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