2.2.4—Reducing and non-reducing sugars
- Syllabus
- 9700–2028–2029
- Objective
- 2.2.4
- Level
- AS
Reducing sugars donate electrons; non-reducing sugars need hydrolysis first
“Reducing” describes a sugar’s chemical reactivity: a reducing sugar can donate electrons and be oxidised, while a non-reducing sugar cannot donate electrons in its original form. This is not a statement about nutritional value or the total amount of sugar.
| Feature | Reducing sugars | Non-reducing sugars |
|---|---|---|
| Reactivity | Can donate electrons; the sugar acts as a reducing agent | Cannot donate electrons in its original form |
| Required examples | Glucose, fructose and maltose | Sucrose |
| Benedict’s test before hydrolysis | Can reduce copper(II) ions, giving a colour change/precipitate when heated | Gives a negative result because the intact sugar cannot reduce the reagent |
| After hydrolysis | Still gives reducing-sugar products | The products are monosaccharides that can reduce copper(II), so Benedict’s can become positive |
In Benedict’s test, a reducing sugar transfers electrons to copper(II) ions during heating; copper(I) oxide forms and the colour changes. A non-reducing sugar such as sucrose must first be hydrolysed to break its glycosidic bond and release monosaccharides. The post-hydrolysis positive result is meaningful only when the untreated sample was tested first and was negative.
Do not infer reducing or non-reducing status from mono- versus disaccharide names alone, and do not treat “non-reducing” as “no sugar”. A negative Benedict’s result for intact sucrose means no reducing sugar was detected before hydrolysis, not that the sample contains no carbohydrate.
ConceptA-Level CAIE Biology AS