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Following on from this question about tritium water, I was wondering whether the decay of tritium to helium-3 has ever been used in organic synthesis. Such a transition would presumably result in no strong $\ce{C-He}$ interaction and thus radical or cation formation at $\ce{C}$.

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There's actually quite an interesting review article about the effects of decaying tritium atoms in organic compounds. The formation of neutral radicals is very unfavourable, and almost always neutral helium is ejected leaving a carbocation behind instead. Compare the two processes:

$$\ \ce{R-He^+ -> R^. + \ \ He^{+}}$$ $$\ce{R-He^+ -> R^+ + \ He}$$

The latter is far more plausible, as the helium cation has an extremely exothermic electron affinity ($\rm{2370\ kJ\ mol^{-1}}$), the largest of any atomic and likely molecular monocation.

Thus, the article is titled "Tritium for generation of carbocations". The usage of tritium for carbocation formation seems to have been first investigated in 1963. This was perhaps especially important back then because carbocation chemistry had not been yet thoroughly explored until George Olah and investigation of superacidic media.

Here's an interesting excerpt of the article, from the section "Outline of the decay technique":

To minimize the extent of unwanted radiolytic processes, the activity of the tritiated precursors used in the decay experiments must be kept as low as possible, typically below 10 mCi per mol. As a consequence, the number of the decay ions, hence the amount of their neutral decay products formed in any reasonable period of time, is comparatively quite small. Even in a storage period as long as 1 year, the decay of 10 mCi of a monotritiated precursor in 1 mol of an inactive gas (“bulk gas”) yields only about $\rm{10^{16}}$ nucleogenic ions. Under the favorable assumption that all ions give a single end product, its concentration would be of the order of only a few parts per billion (ppb), posing formidable problems even to very advanced, highly sophisticated analytical methodologies. Even more seriously, the decay products would be swamped by those arising from the radiolysis of the bulk gas, promoted by the $\beta^-$ particles of tritium, since each decay event produces, together with a single nucleogenic ion, over 200 radiolytic ions.

While the reactions triggered by nucleogenic carbocations will be different from those of radiolytic ions (secondary ions formed by impacts with the beta decay electron), given that the overwhelming majority of reactions will be caused by the latter, precise application of such tritium-based carbocation production for synthetic purposes seems all but futile. It would be far simpler to just target an energetic electron beam at the sample, simulating the beta decay electron and obtaining essentially the same products.


Edit: Here's my previous answer, which is not quite as related but I thought interesting enough to share.

Such a transition would presumably result in no strong $\ce{C-He}$ interaction.

While in practice you are correct, I thought I'd point out something interesting. Some computational investigations have found that certain compounds such as $\ce{He^+ -C\equiv C-H}$ have a considerable bond between helium and carbon, with a bond dissociation energy of up to $\rm{92\ kJ\ mol^{-1}}$ at the B3LYP/6-31G** level of theory. Even the neutral molecule $\ce{HHeCCH}$ is expected to be bound, though with a small kinetic barrier to decomposition into $\ce{He + HCCH}$ of approximately $\rm{63\ kJ\ mol^{-1}}$. Such calculations have motivated the usage of tritiated molecules as a possible (and perhaps a bit desperate) synthetic pathway to helium compounds, which curiously are expected to be more stable than neon compounds.

However, as has been mentioned, the slow decay of tritium (only 0.015% of a sample decays during the course of a day) precludes synthetic usage of its transmutation except perhaps for extremely specific catalytic purposes. Increasing the tritium content in a sample could quickly make costs exorbitant, and all sorts of crazy secondary reactions pathways (having their activation energies met from the abundant energy of nuclear decay) would soon turn organic reactants into a goopy radioactive mess.

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Here are two reasons why this approach would be impractical for organic synthesis:

(1) The half life of tritium is 12.3 years, so the decay is too slow to be practical as part of a synthetic method

(2) While on a scale of nuclear reactions, tritium decay is relatively low in energy emitted, it still dwarfs a typical chemical reaction (18.6 keV or 429,000 kCal/mole) and may cause degradation of the molecule (or the bulk material) in unpredictable ways.

But it could be useful for other types of chemical studies. I will defer to others in the forum to elaborate.

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  • $\begingroup$ Regarding the first point I guess you would be using catalytic tritium both due to cost and the desire not to photolyze your material/molecules to bits. I can't think of any applications where this sort of extreme radical initiation would be useful though. $\endgroup$ – J. LS May 18 '15 at 14:09

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