Does Sucrose Reduce Fehling's Solution? The Chemistry Explained

December 4, 2025 •

2 min read

Sucrose, the common table sugar, is classified as a non-reducing sugar, a fact that dictates its chemical behavior. When subjected to a standard Fehling's test, a solution containing only sucrose will not reduce the reagent and will yield a negative result. This outcome is a fundamental principle used to distinguish between different types of sugars in chemistry.

Quick Summary

Sucrose is a non-reducing sugar and does not reduce Fehling's solution because its structure lacks a free aldehyde or ketone group. The test remains blue, showing a negative result. This is in contrast to reducing sugars like glucose, which form a brick-red precipitate.

Key Points

No Reduction: Sucrose does not reduce Fehling's solution, resulting in a negative test with no color change from blue.
Non-Reducing Sugar: Sucrose is classified as a non-reducing sugar because it lacks a free aldehyde or ketone functional group.
Anomeric Carbon Bond: The glycosidic bond in sucrose connects the anomeric carbons of its glucose and fructose units, preventing the molecule from opening into a reactive chain form.
Contrast with Glucose: In contrast to sucrose, reducing sugars like glucose give a positive Fehling's test, producing a brick-red precipitate.
Hydrolysis Required: For sucrose to give a positive result, it must first be hydrolyzed with acid or an enzyme to break it down into its reducing monosaccharides (glucose and fructose).
Test Indicator: The Fehling's test is a useful tool for distinguishing between reducing and non-reducing sugars based on this fundamental chemical difference.

What is a Reducing Sugar?

To understand why sucrose behaves as it does, it is essential to first grasp the concept of a reducing sugar. A reducing sugar is any sugar that, in an alkaline solution, possesses a free aldehyde ($–CHO$) or ketone ($–C=O$) functional group. It is this free group that allows the sugar to act as a reducing agent. Examples include all monosaccharides like glucose and fructose, and some disaccharides such as lactose and maltose.

How the Fehling's Test Works

Fehling's test identifies reducing sugars. The reagent is made by mixing Fehling's A ($CuSO_4$ solution) and Fehling's B (potassium sodium tartrate in $NaOH$) just before use. The mixture is a deep blue solution containing copper(II) ions ($Cu^{2+}$). When a reducing sugar is heated with this solution, the sugar's aldehyde group is oxidized, and the copper(II) ions are reduced to copper(I) oxide ($Cu_2O$), which forms a brick-red precipitate.

The Non-Reducing Nature of Sucrose

Sucrose is a disaccharide of glucose and fructose. It is a non-reducing sugar because its glucose and fructose units are linked by a glycosidic bond involving their anomeric carbons (glucose's C1 and fructose's C2). This linkage prevents the formation of free aldehyde or ketone groups. Without these groups, sucrose cannot reduce copper(II) ions, leading to a negative Fehling's test.

What Happens During a Standard Fehling's Test with Sucrose?

When sucrose is heated with Fehling's reagent, there is no reaction. The solution stays blue, and no brick-red precipitate forms. This negative result distinguishes sucrose from reducing sugars.

Hydrolysis: The Exception to the Rule

Hydrolysis can make sucrose give a positive Fehling's test. Heating sucrose with dilute acid breaks the glycosidic bond, yielding glucose and fructose. Since both are reducing sugars, they will then reduce Fehling's solution to form the brick-red precipitate.

Comparative Analysis of Sugar Types


Feature	Reducing Sugars	Non-Reducing Sugars
Free Carbonyl Group	Have a free aldehyde or ketone group.	No free aldehyde or ketone group due to bond formation.
Fehling's Test Result	Positive, produces a brick-red precipitate.	Negative, solution remains blue.
Examples	Glucose, Fructose, Lactose, Maltose.	Sucrose, Trehalose.
Anomeric Carbon	At least one anomeric carbon has a free -OH group.	Both anomeric carbons are involved in the glycosidic bond.
Hydrolysis	Not required for a positive Fehling's test.	Required to break the glycosidic bond and expose reducing groups.

Conclusion

Sucrose does not reduce Fehling's solution because its structure lacks the free aldehyde or ketone groups necessary for the reaction. The Fehling's test with sucrose results in the solution remaining blue. However, hydrolysis breaks sucrose into glucose and fructose, which are reducing sugars and will yield a positive Fehling's test. For further reading, consult resources like Chemistry LibreTexts on Nonreducing Sugar.

Frequently Asked Questions

A reducing sugar has a free aldehyde or ketone group that can donate electrons, while a non-reducing sugar, like sucrose, does not have these free reactive groups.

In sucrose, the bond between the glucose and fructose units involves the anomeric carbons of both molecules, which are the sites where the aldehyde or ketone groups would be located. This locks the structure and prevents the ring from opening.

Yes, indirectly. A negative Fehling's test can indicate the presence of a non-reducing sugar like sucrose, while a positive test suggests a reducing sugar is present.

If you boil sucrose with Fehling's solution alone, no reaction will occur. The solution will remain its characteristic blue color, and no precipitate will form.

To get a positive result, you must first hydrolyze the sucrose solution by boiling it with dilute acid. This breaks the glycosidic bond, releasing the reducing monosaccharides (glucose and fructose).

Although fructose is a ketose, it is considered a reducing sugar because, under the alkaline conditions of the Fehling's test, it isomerizes to an aldose, allowing it to reduce the copper ions.

Fehling's solution is prepared freshly before use because the mixture is unstable over time and can decompose, making it ineffective for the test.