From Bruice's *Organic Chemistry*, section *18.6 Friedel–Crafts acylation of benzene* [1, pp. 876–877]:

> 
> *Friedel–Crafts acylation* places an acyl group on a benzene ring, and *Friedel–Crafts alkylation* places an alkyl group on a benzene ring.
> An acyl chloride or an acid anhydride is used as the source of the acyl group needed for a Friedel–Crafts acylation.
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> [![Friedel–Crafts acylation](https://i.stack.imgur.com/fK3zY.png)](https://i.stack.imgur.com/fK3zY.png)
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> […]
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> The synthesis of benzaldehyde from benzene poses a problem because formyl chloride, the acyl halide required for the reaction, is unstable and must be generated in situ.
> The **Gatterman–Koch reaction** uses a high-pressure mixture of carbon monoxide and HCl to generate formyl chloride and uses an aluminum chloride–cuprous chloride catalyst for the acylation.
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> [![Gatterman–Koch reaction](https://i.stack.imgur.com/lS0qt.png)](https://i.stack.imgur.com/lS0qt.png)
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From this excerpt we infer that formyl chloride is unstable.
Is it just a remark, or does it affect the reaction?
If *formyl* chloride is unstable, why is *acyl* chloride stable?

The answer by TheSimpliFire to [Stability of formyl chloride](https://chemistry.stackexchange.com/questions/107557/stability-of-formyl-chloride) suggests vigorous decomposition of formyl chloride by water, but the Gatterman–Koch reaction proceeds under anhydrous conditions. So, why is formyl chloride unstable even in water-free environment?

### Reference

1. Bruice, P. Y. *Organic Chemistry*, 8th ed.; Pearson: Upper Saddle River, NJ, **2016**. ISBN 978-0-13-404228-2.