Why is acetyl cyanide not obtained from acetyl chloride?

Question

Why doesn't acetyl chloride or any other aliphatic acyl chloride react with cyanide ions and undergo nucleophilic addition elimination to form aliphatic acyl cyanide, even though this reaction occurs for aromatic acyl chloride?

This was the related question in my workbook:

Which of the following compounds will react with ethanolic $\ce{KCN}$?
A) ethyl chloride B) acetyl chloride C) chlorobenzene D) benzaldehyde

My answer is A, B, D undergoing SN2, nucleophilic addition-elimination, and benzoin condensation respectively. Answer is given as only A,D.

Searching the internet led me to this article, but I am not able to understand why.

Are you sure about Chlorobenzene reacting with cyanide under these conditions? — Waylander, Feb 23 '18 at 10:31
@Waylander But isn't CN- a stronger nucleophile than ethanol? — Govind Balaji S, Feb 23 '18 at 11:25
It is - but if acetyl cyanide is formed at all under the reaction conditions that just makes it more reactive than acetyl chloride (better leaving group) so it forms ethyl acetate even more readily. — Waylander, Feb 23 '18 at 11:54
@Waylander Thanks. That helps me understand a lot. I think you can post as answer — Govind Balaji S, Feb 23 '18 at 12:25
Depends on what they are are attached to. When it comes to acyl cyanides/chlorides the cyanides are more reactive. — Waylander, Apr 26 '19 at 11:51

score 4 · Accepted Answer · edited Feb 24 '18 at 14:16

4

Acetyl chloride on reacting with $\ce{KCN}$ in ethanol will give ethyl acetate. If acetyl cyanide forms at all, it will immediately react with the ethanol solvent as it is a more reactive species than acetyl chloride.

Acetyl cyanide is known and may be formed by catalytic reaction of hydrogen cyanide and ketene at elevated temperature. Details.

Aromatic acyl cyanides are less reactive than alkyl acyl cyanides much as aromatic acyl chlorides are less reactive. They may be formed by reaction of the neat aromatic acyl chloride with cuprous cyanide (details) and are stable to a wash with cold aq. sodium bicarbonate.

edited Feb 24 '18 at 14:16

Gaurang Tandon

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answered Feb 23 '18 at 12:30

Waylander

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See the last step in this mechanism where an acyl cyanide reacts with water. https://chemistry.stackexchange.com/questions/86061/what-is-the-mechanism-of-the-reaction-between-hexane-2-5-dione-and-nitrous-acid/86090#86090 – user55119 Feb 24 '18 at 00:05
Waylander, could you please elaborate why aromatic acyl chloride undergoes nucleophilic addition elimination with KCN, while aliphatic acyl chloride does not? (this is mentioned by OP in first paragraph of his post) – Gaurang Tandon Feb 24 '18 at 08:21
I have edited my answer to include comment on aromatic acyl cyanides – Waylander Feb 24 '18 at 14:09
Actually, my question was whether or not aromatic acyl cyanides can react with KCN. I don't see how you've answered that :/ – Gaurang Tandon Feb 24 '18 at 14:18
The paper cited by the OP http://www.journal.csj.jp/doi/abs/10.1246/bcsj.60.1085 details reaction of aromatic acyl chlorides with sodium and potassium cyanides impregnated on resins and contains other references to their formation. It also notes that aromatic acyl cyanides in liquid solution are prone to dimerise. – Waylander Feb 24 '18 at 14:39

Why is acetyl cyanide not obtained from acetyl chloride?

1 Answers1