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Oxidation and Reduction of Aldehydes & Ketones

Reactions: Chemical Tests, Deoxygenation, and Hydride Reduction.

By chemca Team • Updated Jan 2026

Aldehydes and Ketones exhibit significantly different behaviors towards oxidizing agents. Aldehydes are easily oxidized to carboxylic acids due to the presence of an H-atom on the carbonyl group. Ketones resist oxidation and require drastic conditions. Both, however, can be reduced to alcohols or hydrocarbons.

1. Oxidation Reactions

A. Oxidation of Aldehydes

Aldehydes are oxidized to carboxylic acids by common oxidizing agents ($KMnO_4, K_2Cr_2O_7$) as well as mild agents like Tollens' and Fehling's reagents.

$$ R-CHO + [O] \xrightarrow{KMnO_4/H^+} R-COOH $$

B. Distinguishing Tests (Mild Oxidation)

1. Tollens' Test (Silver Mirror Test):
Reagent: Ammoniacal Silver Nitrate ($[Ag(NH_3)_2]^+$).
Aldehydes reduce $Ag^+$ to metallic Silver ($Ag$). Ketones do not react.

$$ RCHO + 2[Ag(NH_3)_2]^+ + 3OH^- \rightarrow RCOO^- + \underset{\text{Silver Mirror}}{2Ag \downarrow} + 4NH_3 + 2H_2O $$

2. Fehling's Test:
Reagent: Fehling A (CuSO4) + Fehling B (Alkaline Sodium Potassium Tartrate).
Aliphatic Aldehydes reduce $Cu^{2+}$ (Blue) to $Cu_2O$ (Red ppt).
Important: Aromatic aldehydes (Benzaldehyde) and Ketones do NOT respond to this test.

$$ R-CHO + 2Cu^{2+} + 5OH^- \rightarrow RCOO^- + \underset{\text{Red ppt}}{Cu_2O \downarrow} + 3H_2O $$

C. Oxidation of Ketones

Ketones require drastic conditions (strong oxidizers + heat). Oxidation involves C-C bond cleavage.

Popoff's Rule (For unsymmetrical ketones): The keto group ($>C=O$) stays with the smaller alkyl group during cleavage.

$$ CH_3-CH_2-CO-CH_2-CH_3 \xrightarrow{[O]} 2 CH_3CH_2COOH $$ $$ CH_3-CO-CH_2-CH_2-CH_3 \xrightarrow{[O]} \underset{\text{Major}}{2 CH_3COOH} + CO_2 + H_2O $$

2. Haloform Reaction (Oxidation)

Iodoform Test

Specific for compounds containing Methyl Ketone ($CH_3-CO-$) or Methyl Carbinol ($CH_3-CH(OH)-$) groups.

Reaction with Sodium Hypohalite ($NaOX$) or $X_2 + NaOH$.

$$ R-CO-CH_3 + 3I_2 + 4NaOH \rightarrow R-COONa + \underset{\text{Yellow ppt}}{CHI_3 \downarrow} + 3NaI + 3H_2O $$

Applications:
1. Distinguishes Acetone vs Diethyl Ketone.
2. Distinguishes Ethanol vs Methanol.
3. Converts Methyl Ketone to Carboxylic Acid (with one less carbon).

3. Reduction Reactions

A. Reduction to Alcohols

Reduction is carried out using catalytic hydrogenation or metal hydrides.

$$ R-CHO \xrightarrow{H_2/Ni \text{ or } NaBH_4} R-CH_2OH \text{ (Primary Alcohol)} $$ $$ R-CO-R' \xrightarrow{LiAlH_4 \text{ or } NaBH_4} R-CH(OH)-R' \text{ (Secondary Alcohol)} $$

B. Reduction to Hydrocarbons (Deoxygenation)

The carbonyl group ($>C=O$) is converted directly to a methylene group ($-CH_2-$).

1. Clemmensen Reduction (Acidic Medium):
Reagent: Zinc Amalgam and Concentrated HCl ($Zn-Hg/HCl$).
Suitable for compounds sensitive to bases.

$$ >C=O + 4[H] \xrightarrow{Zn-Hg/HCl} >CH_2 + H_2O $$

2. Wolff-Kishner Reduction (Basic Medium):
Reagent: Hydrazine ($NH_2NH_2$) followed by heating with KOH in ethylene glycol.
Suitable for compounds sensitive to acids.

$$ >C=O \xrightarrow{NH_2NH_2} >C=N-NH_2 \xrightarrow{KOH, \Delta} >CH_2 + N_2 \uparrow $$

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