Reducing and Non-reducing Sugars |
Key Concepts
- Sugars exist in solution as an equilibrium mixture of open-chain and closed-ring (or 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 (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 because the oxidising agent is reduced in the reaction.
- A non-reducing sugar 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
- Fehling's Solution
- Tollen's Reagent
Structure of Monosaccharides
When the closed-ring (cyclic) structure of a monosaccharide opens to form a chain, the result may be either:
an aldehyde (alkanal)
Sugars that are aldehydes are known as aldoses.
|
or |
a ketone (alkanone)
Sugars that are ketones are known as ketoses.
|
A monosaccharide containing 5 carbon atoms is known as a pentose. The open-chain form is therefore either:
- an aldopentose if it is an aldehyde (alkanal)
- or a ketopentose if it is a ketone (alkanone)
A monosaccharide containing 6 carbon atoms is known as a hexose. The open-chain form is therefore either:
- an aldohexose if it is an aldehyde (alkanal)
- or a ketohexose if it is a ketone (alkanone)
| Monosaccharide |
Glucose |
Fructose |
|---|
| Closed-ring (cyclic) Structure |
 |
 |
|---|
| Open-chain Structure |
 |
 |
|---|
| Classification |
aldohexose |
ketohexose |
|---|
Testing for the Presence of a Reducing Sugar
The common oxidising agents used to test for the presence of a reducing sugar:
| Oxidising Reagent |
Benedict's Solution |
Fehling's Solution |
Tollen's Reagent |
|---|
| Composition |
copper sulfate in alkaline citrate |
copper sulfate in alkaline tartrate |
silver nitrate in aqueous ammonia |
|---|
| Colour of Solution |
deep blue |
deep blue |
colourless |
|---|
| Colour After Reaction with a Reducing Sugar |
brick red precipitate
Cu2O(s) |
brick red precipitate
Cu2O(s) |
silver mirror forms
Ag(s) |
|---|
Species Being Reduced
(the oxidant) |
Cu2+
Cu2+ + e ---> Cu+ |
Cu2+
Cu2+ + e ---> Cu+ |
Ag+
Ag+ + e ---> Ag(s) |
|---|
Species Being Oxidised
(the reductant) |
reducing sugar
oxidised to carboxylate |
reducing sugar
oxidised to carboxylate |
reducing sugar
oxidised to carboxylate |
|---|
Oxidation of Monosaccharides
Oxidation of Aldoses
Glucose and galactose are both examples of aldoses.
| Oxidation Using Benedict's or Fehling's Solution: |
aldose
|
+ 2Cu2+ + 5OH-
deep blue |
 |
carboxylate
|
+ Cu2O(s)
brick red |
+ 3H2O |
| Oxidation Using Tollen's Reagent |
aldose
|
+ 2Ag(NH3)2+ + 3OH-
colourless |
|
carboxylate
|
+ 2Ag(s)
silver mirror |
+ 4NH3(g) + 2H2O |
Oxidation of Ketoses
Fructose is an example of a ketose.
| Oxidation Using Benedict's or Fehling's Solution: |
ketose
|
+ 2Cu2+ + 5OH-
deep blue |
|
hydroxy carboxylate
|
+ Cu2O(s)
brick red |
+ 3H2O |
| Oxidation Using Tollen's Reagent |
ketose
|
+ 2Ag(NH3)2+ + 3OH-
colourless |
|
hydroxy carboxylate
|
+ 2Ag(s)
silver mirror |
+ 4NH3(g) + 2H2O |
Structure and Oxidation of Disaccharides
| Sugar |
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 |
|---|
| Type |
non-reducing sugar |
reducing sugar |
reducing sugar |
|---|
|
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 |
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