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

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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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?

### Reference

1. Bruice, P. Y. Organic Chemistry, 8th ed.; Pearson: Upper Saddle River, NJ, 2016. ISBN 978-0-13-404228-2.
• 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 Jun 5, 2019 at 4:40
• The textbook has a typo: it's Gattermann, not Gatterman (doi.org/10.1002/cber.18980310281). Jul 3 at 6:21

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.

• 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)
– IanC
Jun 5, 2019 at 4:59
• Anybody, feel free to write a better answer, I am not an organic chemist, I am not sure now why I did not left it to them.... Jul 3 at 4:14

More stable formyl compounds

While formyl chloride is unstable, formyl fluoride, with a stronger bond from the carbonyl carbon to the halogen, is marginally more stable. It can be made by a double displacement reaction:

$$\ce{HCOONa + C6H5C(O)F → FC(O)H + C6H5COONa}$$[1]

The compound is still unstable enough at ambient temperature to require special procedures to prepare and store, as it decomposes autocatalytically. Low temperature preparation and storage over alkali metal fluorides (which absorb the hydrogen fluoride catalyst) are employed to stabilize the product.

Formyl cyanide, which does not have a good proton-acceptor atom, is stable enough to be made at high temperature. Various synthetic methods are available [2][3][4].

Cited Reference

1. Olah, G. A.; Ohannesian, L.; Arvanaghi, M. (1987). "Formylating Agents". Chemical Reviews. 87 (4): 671–686. https://doi.org/10.1021%2Fcr00080a001.

2. Lewis-Bevan, Wyn; Gaston, Rick D.; Tyrrell, James; Stork, Wilmer D.; Salmon, Gary L. (March 1992). "Formyl cyanide: a stable species. Experimental and theoretical studies". Journal of the American Chemical Society. 114 (6): 1933–1938. https://doi.org/10.1021/ja00032a001.

3. Bogey, M.; Destombes, J.L.; Vallee, Y.; Ripoll, J.L. (May 1988). "Formyl cyanide: Efficient production from allyloxyacetonitrile and its millimeter-wave spectrum". Chemical Physics Letters. 146 (3–4): 227–229. Bibcode:1988CPL...146..227B. https://doi.org/10.1016/0009-2614(88)87435-9.

4. Bogey, M.; Demuynck, C.; Destombes, J.L.; Vallee, Y. (August 1995). "Millimeter-Wave Spectrum of Formyl Cyanide, HCOCN: Centrifugal Distortion and Hyperfine Structure Analysis". Journal of Molecular Spectroscopy. 172 (2): 344–351. Bibcode:1995JMoSp.172..344B. https://doi.org/10.1006/jmsp.1995.1183