Advanced Topic 825 of 1102 Organic Chemistry
Reducing Sugars
Sugars with free hemiacetal groups reduce Tollens' and Fehling's reagents.
In-depth explanation of Reducing Sugars
A reducing sugar is one that has a free hemiacetal or hemiketal group, meaning its anomeric carbon can open to an aldehyde or ketone. The open form can then reduce mild oxidizing agents such as Tollens' and Fehling's reagents, and it is oxidized in the process. Glucose, maltose and lactose are reducing sugars because one of their anomeric carbons remains free. Sucrose is not a reducing sugar, because both of its anomeric carbons are locked together in the glycosidic bond and cannot open.
Going deeper: Reducing Sugars
Key ideas: A sugar is reducing if it has a free hemiacetal or hemiketal that can open to the aldehyde form and respond to Tollens, Fehling or Benedict reagents. Glucose, fructose (via enediol isomerisation), maltose and lactose are reducing. Sucrose is non-reducing because the glycosidic bond uses both anomeric carbons. Starch and cellulose are non-reducing for practical purposes since only one end is free.
Common mistakes and connections: Students think ketoses are not reducing; fructose is, because of tautomerisation under basic conditions. Another error is saying sucrose reduces Tollens reagent; it only does after acid hydrolysis to invert sugar. Links to mutarotation, glycosidic bonds and the Tollens or Fehling test.
Visualization of Reducing Sugars
Sugars with a free hemiacetal reduce Tollens or Fehling reagent; sucrose has its anomeric carbons tied in the glycosidic bond and does not. The interactive lesson shows this as a live animation or chart.
Worked example and use case
Table sugar, which is sucrose, gives a negative result in the Benedict test.
Frequently asked questions about Reducing Sugars
What is Reducing Sugars?
Sugars with free hemiacetal groups reduce Tollens' and Fehling's reagents.
Is Reducing Sugars a beginner, intermediate or advanced topic?
Reducing Sugars is a advanced topic in Organic Chemistry. It builds on beginner and intermediate topics, so review those first if needed.
What should I learn before Reducing Sugars?
Study these earlier topics in Organic Chemistry first: Thiols, Carbohydrate Structure, Glucose Ring Forms.
What comes after Reducing Sugars?
The next topics in Organic Chemistry are Amino Acids and Zwitterions.
Can I practise Reducing Sugars with quizzes?
Yes. The interactive lesson includes a self-check question on Reducing Sugars, and the Organic Chemistry quiz draws on every topic in this area.
Where Reducing Sugars fits in Organic Chemistry
Reducing Sugars is topic 97 of 148 in Organic Chemistry.