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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:
  1. Benedict's Solution
  2. Fehling's Solution
  3. 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)
  O  
  ||  
R - C - H

Sugars that are aldehydes are known as aldoses.
or a ketone (alkanone)
  O  
  ||  
R - C - R'

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:
  O  
  ||  
R - C - H

aldose
+ 2Cu2+ + 5OH-
deep blue
  O  
  ||  
R - C - O-

carboxylate
+ Cu2O(s)
brick red
+ 3H2O

Oxidation Using Tollen's Reagent
  O  
  ||  
R - C - H

aldose
+ 2Ag(NH3)2+ + 3OH-
colourless
  O  
  ||  
R - C - O-

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:
  O  
  ||  
R- C -CH2OH

ketose
+ 2Cu2+ + 5OH-
deep blue
  OH  
  |  
R- C -COO-
  |
H
 

hydroxy carboxylate
+ Cu2O(s)
brick red
+ 3H2O

Oxidation Using Tollen's Reagent
  O  
  ||  
R- C -CH2OH

ketose
+ 2Ag(NH3)2+ + 3OH-
colourless
  OH  
  |  
R- C -COO-
  |
H
 

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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