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12.16: Aldehydes and Ketones with HCN: Cyanohydrin Formation Mechanism

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Organic Chemistry

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Aldehydes and Ketones with HCN: Cyanohydrin Formation Mechanism
 
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12.16: Aldehydes and Ketones with HCN: Cyanohydrin Formation Mechanism

Cyanohydrins are formed when cyanide nucleophiles and carbonyl compounds like aldehydes and ketones react. A strong base, the cyanide ion, catalyzes cyanohydrin formation. The ions are generated from HCN under aqueous conditions. Once the cyanide ions are generated, the first step involves the nucleophilic attack of the cyanide ions on the electrophilic carbonyl carbon. This attack shifts the π electrons from the C=O to the oxygen atom forming the alkoxide ion intermediate. The alkoxide anion thus formed is highly basic and abstracts a proton from another molecule of HCN to form cyanohydrin. This reaction also regenerates the cyanide catalyst.

Figure1

The strongly basic cyanide ion also functions as a good leaving group and can easily give back the aldehydes and ketones. In the presence of a strong base, the hydroxyl group on the cyanohydrin gets deprotonated to form the alkoxide anion. The alkoxide anion undergoes internal rearrangement of the delocalized electrons and loses the cyanide ion to form the carbonyl compound.

Figure2

Tags

Aldehydes Ketones HCN Cyanohydrin Formation Mechanism Nucleophilic Attack Electrophilic Carbonyl Carbon Alkoxide Ion Intermediate Proton Abstraction Cyanide Catalyst Leaving Group Strong Base Deprotonation Internal Rearrangement Delocalized Electrons

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