Carbohydrates
Prepared by:
Mona Abo Hasera
Carbohydrates
• Poly hydroxy aldehydes or poly hydroxy-ketones of formula
(CH2O)n
And are usually classified according to their structure:
 Monosaccharides - simple sugars with multiple OH groups.
Based on number of carbons (3, 4, 5, 6), a monosaccharide is
a triose, tetrose, pentose or hexose, cannot be hydrolyzed to
simpler carbohydrates; eg. Glucose or fructose.
 Disaccharides - 2 monosaccharides covalently linked can be
hydrolyzed into two monosaccharide units; eg. Sucrose, which
is hydrolyzed into glucose and fructose.
 Oligosaccharides - a few monosaccharides covalently linked.
 Polysaccharides - polymers consisting of chains of
monosaccharide or disaccharide units. eg Starch or cellulose.
Monosaccharides
Aldoses (e.g., glucose) have an
aldehyde group at one end.
H
Ketoses (e.g., fructose) have
a keto group, usually at C2.
O
CH2OH
C
C
O
HO
C
H
OH
H
C
OH
OH
H
C
OH
H
C
OH
HO
C
H
H
C
H
C
CH2OH
CH2OH
D-glucose
D-fructose
D vs L Designation
CHO
CHO
D & L designations are
H C OH
based
on
the
CH2OH
configuration about
the single asymmetric D-glyceraldehyde
C in glyceraldehyde.
HO
H
C
OH
CH2OH
D-glyceraldehyde
H
CH2OH
L-glyceraldehyde
CHO
The
lower
representations are
Fischer Projections.
C
CHO
HO
C
H
CH2OH
L-glyceraldehyde
Sugar Nomenclature
For sugars with more
H
O
than one chiral center,
C
D or L refers to the
H – C – OH
asymmetric C farthest HO – C – H
from the aldehyde or
H – C – OH
keto group.
H – C – OH
Most naturally occurring
sugars are D isomers.
CH2OH
D-glucose
O
H
C
HO – C – H
H – C – OH
HO – C – H
HO – C – H
CH2OH
L-glucose
Hemiacetal & hemiketal formation
H
An aldehyde can
OH
C O + R'
react with an
R
alcohol to form
aldehyde
alcohol
a hemiacetal.
A ketone can
react with an
alcohol to form
a hemiketal.
H
R'
O
OH
R
hemiacetal
R
C
C
R
O
+
"R
OH
R'
ketone
"R
O
C
R'
alcohol
hemiketal
OH
Pentoses
and
hexoses can cyclize
as the ketone or
aldehyde
reacts
with a distal OH.
Glucose forms an
intra-molecular
hemiacetal, as the
C1 aldehyde & C5
OH react, to form
a
6-member
pyranose ring.
1
H
HO
H
H
2
3
4
5
6
CHO
C
OH
C
H
C
OH (linear form)
C
OH
D-glucose
CH2OH
6 CH2OH
6 CH2OH
5
H
4
OH
H
OH
3
H
O
H
H
1
2
OH
-D-glucose
OH
5
H
4
OH
H
OH
3
H
O
OH
H
1
2
OH
-D-glucose
These representations of the cyclic sugars are called
Haworth projections.
H
6 CH2OH
6 CH2OH
5
H
4
OH
O
H
OH
3
H
H
2
OH
-D-glucose
H
1
OH
5
H
4
OH
H
OH
3
H
O
OH
H
1
2
H
OH
-D-glucose
Cyclization of glucose produces a new asymmetric center
at C1. The 2 stereoisomers are called anomers,  & .
Haworth projections represent the cyclic sugars as having
essentially planar rings, with the OH at the anomeric C1:
  (OH below the ring)
  (OH above the ring).
Glycosidic Bonds
The anomeric hydroxyl and a hydroxyl of another sugar
or some other compound can join together, splitting out
water to form a glycosidic bond:
R-OH + HO-R'  R-O-R' + H2O
E.g., methanol reacts with the anomeric OH on glucose
to form methyl glucoside (methyl-glucopyranose).
H OH
H OH
H2O
H O
HO
HO
H
H
H
+
CH3-OH
H O
HO
HO
H
OH
H
OH
-D-glucopyranose
methanol
H
OH
OCH3
methyl--D-glucopyranose
Disaccharides:
Maltose, a cleavage
product of starch
(e.g., amylose), is a
disaccharide with an
(1 4) glycosidic
link between C1 - C4
OH of 2 glucoses.
6 CH2OH
6 CH2OH
H
5
O
H
OH
4
OH
3
H
H
H
1
H
4
4
maltose
OH
H
H
H
H
H
1
OH
2
OH
1
O
4
5
O
H
OH
H
H
3
H
6 CH2OH
O
H
OH
H
OH
3
OH
5
O
O
2
6 CH2OH
H
5
2
OH
3
cellobiose
H
2
OH
1
H
OH
Cellobiose, a product of cellulose breakdown, is the
otherwise equivalent  anomer (O on C1 points up).
The (1 4) glycosidic linkage is represented as a zig-
zag.
Other disaccharides include:
 Sucrose, common table sugar, has a glycosidic bond
linking the anomeric hydroxyls of glucose & fructose.
Because the configuration at the anomeric C of glucose
is  (O points down from ring), the linkage is (12).
 Lactose, milk sugar, is composed of galactose & glucose,
with (14) linkage from the anomeric OH of galactose.
CH2OH
H
O
H
OH
H
H
H
1
O
OH
6CH OH
2
5
O
H
4 OH
3
H
OH
H
H
H
H 1
O
H
OH
CH2OH
CH2OH
CH2OH
H
H
H
O
H
OH
H
O
O
H
H
O
H
OH
H
H
O
OH
2
OH
H
OH
H
OH
H
OH
amylose
Polysaccharides:
Plants store glucose as amylose or amylopectin, glucose
polymers collectively called starch.
Glucose storage in polymeric form minimizes osmotic
effects.
Amylose is a glucose polymer with (14) linkages.
The end of the polysaccharide with an anomeric C1 not
involved in a glycosidic bond is called the reducing end.
CH2OH
CH2OH
O
H
H
OH
H
H
OH
H
O
OH
CH2OH
H
H
OH
H
H
OH
H
H
OH
CH2OH
O
H
OH
O
H
OH
H
H
O
O
H
OH
H
H
OH
H
H
O
4
glycogen
H
1
O
6 CH2
5
H
OH
3
H
CH2OH
O
H
2
OH
H
H
1
O
CH2OH
O
H
4 OH
H
H
H
H
O
OH
O
H
OH
H
H
OH
H
OH
Glycogen, the glucose storage polymer in animals, is
similar in structure to amylopectin.
But glycogen has more (16) branches.
The highly branched structure permits rapid glucose
release from glycogen stores, e.g., in muscle during
exercise.
The ability to rapidly mobilize glucose is more essential to
animals than to plants.
CH2OH
H
O
H
OH
H
OH
H
1
O
H
H
OH
6CH OH
2
5
O
H
4 OH
3
H
H
H 1
2
OH
O
O
H
OH
CH2OH
CH2OH
CH2OH
H
H
O
O
H
OH
H
OH
O
H
O
H
OH
H
OH
H
H
OH
cellulose
Cellulose, a major constituent of plant cell walls, consists
of long linear chains of glucose with (14) linkages.
Every other glucose is flipped over, due to  linkages.
This promotes intra-chain and inter-chain H-bonds and
van der Waals interactions,
that cause cellulose chains to
be straight & rigid, and pack
with
a
crystalline
arrangement in thick bundles
- microfibrils.
OH
H
H
H
H
H
Schematic of arrangement of
cellulose chains in a microfibril.
Classification upon reducing end
Reducing
sugars
• can be oxidized by mild oxidizing
agents because the oxidizing agent
is reduced in the reaction
• All monosaccharides
• Maltose, Lactose
Non-reducing
sugars
• is not oxidized by mild oxidizing
agents.
• Sucrose
• All polysaccharides
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
In-lab Experiments
1. Benedict's Test
(positive for reducing sugars)
• Principle:
Benedict's reagent contains cupric ions, which
in an alkaline environment, oxidize the
aldehyde group to a carboxylic acid. Cupric
ions are reduced to cuprous oxide, which
forms a red precipitate
RCHO + 2Cu2+ + 4OH- ----> RCOOH + Cu2O + 2H2O
Procedure
1. Place 1mL of the following 1% carbohydrate solutions
in separate, labeled test tubes: glucose, fructose,
sucrose, lactose, maltose, and starch.
2. Also place 1 ml of distilled water in another tube to
serve as a control.
3. To each tube, add 1 ml of Benedict's reagent and heat
the tubes in a boiling water bath for 5 minutes.
4. Remove the tubes from water bath. Note and record
the results.
 In the presence of a reducing sugar a precipitate
which may be red, yellow or green will form.
2. Barfoed's Test
(Used to distinguish between mono- & di-saccharides)
• Principle
Barfoed's reagent reacts with monosaccharides to produce cuprous oxide at a
faster rate than disaccharides do:
• RCHO + 2Cu2+ + 2H2O -----> RCOOH + Cu2O + 4H+
Procedure
• Procedure:
1. Place 1 mL of the following 1% carbohydrate
solutions in separate, labeled test tubes: glucose,
fructose, sucrose, lactose, and maltose.
2. To each tube, add 1 ml of Barfoed's reagent, and
heat in a boiling water bath for 10 minutes.
3. Remove the tubes from water bath. Note and
record your observations.

A red precipitate will form if the test is
positive.
3. Bial's (Orcinol) Test for pentoses
( for the detection of pentoses)
Principle
• Pentoses are converted to furfural by this
reagent, which forms a blue green color with
orcinol.
Procedure
1. Add about 1 ml of 1% xylose, glucose, fructose, maltose,
arabinose, and xylose solution to their respective labeled
test tubes.
2. Add 1.5 ml of Bial's reagent to each tube and mix well.
3. Carefully heat each tube (with some agitation) directly
over the burner flame. Hold the tube at a diagonal and
heat along the sides of the tube rather than at the bottom
to prevent eruption of the liquid from the tube. Move the
tube diagonally in and out of the flame, until the mixture
just begins to boil. Stop heating when the mixture begins
to boil.
A blue-green color indicates a positive result. Prolonged
heating of some hexoses yields hydroxymethyl furfural
which also reacts with orcinol to give colored complexes.
Bial's reagent (0.1 % orcinol in
concentrated HCl containing 0.1 %
FeCl3.6H2O).
4. Seliwanoff's (Resorcinol) Test
(used for detection of Ketoses)
•
Principle
Ketohexoses (such as fructose) and
disaccharides containing a ketohexose (such
as sucrose) form a cherry-red condensation
product. Other sugars (e.g. aldose) may
produce yellow to faint pink colors.
Procedure
1. Add about 3 ml of Seliwanoff's reagent to each
labeled test tube.
2. Add 1 drop of the respective sugar solution to the
appropriate test tubes, and mix well.
3. Place all the test tubes in the boiling water bath
at the same time and heat for 3 min after the
water begins to boil again. Record your
observations.

A positive result is indicated by the formation
of a red color with or without the separation of a
brown-red precipitate.
Seliwanoff's reagent (0.5 % resorcinol
in 3N HCl).
5. Picric Acid Test
(for reducing sugars)
•
Principle
Picric acid (2,4,6-trinitrophenol) or TNP reacts
with reducing sugars to give a red colored
picramic acid C6H2.OH.NH2(NO2)2
Procedure
1. Into a test tube add 1 ml of maltose solution,
into the second tube, 1ml of sucrose solution.
2. Add into each tube 1 ml of a saturated
solution of picric acid, and then add into each
tube 0.5 ml of sodium hydroxide solution.
3. Heat both samples in a boiling water bath.
 In the presence of reducing sugars, the
solution stains red; a sodium salt of picric acid
is formed.