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Formation of Acetals, Ketals & Cyanohydrins

Detailed mechanisms and applications of Carbonyl Addition Reactions.

By chemca Team • Updated Jan 2026

The carbonyl carbon ($>C=O$) is a hard electrophile. Weak nucleophiles like alcohols and water require acid catalysis, while stronger nucleophiles like cyanide ($CN^-$) or bisulphite ($HSO_3^-$) react directly (often base-catalyzed).

1. Reaction with Water (Hydration)

Formation of Gem-Diols

Aldehydes and ketones react with water to form Geminal Diols (two -OH groups on same carbon). The equilibrium usually favors the carbonyl form.

$$ >C=O + H_2O \rightleftharpoons >C(OH)_2 $$

Stability of Gem-Diols:

• Formaldehyde (HCHO): 99.9% Hydrated in water. Highly stable gem-diol due to lack of steric hindrance.
• Acetone ($CH_3COCH_3$): Negligible hydration. Equilibrium lies to the left.
• Chloral ($CCl_3CHO$): Forms Chloral Hydrate, which is exceptionally stable due to Intramolecular Hydrogen Bonding.
  $$ CCl_3-CH(OH)_2 \text{ (Stable solid)} $$

2. Reaction with HCN

Formation of Cyanohydrins

Reaction with Hydrogen Cyanide yields Cyanohydrins. Since HCN is a weak acid and a poor source of nucleophile ($CN^-$), the reaction is catalyzed by a base ($OH^-$).

$$ HCN + OH^- \rightleftharpoons CN^- + H_2O $$ $$ >C=O + CN^- \rightarrow >C(O^-)CN \xrightarrow{H^+} >C(OH)CN $$

Synthetic Utility: Cyanohydrins are useful intermediates.
1. Hydrolysis ($\xrightarrow{H_3O^+}$) gives $\alpha$-Hydroxy Acids ($R-CH(OH)COOH$).
2. Reduction ($\xrightarrow{LiAlH_4}$) gives $\beta$-Amino Alcohols.

3. Reaction with Sodium Bisulphite ($NaHSO_3$)

Purification of Carbonyls

Aldehydes and unhindered Methyl Ketones react with saturated $NaHSO_3$ solution to form a white crystalline precipitate.

$$ >C=O + NaHSO_3 \rightleftharpoons >C(OH)SO_3Na $$

Reversibility: The addition product is water-soluble but insoluble in excess bisulphite solution (Common Ion Effect). The carbonyl compound can be regenerated by treating the adduct with dilute acid ($HCl$) or alkali ($Na_2CO_3$). This makes it excellent for separation and purification.

Steric Limit: Diethyl ketone and Acetophenone do NOT give this reaction.

4. Reaction with Alcohols (Acetals & Ketals)

Acid Catalyzed Addition

Aldehydes react with 1 equivalent of monohydric alcohol to form Hemiacetals (usually unstable), and with 2 equivalents to form Acetals (stable).

$$ RCHO \xrightarrow{R'OH, H^+} \underset{\text{Hemiacetal}}{RCH(OH)OR'} \xrightarrow{R'OH, H^+} \underset{\text{Acetal}}{RCH(OR')_2} + H_2O $$

Dry HCl Gas: Used as a catalyst to protonate the carbonyl oxygen (increasing electrophilicity) and to absorb water (driving equilibrium forward).

Cyclic Ketals (Protection of Carbonyls)

Ketones do not react easily with monohydric alcohols. However, they react readily with 1,2-diols (Ethylene Glycol) to form stable 5-membered Cyclic Ketals.

$$ R_2C=O + \underset{\text{Ethylene Glycol}}{HO-CH_2-CH_2-OH} \xrightarrow{HCl \text{ gas}} \underset{\text{Cyclic Ketal}}{R_2C(O-CH_2)_2} + H_2O $$

Protection Group: Acetals and Ketals are stable in basic media (oxidizing/reducing agents) but hydrolyze back to aldehydes/ketones in aqueous acidic media. This property is used to "protect" the carbonyl group during synthesis.

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