Free Radical Reactions

By James Ashenhurst

Bond Dissociation Energies = Homolytic Cleavage

Last updated: January 21st, 2021 |

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?

what-is-stronger-bond-o-h-or-c-h-o-h-easier-to-break-with-base-but-c-h-has-smaller-bond-dissociation-energy

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?

Another example:

alkyne-deprotonation-is-easier-than-alkane-deprotonation-even-though-alkyne-c-h-bond-dissociation-energy-is-higher

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.

bond-dissociation-energies-represent-homolytic-bond-cleavage-so-they-are-good-proxies-for-radical-stabiity-tertiary-radicals-the-most-stable.

PS 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.

PPS 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

  1. 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
    DOI:
    1021/jo101127m
    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.
  2. 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
    DOI:
    1021/jo101661c
    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.
  3. 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
    DOI:
    1021/jo102097n
  4. 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
    DOI:
    1021/jo8018768
  5. Bond Dissociation Energies by Kinetic Methods
    A. Kerr
    Chemical Reviews 1966, 66 (5), 465-500
    DOI: 10.1021/cr60243a001
    This paper describes experimental techniques for measuring homolytic BDEs.
  6. III – Bond energies
    Sidney W. Benson
    Journal of Chemical Education 1965, 42 (9), 502
    DOI: 10.1021/ed042p502
    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.
  7. From equilibrium acidities to radical stabilization energies
    Frederick G. Bordwell and Xian Man Zhang
    Accounts of Chemical Research 1993, 26 (9), 510-517
    DOI: 10.1021/ar00033a009
    This paper attempts to correlate the acidity of a proton with the BDE of the corresponding C-H or X-H bond.
  8. 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
    DOI:
    1021/j100092a014
    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.
  9. 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
    DOI:
    10.1021/ja00046a003
    Table I in this paper contains stabilization energies of methyl radicals with various substituents (e.g. ·CH2X).

Comments

Comment section

14 thoughts on “Bond Dissociation Energies = Homolytic Cleavage

  1. Hello,

    I just want to make sure I am understanding this article right…So the tertiary bonds of a organic molecule will have the lowest bond dissociation energies because they produce the most stable radicals (tertiary radical)? Thanks!

    1. Boo! :-) kcal/mol all the way IMO. A C-H eclipsing interaction is about 0.9 kcal/mol and a C-H bond strength is about 100 kcal/mol. So easy to remember, like the Celsius scale.

  2. When atoms combine to form molecules, energy is released as covalent bonds form. The molecules of the products have lower enthalpy than the separate atoms.

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