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Recently, I learnt that molecularity of a reaction is limited to 3 and even that is quite rare. I searched online and asked my teacher about this but the explanation given was that it is difficult for more than 3 molecules to collide simultaneously.

However, I find this explanation to be superficial. I have no deep knowledge myself but I can think of a superficial counter-argument to this explanation. Consider the following diagram:-

enter image description here

Here, we can observe that 8 other molecules are in contact/colliding with the inner molecules. So, going by this superifical model, a molecularity of 8 should be possible.

There, are obviously several flaws to this elementary model but it still proves the need for a better explanation than the trivial one that I found. What is it?

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    $\begingroup$ Reaction molecularity has nothing to do with how many molecules can be simultaneously in contact with central one, geometry-wise. Typical 3 molecular case $\ce{3 ^4He -> ^{12}C}$ is $\ce{2 ^4He <=> ^{8}Be^{*} <=>[+ ^4He]^{12}C^{*} ->^{12}C + \gamma}$ $\endgroup$
    – Poutnik
    Jun 21, 2022 at 13:47
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    $\begingroup$ Usually, only 1 key can be inserted into the lock at the same time, regardless of how many keys can simultaneously touch it. Reactions are more about properly oriented collisions than touches. even trimolecular one are usually hidden forming of intermediate, reacting with yet another one. $\endgroup$
    – Poutnik
    Jun 21, 2022 at 13:50
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    $\begingroup$ Some locks may be made for 2 keys. Asking after sufficient prior thinking improves thinking. Some molecules have 2 reaction centers, some trimolecular reactions are de facto 2 fast subsequent bimolecular reactions with short-time intermediate. $\endgroup$
    – Poutnik
    Jun 21, 2022 at 13:56
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    $\begingroup$ "Key" can be made of two parts. $\endgroup$
    – Mithoron
    Jun 21, 2022 at 14:01
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    $\begingroup$ @NicolauSakerNeto Order versus molecularity. Complex reactions can have high order, consisting of several bimolecular reactions. $\endgroup$
    – Poutnik
    Jun 22, 2022 at 7:01

2 Answers 2

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Imagine vigorously shaking a box of tiny marbles. Pairs will collide often enough, but how often will three collide simultaneously? Or four? Add in the fact others have mentioned that chemical reactions require certain orientations of the colliding molecules, and certain speeds/energies, and hopefully you can see how rare a tetramolecular or even termolecular reaction is. A termolecular reaction requires simultaneous collision of three molecules, of the right relative speeds, at the right angles to each other, in the right spatial orientations.

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    $\begingroup$ Often, there is no right speed, energy, orientation nor place for the 3rd molecule to react at the same time. OTOH, for big molecules, especially polymers like proteins or polysaccharides, multimolecular reactions are possible, as there is multiple potential reaction centers. $\endgroup$
    – Poutnik
    Jun 25, 2022 at 5:22
  • $\begingroup$ @Poutnik That's a good point, I didn't consider macromolecules. I think perhaps one could regard that sort of thing as multiple reactions occuring in different places on the large molecule, rather than a single reaction involving several molecules. $\endgroup$ Jun 25, 2022 at 16:02
  • $\begingroup$ Yes, I suppose that is the case, too. Truly trimolecular reaction is hard to find. I guess this one counts H + H + M -> H2 + M ,as H + H -> H2* -> H + H. $\endgroup$
    – Poutnik
    Jun 25, 2022 at 16:53
  • $\begingroup$ Energy-wise, truly bimolecular reactions of atoms or small molecules, producing a single product are rare. As they have to deal with released energy not to break the product back. It cannot be translational energy due momentum conservation. It cannot usually(unless distributed to other bonds) be vibration energy as it would break the formed bonds. It cannot be rotational energy due angular momentum conservation. It may be electron excitation energy in some cases. But the 3rd reagent is an inert reagent, with which the kinetic energy, linear and angular momentum are shared. $\endgroup$
    – Poutnik
    Jun 26, 2022 at 7:08
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Usually reactions having 3rd order behaviour on further examination show that they form an intermediate complex that then reacts to form products. The chance of three species simultaneously coming into contact in less than a nanosecond being vanishingly small.

Experiments by Nobel prize winner Porter (Proc. Roy Soc. v261, p29,1961) on iodine atom recombination in the vapour phase show that while $\displaystyle \frac{d[I]}{dt}=-k[I]^2[M]$ is observed, M being an inert gas or iodine vapour ($I_2$) itself, the mechanism is

$$\displaystyle I+M\rightleftharpoons IM, \quad I+IM \to I_2+M$$

The observed activation energy is negative which is not possible for a single step reaction.

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