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Source: Paula Bruice Ed. 2017 Pg.877

From this excerpt, we infer that formyl chloride is unstable. But how does that affect the reaction in any way? Is it just mentioned as a point to note or does it have an effect on the reaction. And if acetyl chloride is unstable why is acyl chloride stable? enter image description here

Also note that I do realise that this is a duplicate of Stability of formyl chloride

But we’ve not used water in Gatterman Koch reaction as reasoned by @SimpliFire in the thread. So how does it remain unstable even when water is not involved.

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  • $\begingroup$ Formyl chloride may be much less stable then higher acylchlorides. Similarly acetic acid is stable, but formic acid is unstable ( HCOOH-> H2O + CO) and should be stored with open vent $\endgroup$ – Poutnik Jun 5 at 4:40
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The simplest stable acyl chloride is ethanoyl chloride or acetyl chloride; methanoyl chloride (formyl chloride) is not stable at room temperature, although it can be prepared at –60 °C or below. (Wikipedia)

The instability of $\ce{HCOCl}$ is caused by ease of elimination of HCl from its molecules. Cl is decent leaving group and after it's gone, remaining acylium cation has a very acidic hydrogen instead of alkyl present in other acyl halides. Thus easier breaking of C-H bond vs C-C bond causes drastic difference in thermal stability.

Note also the formyl chloride can be considered not only the derivate of formic acid, but also a derivate of formaldehyde.

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    $\begingroup$ The question got me curious about the decomposition mechanism, every one I thought of involved some weird intermediates. Maybe it's a concerted reaction? The deprotonation, dehalogenation and oxygen dative bonding all happening at once? I think it's worth adding to the answer that very likely the reason formyl chloride is more unstable than other acyl chlorides is the fact the carbon is very positively charged, has a great leaving group and is connected to a very acid hydrogen (due to the charge) $\endgroup$ – IanC Jun 5 at 4:59

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