Tests for Reducing Sugars: Fehling’s, Benedict’s, and Tollen’s Tests

Reducing Sugar Tests

Fehling’s Test for Reducing Sugars

Fehling’s test is a classical chemical method used to detect the presence of reducing sugars in a solution. Reducing sugars are carbohydrates that possess a free aldehyde or ketone group capable of reducing mild oxidizing agents.

• Principle: Reducing sugars reduce the blue copper(II) ions (Cu2+) in Fehling’s solution to red copper(I) oxide (Cu2O), which precipitates out of solution.

• Observation: The solution changes from blue to brick-red if reducing sugars are present.

Reaction:

$\text{Reducing Sugar} + 2\,\text{Cu}^{2+} + 5\,\text{OH}^- \rightarrow \text{Oxidized Sugar} + \text{Cu}_2\text{O} (\text{red ppt}) + 3\,\text{H}_2\text{O}$

• Negative Control: No color change (solution remains blue).

• Positive Control: Brick-red precipitate forms, indicating the presence of reducing sugars.

Benedict’s Test for Reducing Sugars

Benedict’s test is similar to Fehling’s test but uses sodium citrate and sodium carbonate to stabilize the copper(II) ions. It is also used to detect reducing sugars.

• Principle: Reducing sugars reduce Cu2+ to Cu2O, resulting in a color change from blue to green, yellow, orange, or red, depending on the amount of reducing sugar present.

• Comparison: Benedict’s test is less sensitive to the rate of reaction than Fehling’s test and is more commonly used in laboratories.

Reaction:

$\text{Reducing Sugar} + 2\,\text{Cu}^{2+} + 5\,\text{OH}^- \rightarrow \text{Oxidized Sugar} + \text{Cu}_2\text{O} (\text{red ppt}) + 3\,\text{H}_2\text{O}$

Tollen’s Test for Reducing Sugars

Tollen’s test, also known as the silver mirror test, uses ammoniacal silver nitrate as the oxidizing agent instead of copper(II) ions. It is used to detect the presence of reducing sugars, especially aldehydes.

• Principle: Reducing sugars reduce Ag+ ions to metallic silver, which deposits as a shiny mirror on the inner surface of the test tube.

• Observation: Formation of a silver mirror indicates a positive result for reducing sugars.

Reaction:

$\text{Reducing Sugar} + 2\,\text{Ag}^+ + 2\,\text{OH}^- \rightarrow \text{Oxidized Sugar} + 2\,\text{Ag} (\text{mirror}) + 2\,\text{H}_2\text{O}$

Key Definitions and Concepts

• Reducing Sugar: Any sugar that contains a free aldehyde or ketone group capable of acting as a reducing agent.

• Non-reducing Sugar: Sugars that do not have a free aldehyde or ketone group and do not react in these tests (e.g., sucrose).

• Examples of Reducing Sugars: Glucose, fructose, galactose, lactose, and maltose.

Practice and Application

• Glucose is a strong reducing sugar and gives a positive result in all three tests.

• Sucrose is a non-reducing sugar and does not react unless hydrolyzed to glucose and fructose.

• Fructose is a reducing sugar due to its ability to tautomerize to an aldehyde form under basic conditions.

Summary Table: Results of Reducing Sugar Tests

Sugar Molecule	Fehling’s Test	Benedict’s Test	Tollen’s Test	Final Conclusion
Glucose	Color change (red ppt)	Color change (red ppt)	Silver mirror forms	Reducing sugar
Sucrose	No color change	No color change	No silver mirror	Non-reducing sugar
Fructose	Color change (red ppt)	Color change (red ppt)	Silver mirror forms	Reducing sugar

Additional info:

• Some disaccharides (e.g., maltose, lactose) are reducing sugars, while others (e.g., sucrose) are not due to the absence of a free anomeric carbon.

• These tests are important in clinical biochemistry for detecting glucose in urine (diabetes diagnosis) and in food chemistry for sugar analysis.