Thursday

Oxidation Reactions | chemca

Organic Synthesis

Oxidation Reactions

Reactions involving gain of Oxygen or loss of Hydrogen.

By chemca Team • Updated Jan 2026

Oxidation in organic chemistry typically involves increasing the oxygen content or decreasing the hydrogen content of a molecule. The extent of oxidation depends on the strength of the oxidizing agent.

1. Oxidation of Alcohols

Primary & Secondary Alcohols

A. Mild Oxidation (To Aldehydes):

Primary alcohols are oxidized to aldehydes using PCC (Pyridinium Chlorochromate) or Collins Reagent. These reagents prevent further oxidation to carboxylic acids.

$$ R-CH_2OH \xrightarrow{PCC} R-CHO $$

B. Strong Oxidation (To Acids):

Using strong oxidizers like acidic $KMnO_4$ or $K_2Cr_2O_7$, primary alcohols go directly to carboxylic acids.

$$ R-CH_2OH \xrightarrow{KMnO_4/H^+} R-COOH $$

C. Secondary Alcohols:

Oxidized to Ketones by chromic anhydride ($CrO_3$).

$$ R-CH(OH)-R' \xrightarrow{CrO_3} R-CO-R' $$

2. Oxidation of Alkenes

Cleavage & Hydroxylation

A. With Cold Dilute KMnO4 (Bayer's Reagent):

Syn-hydroxylation occurs without breaking the sigma bond.

$$ CH_2=CH_2 \xrightarrow{dil. KMnO_4, 273K} HO-CH_2-CH_2-OH $$

B. With Hot Acidic KMnO4 (Oxidative Cleavage):

The double bond breaks completely. Terminal $=CH_2$ becomes $CO_2 + H_2O$. Non-terminal groups become Acids or Ketones.

$$ CH_3-CH=C(CH_3)_2 \xrightarrow{KMnO_4/H^+, \Delta} CH_3COOH + (CH_3)_2C=O $$

3. Oxidation of Carbonyls

Distinguishing Tests

Aldehydes are easily oxidized to acids, while ketones require drastic conditions.

Tollens' Test (Silver Mirror): Aldehydes reduce ammoniacal silver nitrate to metallic silver.

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

Fehling's Test: Aliphatic aldehydes reduce Fehling's solution ($Cu^{2+}$ complex) to a red precipitate of $Cu_2O$.

$$ R-CHO + 2Cu^{2+} + 5OH^- \rightarrow R-COO^- + Cu_2O \downarrow (\text{Red}) + 3H_2O $$

4. Oxidation of Alkyl Benzenes

Side Chain to Acid

Strong oxidation of alkyl benzenes with alkaline $KMnO_4$ converts the entire alkyl chain to a carboxyl group, provided there is at least one benzylic hydrogen.

$$ C_6H_5-CH_2-CH_3 \xrightarrow{1. KMnO_4/KOH, \Delta} \xrightarrow{2. H_3O^+} C_6H_5-COOH $$

Exception: Tert-butyl benzene resists oxidation because it lacks benzylic hydrogen.

Knowledge Check

Test your understanding of Oxidation Reactions

Search This Blog

Thursday