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Chemical Properties of Aldehydes and Ketones

Reactivity of the Carbonyl Group: Nucleophilic Addition, Oxidation, and Condensation.

By chemca Team • Updated Jan 2026

The carbonyl group ($>C=O$) is highly reactive due to the polarity of the C-O bond. Aldehydes are generally more reactive than ketones towards nucleophilic addition due to steric and electronic reasons.

1. Nucleophilic Addition Reactions

General Mechanism

Nucleophiles attack the electrophilic carbonyl carbon.

A. Addition of HCN: Forms Cyanohydrins.

$$ >C=O + HCN \rightleftharpoons >C(OH)CN \text{ (Cyanohydrin)} $$

B. Addition of Sodium Bisulphite ($NaHSO_3$): Forms a crystalline addition product (Used for separation/purification).

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

C. Addition of Alcohols (Hemiacetals & Acetals): Aldehydes react with alcohols in the presence of dry HCl gas.

$$ R-CHO \xrightarrow{R'OH, HCl} \text{Hemiacetal} \xrightarrow{R'OH, HCl} \text{Acetal (1,1-Dialkoxyalkane)} $$

Ketones react with diols (like Ethylene Glycol) to form cyclic Ketals.

2. Addition-Elimination (Ammonia Derivatives)

Reaction with $NH_2-Z$

Carbonyls react with ammonia derivatives in weakly acidic medium to lose a water molecule and form compounds with a $>C=N-$ bond.

$$ >C=O + H_2N-Z \rightleftharpoons [>C(OH)NHZ] \xrightarrow{-H_2O} >C=N-Z $$

• Hydroxylamine ($NH_2OH$): Forms Oximes ($>C=N-OH$).
• Hydrazine ($NH_2NH_2$): Forms Hydrazones ($>C=N-NH_2$).
• 2,4-DNP (Brady's Reagent): Forms orange precipitate of 2,4-Dinitrophenylhydrazone (Test for Carbonyl group).

3. Oxidation Reactions

Distinguishing Aldehydes & Ketones

Aldehydes are easily oxidized to carboxylic acids. Ketones resist mild oxidation.

A. Tollens' Test (Silver Mirror): Aldehydes reduce Tollens' reagent ($[Ag(NH_3)_2]^+$) to metallic Silver.

$$ R-CHO + 2[Ag(NH_3)_2]^+ + 3OH^- \rightarrow R-COO^- + 2Ag \downarrow + 4NH_3 + 2H_2O $$

B. Fehling's Test: Aliphatic Aldehydes reduce Fehling's solution ($Cu^{2+}$) to red $Cu_2O$. (Aromatic aldehydes do not respond).

$$ R-CHO + 2Cu^{2+} + 5OH^- \rightarrow R-COO^- + Cu_2O \downarrow \text{ (Red ppt)} $$

C. Haloform Reaction (Iodoform Test): Given by Methyl Ketones ($CH_3-CO-$) or Methyl Carbinols ($CH_3-CH(OH)-$).

$$ R-CO-CH_3 + 3I_2 + 4NaOH \rightarrow R-COONa + CHI_3 \downarrow \text{ (Yellow)} + 3NaI $$

4. Reactions due to $\alpha$-Hydrogen

Aldol Condensation

Aldehydes and Ketones with at least one $\alpha$-hydrogen undergo self-condensation in the presence of dilute alkali ($NaOH$) to form $\beta$-hydroxy aldehydes (Aldols) or $\beta$-hydroxy ketones (Ketols).

$$ 2CH_3-CHO \xrightarrow{dil. NaOH} \underset{\text{3-Hydroxybutanal (Aldol)}}{CH_3-CH(OH)-CH_2-CHO} \xrightarrow{\Delta, -H_2O} \underset{\text{But-2-enal}}{CH_3-CH=CH-CHO} $$

Cross Aldol: Reaction between two different carbonyl compounds. If both have $\alpha$-H, a mixture of 4 products is obtained.

5. Cannizzaro Reaction

No $\alpha$-Hydrogen

Aldehydes with no $\alpha$-hydrogen (e.g., HCHO, $C_6H_5CHO$) undergo self-oxidation and reduction (disproportionation) in the presence of concentrated alkali ($50\% NaOH$).

$$ 2HCHO + Conc. KOH \rightarrow \underset{\text{Methanol}}{CH_3OH} + \underset{\text{Potassium Formate}}{HCOOK} $$

One molecule is reduced to Alcohol, the other is oxidized to Acid Salt.

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