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Macroscopically, reaction rates can be of varied order. But mechanistically, most reactions are first-order or second-order/binary (e.g. SN2, many catalyst surface reactions, dimers). Most processes that are macroscopically ternary (e.g. protein synthesis) involve the formation of an intermediate (e.g. translation complex).

Are there any true elementary ternary reactions, involving the chance collision of 3 substrates?

Note: for this purpose, intermediate dimers are not ternary. I'm asking if there is any known reaction with a genuine ternary transition state, only stabilized by or requiring the 3 species.

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  • $\begingroup$ You can never prove that a reaction is ternary, even if it really is. There is always a chance of some short-lived intermediate, too unstable to observe. $\endgroup$
    – Ivan Neretin
    Jan 25 at 18:20
  • $\begingroup$ Like $\ce{ 2 ^4He <<=> ^8Be}$ and $\ce{ ^4He + ^8Be -> ^{12}C}$ what is interstellar synthesis of carbon from helium. $\endgroup$
    – Poutnik
    Jan 25 at 18:36
  • $\begingroup$ @IvanNeretin nothing is certain, but pchem could probably provide supporting evidence for a proposed ternary mechanism. $\endgroup$
    – alexchandel
    Jan 25 at 18:59
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    $\begingroup$ The transition state will likely depend on all 3 substrates, perhaps due to a favorable electronic state in 1 caused by the presence of both others. $\endgroup$
    – alexchandel
    Jan 25 at 19:05
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    $\begingroup$ See: chemistry.stackexchange.com/questions/79080/… $\endgroup$
    – Nilay Ghosh
    Jan 26 at 4:46

1 Answer 1

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We cannot really distinguish a ternary collision from two binary ones, because the binary collision takes time for the interaction to run its course and therefore any system has a nonzero chance that the third body will arrive during this time.

If the binary collision pair is stabilized, then the interaction time will be longer and the probability for a ternary interaction grows. In chemistry the poster-child example of this effect is the reduction of nitric oxide by hydrogen given by the reaction and gas-phase rate law:

\begin{align} \ce{2NO + 2H2 &-> N2 + 2H2O},& \text{rate} &= k\ce{[NO]^2[H2]} \end{align}

The simplest explanation for this ternary rare law is that there is some stability in the $\ce{N2O2}$ dimer[1], so a collision between two nitric oxide molecules that produces this dimer "hangs around" long enough for a hydrogen molecule to arrive and react in a significant fraction of cases, thus beginning the reduction.

Reference

1. Stephen G. Kukolich (1982). "The structure of the nitric oxide dimer". J. Am. Chem. Soc. 104, 17, 4715–4716. https://doi.org/10.1021/ja00381a052

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  • $\begingroup$ If an intermediate dimer forms, then I don't think this counts as a "genuine/elementary" ternary reaction as asked. (Not that it isn't an interesting example!) $\endgroup$
    – alexchandel
    Feb 1 at 19:27
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    $\begingroup$ That depends on your point of view. Is the dimer (which can decompose as rapidly as it forms) a separate species or a temporary collision state? And can you really tell? Therein lies the rub. $\endgroup$
    – Oscar Lanzi
    Feb 1 at 19:32
  • $\begingroup$ Yes, for the purposes of this question intermediate collision states (e.g. dimers) are separate. QM will distinguish between a genuine ternary stabilized transition state and a dimer that rapidly collides with a third species. $\endgroup$
    – alexchandel
    Feb 10 at 18:50

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