Lab 5: Qualitative Analysis Test
for Carbohydrates
Qualitative Analysis Test
• Is concerned with determining, the identity of
a substance.
• Enables us to detect the presence of things
which may be beyond the reach of our senses.
Simple and complex carbohydrates
• Simple are found in foods such as fruits, milk,
and vegetables and provide energy but lack
vitamins, minerals, and fibers.
• Needs short time for digestion.
• Complex are provide energy, vitamins,
minerals and fibers such as rice and starchy
vegetables'
• Need longer time to digest
Types of Carbohydrates
• Most naturally occurring sugars are D isomers.
• Monosaccharides are the simplest carbohydrates
- Also called “simple sugars”, cannot be hydrolyzed to simpler carbohydrates
- Examples: glucose, fructose, galactose, ribose
• Disaccharides are two monosaccharides bonded together.
- Examples: sucrose (table sugar), lactose (milk sugar).
•
Oligosaccharides - a few monosaccharides covalently linked.
• Polysaccharides are polymers of monosaccharides
- Can be split into many monosaccharides with acid or enzymes
- Examples: starch, cellulose, glycogen
• The joining of two hexoses by the glycosidic
bond causes the formation of disaccharide.
• Longer
chains
composed
of
3-10
monosaccharide
units
are
called
oligosaccharides.
• Polysaccharides usually contain hundreds or
thousands of monosaccharide units.
Sugar Nomenclature
For sugars with more than one chiral center, D
or L refers to the asymmetric C farthest from the
aldehyde or keto group.
Most naturally occurring sugars are D isomers.
O
H
C
H – C – OH
HO – C – H
H – C – OH
H – C – OH
CH2OH
D-glucose
O
H
C
HO – C – H
H – C – OH
HO – C – H
HO – C – H
CH2OH
L-glucose
Cyclization of glucose produces a new asymmetric
center at C1. The 2 stereoisomers are called anomers, a
& b.
Haworth projections represent the cyclic sugars as
having essentially planar rings, with the OH at the
anomeric C1:
 a (OH below the ring)
 b (OH above the ring).
6 CH2OH
6 CH2OH
5
H
4
OH
O
H
OH
3
H
H
2
OH
a-D-glucose
H
1
OH
H
4
OH
5
O
OH
H
OH
H
1
3
H
2
OH
b-D-glucose
H
The active part in sugars is aldehyde or ketone group.
The presence of aldehyde or ketone groups and hydroxyl groups
causes that the sugars have typical reactions for
aldehydes/ketones and alcohols.
Opend and closed form of
monosaccarides
Classification upon reducing end
1. Reducing Sugars
•
•
•
•
Oxidation: loss of electrons
Reduction: gain of electrons
Have aldehyde group
Sugars that can be oxidized by mild oxidizing
agents and the oxidizing agent is reduced in the
reaction.
( Can be oxidized to acid,Reduces another
compound)
• All monosaccharides
• Maltose, Lactose
• A reducing sugar is a carbohydrate possessing
either a free aldehyde or free ketone
functional group as part of its molecular
structure.
A functional groups are the regions of a
molecule that gives it particular properties.
A single molecule can have more than one
functional group as part of its structure.
• All of the monosaccharides and most of the
disaccharides can be oxidized.
• Sugars exist in solution as an equilibrium mixture
of open-chain and closed-ring (or cyclic)
structures.
• All monosaccharides have an open and closed
form structure, but oligosaccharides have only
closed structure.
• When the cyclic structure opens, the aldehyde
group is available for oxidation.
Lactose is a reducing sugar, why?
• ( one of the carbonyle groups are free)
• a carbonyl group is a functional group
composed of a carbon atom double-bonded
to an oxygen atom: C=O.
2. NON Reducing Sugars
• is not oxidized by mild oxidizing agents.
• Sucrose
• All polysaccharides
Sucrose is not a reducing sugar, why?
• Sucrose is not a reducing sugar because it
cannot revert to the open-chain form that
would provide the aldehyde group needed to
reduce the cupric ion. (the carbonyle groups
are busy in the to side)
•
• Common oxidizing agents used to test for the
presence of a reducing sugar are: Benedict's solution,
Fehling's solution and picric acid solution.
• An oxidizing agent (also called an oxidizer or oxidant) is
referred to as a chemical compound that readily
transfers oxygen atoms or a substance that gains
electrons in a redox chemical reaction.
• Aldose(glucose) ---- oxidation ----- carboxylate
• Ketose(fructose) ----- oxidation----- hydroxyl
carboxylate
Oxidizing Reagent
Benedict's Solution
Fehling's Solution
Tollen's Reagent
copper sulfate in
copper sulfate in alkaline
silver nitrate in aqueous
alkaline citrate
tartrate
ammonia
Color of Solution
deep blue
deep blue
colorless
Color After Reaction with a
brick red precipitate
brick red precipitate
silver mirror forms
Reducing Sugar
Cu2O(s)
Cu2O(s)
Ag(s)
Species Being Reduced
Cu2+
Cu2+
Ag+
(the oxidant)
Cu2+ + e ---> Cu+
Cu2+ + e ---> Cu+
Ag+ + e ---> Ag(s)
Species Being Oxidized
reducing sugar
reducing sugar
reducing sugar
(the reductant)
oxidized to carboxylate
oxidized to carboxylate
oxidized to carboxylate
Composition