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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.

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  • $\begingroup$ Are you sure about Chlorobenzene reacting with cyanide under these conditions? $\endgroup$
    – Waylander
    Feb 23, 2018 at 10:31
  • $\begingroup$ Acetyl chloride in ethanolic KCN will give ethyl acetate $\endgroup$
    – Waylander
    Feb 23, 2018 at 10:35
  • $\begingroup$ @Waylander Oh I wanted to type B. Thanks for pointing out. $\endgroup$
    – Govind Balaji S
    Feb 23, 2018 at 11:22
  • $\begingroup$ @Waylander But isn't CN- a stronger nucleophile than ethanol? $\endgroup$
    – Govind Balaji S
    Feb 23, 2018 at 11:25
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    $\begingroup$ 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. $\endgroup$
    – Waylander
    Feb 23, 2018 at 11:54

1 Answer 1

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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.

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    $\begingroup$ See the last step in this mechanism where an acyl cyanide reacts with water. chemistry.stackexchange.com/questions/86061/… $\endgroup$
    – user55119
    Feb 24, 2018 at 0:05
  • $\begingroup$ 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) $\endgroup$
    – Gaurang Tandon
    Feb 24, 2018 at 8:21
  • $\begingroup$ I have edited my answer to include comment on aromatic acyl cyanides $\endgroup$
    – Waylander
    Feb 24, 2018 at 14:09
  • $\begingroup$ Actually, my question was whether or not aromatic acyl cyanides can react with KCN. I don't see how you've answered that :/ $\endgroup$
    – Gaurang Tandon
    Feb 24, 2018 at 14:18
  • $\begingroup$ The paper cited by the OP 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. $\endgroup$
    – Waylander
    Feb 24, 2018 at 14:39

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