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Shweta Thakur posted an Question
February 05, 2021 • 21:26 pm 30 points
  • IIT JAM
  • Chemistry (CY)

Why aldehyde and ketone react with weak acid such as hcn and but not with strong acid example hcl and sulfuric acid

why aldehyde and Ketone react with weak acid such as HCL and but not with strong acid example HCL and sulfuric acid

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    Dinesh khalmaniya 1 best-answer

    Actually, hydrogen cyanide reacts with aldehyde but not alone, it requires some additional reagent. Pure HCN can not react because, the HCN is very weak acid such taht it does not provide sufficient amount of cyanide ion which could alone attack over slightly positive carbon atom. HCN being an acid 1st protonates the C=O oxygen and then CN- attacks the Carbonyl carbon because after protonation of the oxygen, the carbon becomes more Electrophilic and then the CN- ion attacks the carbon to finally yield cyanohydrin. But as HCN is a very weak acid having pka= 9.2 so to initiate the dissociation or to get the conjugate base i.e CN- we use an alkaline medium sometimes, but this purpose can be fulfilled by water also. In aqueous medium HCN partially dissociates to give H+ and CN- . Now if you we use a strong acid medium then Hydrolysis of -CN will occur to form -COOH. So to prevent the further Hydrolysis we only use HCN or KCN/ weak acid or NaCN/ weak acid to form cyanohydrin.

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