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Grignard acts as a base with compounds containing acidic hydrogen:

$$\ce{RMgX + R'-OH -> R-H + R'-O-Mg-X}$$ and it acts as a nucleophile with others with an electrophilic site:

$$\ce{RMgX + R'-CHO -> R-COH-R' + HO-Mg-X}$$

Now the compound of interest is hydrogen cyanide:

$$\ce{H-CN}$$

This compound has both an acidic hydrogen and an electrophilic site(carbon). Also its worth mentioning that the alkyl counterparts of hydrogen cyanide go through the nucleophilic action of Grignard on the carbon atom.

So how does the reaction proceed here?

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    $\begingroup$ Compare the pKa's of R-H and HCN to get an answer. $\endgroup$
    – user55119
    Commented Feb 9, 2022 at 15:57
  • $\begingroup$ An old name for hydrogen cyanide is prussic acid. Does that gives you a clue? $\endgroup$
    – Waylander
    Commented Feb 10, 2022 at 9:39

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A Grignard reagent will certainly react with water to form a hydrocarbon, and based on $pK_a$ values $\ce{HCN}$ is a much stronger B-L acid than water. While a nucleophilic attack on the cyanide function is theoretically possible, it would be kinetically less favored than extraction of the sufficiently available protic hydrogen forming the hydrocarbon instead of an eventual aldehyde.

There are various other ways to make aldehydes, but this answer reports the existence of $\ce{HMgX}$ compounds which can serve as hydride-ion sources. If that hydride transfer works with nitriles, the reaction would yield a product that could be worked up to an aldehyde.

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    $\begingroup$ This question is best answered by research in Chemical Abstracts and actual research in a properly equipped Lab if nothing is found. I can think of addition schemes to remove the acidic H then possibly react with the with the CN moiety. $\endgroup$
    – jimchmst
    Commented Apr 21, 2023 at 15:24

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