Oxidation

🔬 1. Oxidation of Aldoses by Mild Oxidizing Agents

✅ Hemiacetal and Open-Chain Forms:

  • Most monosaccharides (like D-glucose) exist mainly in a cyclic hemiacetal form.

  • But at equilibrium, a small percentage exists in the open-chain form, which contains an aldehyde group (–CHO).

✅ Oxidation of the Aldehyde Group:

  • The aldehyde group in the open-chain form can be oxidized by mild oxidizing agents (e.g. aqueous Br₂).

  • This oxidation converts the aldehyde to a carboxylic acid, forming an aldonic acid.

🧪 Example:
D-glucose → oxidized by Br₂ → D-gluconic acid (an aldonic acid)

🔁 Equilibrium Shifts:

  • As aldehyde is consumed, more cyclic glucose converts to open-chain form to restore equilibrium.

  • This cycle continues until nearly all glucose is oxidized.

⚠ 2. Ketoses Are Not Oxidized by Mild Oxidants

🧬 Why Not?

  • Ketoses (like fructose) lack the aldehydic hydrogen needed for this mild oxidation.

  • So, they are not oxidized by aqueous bromine under mild (pH ~6) conditions.

🔬 This provides a test to distinguish aldoses from ketoses.

🔁 3. Can Ketoses Become Aldoses?

🔄 Yes, But...

  • Ketoses can isomerize to aldoses via an enediol intermediate, similar to the epimerization seen in base.

  • But this requires basic or strongly acidic conditions.

🚫 Not Under Mild Conditions:

  • In buffered Br₂ solution (pH ~6), isomerization is too slow, so ketoses remain unreactive.

🧪 4. Oxidation with Stronger Reagents:

  • Stronger oxidizing agents like:

    • Tollens' reagent (Ag⁺/NH₃)

    • Fehling’s solution (Cu²⁺/sodium tartrate)

    • Benedict’s reagent (Cu²⁺/sodium citrate)

✅ Can Oxidize Both Aldoses and Ketoses

  • Because under these basic conditions, ketoses can isomerize to aldoses, which are then oxidized.

  • These are qualitative tests for reducing sugars.

💡 A reducing sugar is one that can open its ring to expose a free aldehyde or can isomerize into one.

🔍 5. Glycosides Are Not Reducing Sugars

❌ Why Not?

  • Glycosides have their anomeric OH replaced with –OR (ether-like), locking the ring.

  • They cannot open to form an aldehyde → cannot reduce oxidizing agents.

  • So, they give negative results in all the above tests.

⚡ 6. Strong Oxidation with Nitric Acid (HNO₃)

  • HNO₃ is a strong oxidizing agent.

  • It oxidizes both:

    • The aldehyde group at C1 → carboxylic acid

    • The primary alcohol group at C6 → carboxylic acid

🎯 This gives a dicarboxylic acid, known as an aldaric acid.

🧠 Summary:

Type

Oxidized by Br₂

Oxidized by Tollens/Fehling/Benedict

Oxidized by HNO₃

Reducing Sugar?

Aldose

✅ Yes

✅ Yes

✅ Yes (to aldaric acid)

✅ Yes

Ketose

❌ No

✅ Yes (via isomerization)

✅ Yes

✅ Yes (indirectly)

Glycoside

❌ No

❌ No

❌ No

❌ No