Reducing and Non-reducing Sugars Chemistry Tutorial

Key Concepts

• Sugars exist in solution as an equilibrium mixture of open-chain and closed-ring structures.
  Closed-ring structures are also known as cyclic structures.
• In the open-chain form, the carbon atom that contains the C=O bond is called the carbonyl carbon.
• In the closed-ring structure (cyclic structure), the carbonyl carbon is the one which is attached to the O of the ring and an OH group.
• Sugars that can be oxidised by mild oxidising agents are called reducing sugars.
• A non-reducing sugar is a sugar that is NOT oxidised by mild oxidising agents.
• All common monosaccharides are reducing sugars.
• The disaccharides maltose and lactose are reducing sugars.
• The disaccharide sucrose is a non-reducing sugar.
• Common oxidising agents used to test for the presence of a reducing sugar are:

  ⚛ Benedict's Solution ⁽¹⁾

  ⚛ Fehling's Solution ⁽²⁾

  ⚛ Tollen's Reagent ⁽³⁾

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Structure and Classification of Monosaccharides: Aldoses and Ketoses

We can classify a monosaccharide on the basis of its open ring or chain structure.

This structural classification of the open-ring, or chain, form of a monosaccharide depends on:

• location of the carbonyl (C=O) functional group
• the number of carbon atoms in the chain

When the closed-ring structure (cyclic structure) of a monosaccharide opens to form a chain, the result may be either

• an aldehyde (terminal carbonyl, C=O, functional group)
• a ketone (non-terminal carbonyl, C=O, functional group)

Monosaccharides that are aldehydes are known as aldoses.
("ald" from aldehyde + "ose" for a reducing sugar.)

Sugars that are ketones are known as ketoses.
("ket" from ketone + "ose" for a reducing sugar.)

There are usually 3 to 7 carbon atoms in a monosaccharide.
The number of carbon atoms is indicated by the prefixes:

• tri (3)
• tetr (4)
• pent (5)
• hex (6)
• hept (7)

For example, a monosaccharide containing 5 carbon atoms is known as a pentose.
"pent" for 5 + "ose" for a reducing sugar.

The location of the carbonyl carbon (C=O) then decides if it is an aldehyde with the "ald" prefix or a ketone with the "ket" prefix.

For example, the open-chain form of a pentose could be either:

• an aldopentose if it is an aldehyde
• or a ketopentose if it is a ketone

A monosaccharide containing 6 carbon atoms is known as a hexose.
The open-chain form could be either:

• an aldohexose if it is an aldehyde (alkanal)
• or a ketohexose if it is a ketone (alkanone)

The table below gives an overview of how we classify monosaccharides:

Glucose and fructose are both monosaccharides with 6 carbon atoms in the open-ring, or chain, form of the structure.
Glucose and fructose are therefore both classified as hexoses.

However, the chain structure of glucose contains a terminal carbonyl (C=O) functional group which makes it an aldohexose.
Fructuose, on the other hand, has a non-terminal carbonyl (C=O) functional group which makes it a ketohexose.

Compare the closed-ring and open-ring (or chain) structures of glucose and fructose as shown in the table below:

Hexose	Glucose	Fructose
Closed-ring (cyclic) Structure
Open-chain Structure
Classification	aldohexose	ketohexose

Testing for the Presence of a Reducing Sugar

Another to classify sugars is on the basis of a chemical reaction, that is, on whether the sugar can be oxidised or not.

If a carbonyl group can undergo mild oxidation it will produce a carboxylate ion (COO^-).

A sugar that can be oxidised is known as a reducing sugar.
A reducing agent is a species that causes a different species to be reduced.
In the process, the reducing agent is itself oxidised.
Therefore a sugar that is oxidised must be causing another species to be reduced, so the sugar is a reducing agent (or reductant), or a reducing sugar.

A sugar that can NOT be oxidised is known as a non-reducing sugar.

In order to test whether a sugar can be oxidised or not, we need to add a species that can undergo reduction.
A species that undergoes reduction is known as an oxidising agent, or oxidant, because it causes the other species (the sugar) to be oxidised.

Blue solutions of copper ions, Cu²⁺, are popular oxidising agents for sugars because they produce an insoluble Cu₂O precipitate which is a brick-red colour that is easy to see.
The oxidation state of Cu in Cu²⁺ is +2
The oxidation state of Cu in Cu₂O is +1
Since the oxidation state of Cu has decreased, the copper "atoms" have gained an electron, that is the copper "atoms" have been reduced.

Cu²⁺ is therefore an oxidising agent (also known as an oxidant).

Cu²⁺ is found in both Benedict's solution and in Fehling's solution which are two solutions that are used to test for the presence of a reducing sugar.

Another solution that is commonly used to test for a reducing a sugar is Tollen's reagent.
Tollen's reagent does not contain Cu²⁺, instead it contains colourless silver ions, Ag⁺.
When silver ions gain an electron (are reduced), the result is solid silver:

The oxidation state of Ag in Ag⁺ is +1
The oxidation state of Ag in Ag_(s) is 0
The oxidation state of Ag has decreased from +1 to 0, Ag⁺ has gained an electron, so Ag⁺ has been reduced.

Ag⁺ is the oxidising agent, or oxidant.
The solid silver forms a visible "mirror" which makes it easy to determine whether the sugar has been oxidised.

The table below compares these three solutions commonly used to test for the presence of a reducing sugar: Benedict's solution, Fehling's solution and Tollen's reagent:

In general, an aldehyde functional group is easily oxidised to a carboxyl functional group.
We expect aldose sugars to be reducing sugars.
If one of the mild oxidising reagents above is added to a monosaccharide aldose sugar you expect to see the relevant colour change.
Although fructose is a ketose sugar containing the ketone functional group, it is also a reducing sugar.
Of the disaccharides, maltose and lactose are reducing sugars, but sucrose is NOT.
Sucrose is a non-reducing sugar. If you add one of the mild oxidising agents above to sucrose, there will be NO reaction, no colour change.

Reducing Sugars: Mild Oxidation of Monosaccharides

The carbonyl group (C=O) in an aldose is readily oxidised to a carboxylate group
Glucose and galactose are both examples of aldose sugars.
Glucose and galactose can be oxidised by a mild oxidising agent.

Consider the reaction between an aldose such as glucose or galactose and Benedict's solution (or Fehling's solution) as shown below:

The aldehyde C=O group has been oxidised to a carboxylate ion (COO^-).
The copper has been reduced.
You will observe a brick-red precipitate.
You will conclude that the aldose is a reducing sugar.

Now consider the reaction between an aldose and Tollen's Reagent as shown below:

The aldehyde C=O group has been oxidised to a carboxylate ion (COO^-).
The silver ions have been reduced.
You will observe the precipitation of solid silver as a silver mirror.
You will conclude that the aldose is a reducing sugar.

Although the carbonyl group of a ketone is usually not easily oxidised, there are exceptions.
For example, fructose is an example of a ketose that is a reducing sugar.
Fructose can be oxidised by a mild oxidising agent.
The product of the oxidation reaction is a hydroxy carboxylate.
The products of the oxidation reaction contains both the hydroxyl (OH) group and the carboxylate (COO^-) group.

Consider the reaction between fructose and Benedict's solution or Fehling's solution as shown below:

The copper has been reduced.
You will observe a brick-red precipitate.
You will conclude that this ketose, fructose, is a reducing sugar.

Now consider the reaction between Tollen's reagent and fructose as shown below:

Silver ions have been reduced.
You will observe a "silver mirror" effect.
You will conclude that this ketose, fructose, is a reducing sugar.

Structure and Mild Oxidation of Disaccharides

Some disaccharides are reducing sugars, they can be oxidised by mild oxidising agents.
Examples are maltose and lactose.

Some disaccharides are non-reducing sugars, they can NOT be oxidised by mild oxidising agents.
Sucrose is an example of a non-reducing sugar.

The table below gives the key structural features of reducing and non-reducing disaccharides as well as the results of reactions with mild oxidising agents like Benedict's solution, Fehling's solution and Tollen's reagent:

Disaccharide	sucrose	maltose	lactose
Cyclic Structure
Glycosidic Link through	2 carbonyl carbons	1 carbonyl carbon	1 carbonyl carbon
Open-chain structure	no	yes	yes
+Benedict's Solution	no reaction	forms brick red solid	forms brick red solid
+Fehling's Solution	no reaction	forms brick red solid	forms brick red solid
+Tollen's Reagent	no reaction	forms silver mirror	forms silver mirror
Classification	non-reducing sugar	reducing sugar	reducing sugar

Mild Oxidation of Polysaccharides

Polysaccharides such as starch and cellulose can not be oxidised using mild oxidising agents like Benedict's solution, Fehling's solution or Tollen's reagent.

If you test a polysaccharide with each of these reagents there will be no reaction.

Worked Example of a Reducing Sugars Problem

Problem Solving using the StoPGoPS approach.