[thetada]

I had an interesting discussion with my students last week, which has caused me to wonder about what does and what does not count as heterolytic or homolytic fission.

We were discussing whether the propagation stages of radical reactions can be considered to demonstrate homolytic fission. In their text book, homolytic fission is defined as the formation of two radicals by the cleavage of a covalent bond in which each of the resulting particles retains one of the bonding electrons. The caveat that two radicals should be produced means that propagation stage bond cleavage is not homolytic. I have read other definitions, which state that homolytic fission is the cleavage of a covalent bond in which each of the resulting particles retains one of the bonding electrons. That makes more sense to me.

It got more interesting today when we started discussing heterolytic fission. Two ways this might happen are spontaneously, as in the first stage of S_N1, or as a result of S_N2 attack. I see a parallel to the homolytic fission quandary. The question mark is whether it can be considered homolytic fission if the old bond breaks as a result of the formation of the new bond with the incoming radical. In that scenario, only one new radical is formed. But if we applied the same logic to heterolytic fission, it would mean that S_N1 cleavage was heterolytic fission, whereas S_N2 cleavage was not.

Looking at their definition of heterolytic fission, it states: “Happens when one bonding atoms receives both electrons from the bonding pair.” Now clearly this would be applicable in both the S_N1 and the S_N2 scenarios.

It might seem like quite a quibbling point but precise use of language is effectively part of the grading criteria. What does anyone else think? Should the production of 2 radicals form part of the definition of homolytic cleavage, or is it more usefully defined solely in terms of how the bonding pair is distributed?

(Ha! I’ve just been consulting the definitions in the text book and they have them the wrong way round!)

[mjc123]

As far as I'm concerned, all four reactions involve breaking a bond; the question is whether one atom gets both the bond electrons or whether they are split one each. So propagation is homolytic, and S_N2 is heterolytic, just as much as initiation and S_N1 respectively. I see no sense in saying anything else. It is of importance to know that in the propagation step the R gets one of the electrons to form an R· radical, and the H takes one to form a bond with Cl·, and I don't know how to express this other than by saying the bond is cleaved homolytically. Similarly with S_N2/heterolytic.
The only nuance I can think of is if "fission" is narrowly defined as one molecule splitting apart into two moieties without concomitant bond formation, so that initiation and S_N1 are "fission" reactions, but propagation and S_N2 are "displacement" reactions or something. Then "the caveat that two radicals should be produced" does not mean that "propagation stage bond cleavage is not homolytic", but that it is not "fission". But I'm not familiar with this usage; I think fission simply means bond breaking. I suspect that the textbook definition is simply carelessly narrow, assuming that homolytic bond cleavage will result in two radicals, as one might lazily assume heterolytic fission results in two ions (but what about e.g. R-OH₂⁺  R⁺ + H₂O?). But look a bit closer at the context of your textbook definitions, to see whether that sheds any light on their meaning.

[thetada]

Thanks for your reply. We agree on the most sensible usage. I've just looked again at one of the other sources I consulted, and I realised I misinterpreted it. I think it is simply a case of the definition in my textbook being a bit limited.