How to find a disaccharide?

January 2, 2026 •

3 min read

Disaccharides are double sugars, formed when two monosaccharides join through a glycosidic bond. Learning how to find a disaccharide involves distinguishing it from other carbohydrates, a process that relies on specific chemical tests and hydrolysis techniques.

Quick Summary

This guide explains laboratory procedures to identify a disaccharide. It covers differentiation from monosaccharides and detecting reducing versus non-reducing sugars.

Key Points

Start with a general test: Molisch's test can confirm if a carbohydrate is present in the initial sample.
Use Benedict's test for reducing sugars: This test distinguishes reducing sugars (all monosaccharides and some disaccharides) from non-reducing sugars (like sucrose).
Hydrolyze non-reducing sugars: If Benedict's is negative, use acid hydrolysis followed by a second Benedict's test to confirm a non-reducing disaccharide.
Distinguish monosaccharides from disaccharides with Barfoed's test: This test differentiates between monosaccharides and reducing disaccharides based on reaction time.
Identify specific disaccharides with Osazone test: The characteristic crystal shapes formed in the Osazone test can help confirm the identity of specific disaccharides like lactose and maltose.
Consider the glycosidic bond: A disaccharide's reducing status depends on whether its anomeric carbons are both involved in the glycosidic bond.

Understanding the Basics: What is a Disaccharide?

A disaccharide is a carbohydrate consisting of two monosaccharide units joined together by a glycosidic bond. These covalent bonds are formed through a dehydration (or condensation) reaction, which releases a water molecule. Common examples include sucrose, lactose, and maltose. Their properties vary based on their constituent monosaccharides and the glycosidic linkage type.

Reducing vs. Non-Reducing Disaccharides

A key aspect of disaccharides is whether they are 'reducing' or 'non-reducing'. This property depends on the presence of a free anomeric carbon.

Reducing Disaccharides: Contain a free hemiacetal unit, allowing them to act as a reducing agent. Examples include lactose and maltose.
Non-Reducing Disaccharides: Lack a free hemiacetal or ketone group because the glycosidic bond links both anomeric carbons. Sucrose is a common example.

Laboratory Tests to Find a Disaccharide

Identifying a disaccharide in a lab involves a series of tests to first detect carbohydrates generally, then differentiate them, and finally confirm the specific sugar.

Molisch's Test: General Carbohydrate Detection

Molisch's test is a preliminary test for all carbohydrates. A positive result, indicated by a purple ring, confirms a carbohydrate is present but doesn't specify the type.

Benedict's Test for Reducing Sugars

Benedict's test identifies reducing sugars. The blue Benedict's reagent changes color when heated with a reducing sugar, progressing through green, yellow, orange, and finally a brick-red precipitate, depending on concentration.

A positive test suggests a reducing sugar (monosaccharide or reducing disaccharide).
A negative test suggests a non-reducing sugar (like sucrose) or no sugar.

Hydrolysis for Non-Reducing Disaccharides

If Benedict's test is negative, hydrolysis can reveal if a non-reducing disaccharide like sucrose is present.

Acid Hydrolysis: Heating with dilute hydrochloric acid breaks glycosidic bonds.
Neutralization: Sodium bicarbonate neutralizes the acid.
Second Benedict's Test: A positive result after hydrolysis confirms the presence of a non-reducing disaccharide in the original sample.

Barfoed's Test for Differentiation

Barfoed's test differentiates monosaccharides from disaccharides. It uses copper acetate in a slightly acidic solution.

Monosaccharides produce a reddish-brown precipitate quickly (1-2 minutes). Reducing disaccharides react more slowly (7-8 minutes).

Key Disaccharides: A Comparative Table


Disaccharide	Monosaccharide Units	Type of Glycosidic Linkage	Reducing Status
Sucrose	Glucose + Fructose	α(1→2)β	Non-Reducing
Lactose	Galactose + Glucose	β(1→4)	Reducing
Maltose	Glucose + Glucose	α(1→4)	Reducing
Cellobiose	Glucose + Glucose	β(1→4)	Reducing
Trehalose	Glucose + Glucose	α(1→1)α	Non-Reducing

Step-by-Step Procedure to Find a Disaccharide

Molisch's Test: Confirm carbohydrate presence with Molisch's reagent; a purple ring is positive.
Benedict's Test: Check for reducing sugars; negative means non-reducing, positive means reducing.
Hydrolysis (if Benedict's negative): Hydrolyze with HCl, neutralize, and re-test with Benedict's; a positive result confirms a non-reducing disaccharide.
Barfoed's Test (if Benedict's positive): Use Barfoed's test to distinguish monosaccharides from reducing disaccharides based on reaction speed.

Conclusion: A Systematic Approach to Identification

Identifying a disaccharide involves a systematic approach with chemical tests. Starting with a general test like Molisch's and moving to specific tests like Benedict's and Barfoed's allows for accurate classification. Hydrolysis is essential for non-reducing sugars like sucrose. This sequence helps accurately characterize and find a disaccharide. For detailed information on carbohydrate chemistry, refer to resources like Chemistry LibreTexts: 24.8: Disaccharides and Glycosidic Bonds.

Frequently Asked Questions

The key difference is that a disaccharide consists of two monosaccharide units and is generally detected using a longer test procedure, including hydrolysis if non-reducing, while monosaccharides react more directly and quickly, especially in tests like Barfoed's.

The Benedict's test can help identify reducing disaccharides (like lactose and maltose) which give a positive result. For non-reducing disaccharides (like sucrose), a negative initial result followed by a positive result after hydrolysis is the key indicator.

Sucrose is a non-reducing sugar, meaning it won't react with Benedict's reagent directly. Hydrolysis with acid breaks the glycosidic bond, releasing the reducing monosaccharides (glucose and fructose) which then give a positive Benedict's test.

No, the Barfoed's test distinguishes between reducing monosaccharides and reducing disaccharides based on reaction time. It does not work on non-reducing disaccharides like sucrose without prior hydrolysis.

The Osazone test is a confirmatory test where the specific carbohydrate forms characteristic yellow crystals upon heating with phenylhydrazine. Observing the shape of the crystals under a microscope can identify the disaccharide, such as the cotton-ball shape for lactose or sunflower shape for maltose.

This depends on the anomeric carbon. A disaccharide is reducing if one of its two monosaccharide units has a free anomeric carbon. If the anomeric carbons of both monosaccharide units are tied up in the glycosidic bond, the disaccharide is non-reducing.

A glycosidic bond is a type of covalent bond that links a carbohydrate molecule to another group. In disaccharides, it specifically links the two monosaccharide units together, formed by a dehydration reaction.