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Q8.1(i):
What is meant by the following term? Give an example of the reaction. (i) Cyanohydrin

Solution :

1. Definition of Cyanohydrin

A cyanohydrin is a specific class of organic compounds characterized by the presence of both a hydroxyl group ($-OH$) and a cyano group ($-CN$) attached to the same carbon atom. Their general structural formula can be represented as $R_2C(OH)CN$, where $R$ can represent hydrogen atoms or alkyl/aryl groups [Per IUPAC nomenclature and functional group classification].

2. Reaction Mechanism: Nucleophilic Addition to Carbonyls

Cyanohydrins are synthesized via the nucleophilic addition of hydrogen cyanide ($HCN$) to aldehydes or ketones. The reaction is typically base-catalyzed because $HCN$ is a weak acid and does not dissociate sufficiently to provide a high concentration of the nucleophile ($CN^-$).

Step-by-Step Mechanism:

  • Step 1 (Activation): A base ($OH^-$ or $CN^-$) deprotonates $HCN$ to generate the powerful nucleophile, the cyanide ion ($CN^-$).
    $HCN + OH^- \rightleftharpoons CN^- + H_2O$
  • Step 2 (Nucleophilic Attack): The lone pair on the carbon atom of the $CN^-$ ion attacks the electrophilic carbonyl carbon of the aldehyde/ketone, breaking the $\pi$-bond and forming an alkoxide intermediate.
    $R_2C=O + CN^- \rightarrow R_2C(O^-)-CN$
  • Step 3 (Protonation): The alkoxide intermediate abstracts a proton from the solvent (usually $H_2O$ or $HCN$) to yield the final cyanohydrin product.
    $R_2C(O^-)-CN + H_2O \rightarrow R_2C(OH)CN + OH^-$

3. Exemplary Reaction: Acetaldehyde Cyanohydrin

Consider the reaction of acetaldehyde (ethanal) with hydrogen cyanide:

$CH_3CHO + HCN \rightarrow CH_3CH(OH)CN$

CH₃-CHO + HCN CH₃-CH(OH)-CN Base Catalyst

4. Summary Table of Cyanohydrin Synthesis

Substrate Reagent Cyanohydrin Product
Formaldehyde ($HCHO$) $HCN$ Glycolaldehyde cyanohydrin ($HOCH_2CN$)
Acetaldehyde ($CH_3CHO$) $HCN$ Acetaldehyde cyanohydrin ($CH_3CH(OH)CN$)
Acetone ($CH_3COCH_3$) $HCN$ Acetone cyanohydrin ($(CH_3)_2C(OH)CN$)

Final Solution: A cyanohydrin is an organic compound with the general formula $R_2C(OH)CN$, formed by the nucleophilic addition of the cyanide ion to an aldehyde or ketone. An example is the formation of acetaldehyde cyanohydrin ($CH_3CH(OH)CN$) from acetaldehyde and $HCN$.


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