How to Distinguish Glucose and Fructose: Chemical and Structural Differences

December 13, 2025 •

3 min read

Despite sharing the same chemical formula, C6H12O6, glucose and fructose possess distinct functional groups—an aldehyde for glucose and a ketone for fructose—making them structural isomers. Understanding how to distinguish glucose and fructose is crucial in scientific analysis, nutritional science, and quality control, as their metabolic effects on the body are quite different.

Quick Summary

Using simple chemical tests, one can differentiate glucose from fructose by targeting their unique functional groups. Glucose, an aldose, and fructose, a ketose, react differently with reagents like bromine water and Seliwanoff's reagent, allowing for their clear identification in a laboratory setting.

Key Points

Functional Groups: The core difference is glucose's aldehyde group (aldose) versus fructose's ketone group (ketose), which dictates their chemical behavior.
Seliwanoff's Test: Detects ketoses like fructose rapidly with a deep cherry-red color, while aldoses like glucose react slowly, if at all.
Bromine Water Test: Used to identify aldoses; the bromine water is decolorized by glucose but not by fructose.
Cyclic Structure: Glucose typically forms a six-membered pyranose ring, whereas fructose forms a five-membered furanose ring in solution.
Metabolism and Taste: Fructose is significantly sweeter than glucose and is metabolized primarily in the liver, while glucose is the body's main energy source and stimulates insulin.
Confirmation: Using a general carbohydrate test like Molisch's test is a good preliminary step before employing specific differential tests.

Understanding the Structural Differences

At the molecular level, glucose and fructose are isomers, meaning they have the same chemical formula but different atomic arrangements. This seemingly small difference profoundly impacts their properties and how they react with other compounds. Glucose is an aldohexose, containing an aldehyde group (R-CHO) at its first carbon atom. In contrast, fructose is a ketohexose, featuring a ketone group (R-C(=O)-R') at its second carbon atom. In solution, this structural difference leads to different ring forms: glucose typically forms a six-membered pyranose ring, while fructose forms a five-membered furanose ring. These differences in functional groups are the key to distinguishing between them using chemical tests.

Chemical Tests to Distinguish Glucose and Fructose

Several laboratory tests leverage the specific reactivity of glucose's aldehyde group versus fructose's ketone group to tell them apart. Two of the most effective and commonly used are Seliwanoff's test and the Bromine Water test.

Seliwanoff's Test: The Ketose Indicator

Seliwanoff's test is a specific chemical assay used to differentiate ketoses from aldoses. The test is based on the principle that ketoses are more rapidly dehydrated by concentrated hydrochloric acid than aldoses are, forming a furfural derivative.

The Seliwanoff's test procedure:

Add a small amount of the sugar solution to a test tube.
Add Seliwanoff's reagent (resorcinol in concentrated HCl) to the test tube.
Heat the test tube in a boiling water bath for approximately one minute.

Interpreting the results:

Fructose (a ketose): Will produce a rapid, deep cherry-red color within one minute.
Glucose (an aldose): May produce a faint pink or reddish-orange color, but much more slowly. A high concentration of glucose, or prolonged heating, can cause it to produce a false-positive color due to isomerization.

Bromine Water Test: The Aldose Detector

Unlike Seliwanoff's test, which specifically targets ketoses, the Bromine Water test is designed to identify aldoses. Bromine water ($Br_2$/H2O) acts as a mild oxidizing agent that can oxidize the aldehyde group of glucose to a carboxylic acid (gluconic acid). The ketone group of fructose, however, is not affected by this mild oxidizing agent.

The Bromine Water test procedure:

Dissolve the unknown sugar in water.
Add bromine water to the solution.
The color of the bromine water is observed.

Interpreting the results:

Glucose (an aldose): The brownish-yellow color of the bromine water will disappear as it is reduced by the aldehyde group.
Fructose (a ketose): The color of the bromine water will remain unchanged, indicating no reaction.

Comparing Glucose and Fructose


Feature	Glucose	Fructose
Functional Group	Aldehyde (-CHO) at C1	Ketone (C=O) at C2
Category	Aldo-hexose	Keto-hexose
Ring Structure	Typically a 6-membered pyranose ring	Typically a 5-membered furanose ring
Seliwanoff's Test	Slow reaction, faint pink color (or false positive on prolonged heating)	Rapid reaction, deep cherry-red color
Bromine Water Test	Decolorizes the bromine water	No change in bromine water color
Sweetness	Sweet, but less sweet than fructose	Significantly sweeter than glucose
Metabolism	Used by nearly all body cells for energy; triggers insulin release	Primarily metabolized in the liver; does not trigger insulin release directly
Occurrence	Found in fruits, starches, honey; primary blood sugar	Found in fruits, honey, high-fructose corn syrup

Molisch's Test: A General Carbohydrate Test

While Seliwanoff's and Bromine Water tests are specific, it is often necessary to first confirm the presence of carbohydrates using a general test. Molisch's test serves this purpose by detecting the presence of carbohydrates in a sample. In this test, concentrated acid dehydrates the carbohydrate to form an aldehyde, which then condenses with α-naphthol from the Molisch's reagent to form a purple ring at the interface of the two liquids. Both glucose and fructose would yield a positive result for this general test, which is why more specific tests are needed for differentiation. For more information on carbohydrate analysis, see the Virtual Labs resource.

Conclusion

To confidently distinguish glucose and fructose, a combined approach using both general and specific chemical tests is the most reliable method. Observing a positive Molisch's test first confirms the sample is a carbohydrate. Following up with Seliwanoff's test provides rapid identification of ketoses like fructose through a vibrant cherry-red color, while the non-reactivity of aldoses like glucose serves as a negative control. For further confirmation, the Bromine Water test can be used to specifically identify glucose via decolorization. These methods exploit the fundamental structural differences between these two sugars, making their identification straightforward and reliable in a laboratory setting.

Frequently Asked Questions

The primary structural difference is the functional group. Glucose has an aldehyde group, making it an aldose, whereas fructose has a ketone group, classifying it as a ketose. This leads to them forming different ring structures in solution.

Seliwanoff's test is specific for ketoses. Fructose, a ketose, is rapidly dehydrated by the test's reagent, producing a deep cherry-red color. Glucose, an aldose, reacts much more slowly, producing only a faint pink color or none at all.

Yes. Bromine water is a mild oxidizing agent that oxidizes the aldehyde group of glucose, causing the brownish-yellow reagent to lose its color. Fructose, lacking an aldehyde group, does not react, and the color remains unchanged.

Yes, both are reducing sugars. While glucose has a free aldehyde group, fructose's ketone group can isomerize to an aldehyde under alkaline conditions, allowing it to give a positive result in tests like Fehling's or Benedict's.

Glucose is metabolized throughout the body and stimulates insulin production for cellular uptake. Fructose is primarily metabolized in the liver and does not cause a rapid insulin spike, which can lead to different physiological effects.

Yes. If heated for too long, the acid in Seliwanoff's reagent can convert aldoses like glucose into ketoses, causing them to produce a red color and leading to a false-positive result.

Molisch's test is a general screening tool for carbohydrates. It confirms that the sample is a carbohydrate by producing a purple ring. Specific tests like Seliwanoff's are then used to distinguish between different types of carbohydrates.