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In the mechanism of alkene-ozonolysis, the intermediate molozonide adduct, which is a cyclical but symmetric molecule (shouldn't symmetry about the C-C single bond bring stability?), decomposes thermally into a ketone and a peroxy-zwitter ion via cleavage of the O−O and C−C bonds.

However, it seems unlikely, and is against the symmetry argument. Is there some better explanation? Or we don't know the correct mechanism? Please add some references.

Knowing that there are two symmetrically placed O−O bond in the adduct, the cleavage of the only one bond is weird.

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    $\begingroup$ The O-O single bond is weak (142 KJ/mol). See this answer chemistry.stackexchange.com/questions/100110/… $\endgroup$
    – Waylander
    Apr 1, 2021 at 12:04
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    $\begingroup$ Why should symmetry be stable? The "symmetry argument" doesn't exist. Symmetry does not contribute anything to the energy. $\endgroup$
    – orthocresol
    Apr 1, 2021 at 12:46
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    $\begingroup$ Look into the vibrational modes and look for any that might suggest an assymmetric cleavage path. $\endgroup$
    – Buck Thorn ♦
    Apr 1, 2021 at 13:33
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    $\begingroup$ That said, bond breaking is quite a different thing from stability. You seem to also claim that symmetry should confer on a molecule some kind of stability, such that a symmetric molecule is always more stable than its asymmetric isomer (for example). However, this is also easily falsifiable: nitrous oxide, for example, does not have oxygen as the central atom. The stability of a molecule comes from various thermodynamic considerations such as bond strengths and so on. There is no stabilisation term to be added simply based on whether a molecule happens to reflects onto itself. $\endgroup$
    – orthocresol
    Apr 1, 2021 at 14:14
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    $\begingroup$ Are you meaning the mechanism of alkene-ozonolysis (not mechanism of alcohol-ozonolysis)? $\endgroup$
    – Mathew Mahindaratne
    Apr 1, 2021 at 19:33

1 Answer 1

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Why is the molozonide adduct thermally unstable?

The molozonide adduct (1,2,3-trioxalane; the initial adduct by 1,3-dipolar cycloaddition of alkene and ozone) is thermally unstable because it contains two weak $\ce{O-O}$ bonds (Ref.1). Mechanistic studies reveal that the ozonide (1,2,4-trioxalane) is formed by a sequence of three pericyclic steps involving 1,3-dipolar cycloaddition, retro-1,3-dipolar cycloaddition, and again 1,3-dipolar cycloaddition as shown in following scheme:

Ozonide (1,2,4-trioxalane) Formation

Also, I'd like to point out that the initial molozonide adduct is not always symmetric. It is symmetric only if the alkene is symmetric. And, the rearrangement of 1,2,3-trioxalane to 1,2,4-trioxalane is not about the stability of $\ce{C-C}$ in 1,2,3-trioxalane as OP argued. It's about thermal stability of consecutive $\ce{O-O}$ bonds in 1,2,3-trioxalane (recall that one $\ce{O-O}$ bond in peroxy compounds is not thermally stable, e.g., $\ce{HO-OH}$).

Knowing that there are two symmetrically placed $\ce{O-O}$ bond in the adduct, the cleavage of the only one bond is weird.

The accepted mechanism (Criegee Mechanism) says one of either $\ce{O-O}$ bond in the initial adduct can be broken to give a corresponding carbonyl oxide (a dipole) and carbonyl compound (dipolarophile), which undergo second 1,3-dipolar cycloaddition to give stable ozonide (see above scheme). None of the mechanism says one $\ce{O-O}$ bond cleaves selectively. it could be either one:

Criegee Mechanism

However, the second 1,3-dipolar cycloaddition of either set of carbonyl oxide (a dipole) and carbonyl compound (dipolarophile) gives identical ozonide. This second dipolsar cycloaddition $([_\pi2_\mathrm{s} + _\pi\!4_\mathrm{s}])$ has also been confirmed by $\ce{^{17}O}$-NMR studies (Ref.2):

The mechanism of ozonolysis was revisited with the use of $\ce{^{17}O}$-NMR spectroscopy. In a crossover experiment with $\ce{^{17}O}$‐labelled benzaldehyde and the ozonides of styrene and ethylidenecyclohexane it was shown that only the ether bridge of the secondary ozonides is carrying the $\ce{^{17}O}$ label. This is contrary to results reported earlier and confirms the Criegee mechanism.

References:

  1. Dipak K. Mandal, “Chapter 4: Cycloadditions 1: Perturbation Theory of Reactivity, Regioselectivity and Periselectivity,” In Pericyclic Chemistry: Orbital Mechanisms and Stereochemistry; First Edition, Elsevier Inc.: Amsterdam, Netherlands, 2018, pp. 107-190 (ISBN: 978-0-12-814958-4).
  2. Christian Geletneky, Stefan Berger, “The Mechanism of Ozonolysis Revisited by $\ce{^{17}O}$-NMR Spectroscopy,” Eur. J. Chem. 1998, (8), 1625–1627 (DOI: https://doi.org/10.1002/(SICI)1099-0690(199808)1998:8<1625::AID-EJOC1625>3.0.CO;2-L)).
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    $\begingroup$ That's a nasty DOI... $\endgroup$
    – orthocresol
    Apr 4, 2021 at 1:37

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