Chapter 25: Carbohydrates

Reducing Sugars

aldehyde                                                                carboxylate

red precipitate

Summary

• Sugars that contain aldehyde groups that are oxidised to carboxylic acids are classified as reducing sugars.
• Common test reagents are :
• Benedicts reagent (CuSO₄ / citrate)
• Fehlings reagent (CuSO₄ / tartrate)
• They are classified as reducing sugars since they reduce the Cu²⁺ to Cu⁺  which forms as a red precipitate, copper (I) oxide.
• Remember that aldehydes (and hence aldoses) are readily oxidised (review ?)
• In order for oxidation to occur, the cyclic form must first ring-open to give the reactive aldehyde.
• So any sugar that contains a hemi-acetal will be a reducing sugar.
• But glycosides which are acetals are not reducing sugars.

b-D-glucose	methyl b-d-glucopyranoside	sucrose
hemi-acetal => reducing sugar	acetals => non-reducing sugar

• Ketoses can also be reducing sugars because they can isomerise (a tautomerisation) to aldoses via an enediol:

© Dr. Ian Hunt, Department of Chemistry