
Last updated: August 30th, 2022 |
Here’s a point which causes a lot of confusion.
Look at these two reactions. What do you think is the stronger bond, O-H or C-H?
According to this this table (PDF) the bond dissociation energy (BDE) of OH is 460 kJ/mol (110 kcal/mol) and the value for CH is 389 kJ/mol (93 kcal/mol). [For another table, see this page from Reusch]. So why is the stronger bond being broken here?
But the bond strengths here are alkyne C-H (523 kJ/mol or 125 kcal/mol) versus the tertiary C-H bond strength in this case (384 kJ/mol or 93 kcal/mol). So why is the C-H bond with the lower bond dissociation energy formed and the higher C-H bond is broken?
Here’s three clues about bond dissociation energies.
1) For C-H bonds, bond dissociation energies decrease as you add substitution to the carbon.
2) Water can interfere with acid-base reactions, but water tends not to interfere with free radical reactions. If you’ve done a Grignard reaction in the lab, you know how finicky they can be, because you need to remove all traces of water from the solvent in order for it to start. On the other hand, the same restraint doesn’t apply to free radical reactions! It’s possible to run free-radical reactions in the presence of water without any concern that the desired free-radical reaction will be trapped by the H-OH instead.
3) Another clue is in that it is much easier to form alkyl radicals than alkenyl and alkynyl radicals.
The answer is that bond dissociation energy = homolytic cleavage
The measured bond dissociation energies (BDE’s) in tables represent the breaking apart of the bond into two radicals. This is because of the way bond dissociation energies are measured – through calorimetry of radical reactions.
Therefore the bond dissociation energy reflects the stability of the radicals formed! R3C• is a more stable radical than HO• . R3C• is also a more stable radical than an alkynyl radical. It also helps to explain why the order of bond strengths goes primary C-H > secondary C-H > tertiary C-H.
Note 1. Why are homolytic bond strengths measured and not heterolytic? That’s a good question. It’s much easier to break C-H and C-C bonds in alkanes homolytically, for one. Secondly, radicals are neutral and don’t carry around a solvent shell with them, like anions. So they’re less sensitive to solvent effects. For a technical discussion, look here.
Note 2. Why might the OH radical be less stable than R radicals, and stability of alkyl radicals be greater than alkenyl and alkynyl radicals?
(Advanced) References and Further Reading
- Shortcomings of Basing Radical Stabilization Energies on Bond Dissociation Energies of Alkyl Groups to Hydrogen
Andreas A. Zavitsas, Donald W. Rogers, and Nikita Matsunaga
The Journal of Organic Chemistry 2010, 75 (16), 5697-5700
Several textbooks, including some advanced ones, provide radical stabilization energies, and this paper discusses why that may not be the best way to quantify the stability of free radicals.
- On the Advantages of Hydrocarbon Radical Stabilization Energies Based on R−H Bond Dissociation Energies
Matthew D. Wodrich, W. Chad McKee, and Paul von Ragué Schleyer
The Journal of Organic Chemistry 2011, 76 (8), 2439-2447
This paper addresses some of the shortcomings with the approach used in Ref #1 above. The late Prof. Schleyer was a very influential figure in organic chemistry, and was a pioneer in using computational methods to address interesting problems in organic chemistry.
- The Radical Stabilization Energy of a Substituted Carbon-Centered Free Radical Depends on Both the Functionality of the Substituent and the Ordinality of the Radical
Marvin L. Poutsma
The Journal of Organic Chemistry 2011, 76 (1), 270-276
- A Single Universal Scale of Radical Stabilization Energies Does Not Exist: Global Bond Dissociation Energies and Radical Thermochemistries Are Described by Combining Two Universal Scales
Andreas A. Zavitsas
The Journal of Organic Chemistry 2008, 73 (22), 9022-9026
- Bond Dissociation Energies by Kinetic Methods
Chemical Reviews 1966, 66 (5), 465-500
This paper describes experimental techniques for measuring homolytic BDEs.
- III – Bond energies
Sidney W. Benson
Journal of Chemical Education 1965, 42 (9), 502
This paper describes the empirical measurement of homolytic bond dissociation energies. This paper was written by Prof. Benson while at the Stanford Research Institute (now SRI International), a non-profit research center very close to Stanford University. In 1978, Prof. Benson joined Prof. George Olah at USC and helped established the Loker Hydrocarbon Research Institute there.
- From equilibrium acidities to radical stabilization energies
Frederick G. Bordwell and Xian Man Zhang
Accounts of Chemical Research 1993, 26 (9), 510-517
This paper attempts to correlate the acidity of a proton with the BDE of the corresponding C-H or X-H bond.
- 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).