# Why is formyl chloride unstable while higher acyl chlorides are stable?

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?

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.

• 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 – Poutnik Jun 5 '19 at 4:40

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.