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Chemistry 20
Chapter 12
Carbohydrates
Carbohydrates
• Produced by photosynthesis
in plants.
• The major source of energy
from our diet.
• Composed of the elements C,
H, and O.
Cn(H2O)n
6CO2 + 6H2O + energy
Photosynthesis
Respiration
C6H12O6 + 6O2
glucose
Carbohydrates
- The most abundant organic compounds in the plant world.
- 3/4 of the weight of plants.
- 1% of the weight of animals and humans (they do not store).
- 65% of the foods in our diet.
Carbohydrates
1. Monosaccharide + H2O
2. Disaccharide + H2O
H+ or enzyme
H+ or enzyme
no hydrolysis
two monosaccharide units
+
3. Polysaccharide + many H2O
H+ or enzyme
many monosaccharide units
Monosaccharides
A carbohydrate that cannot be split or hydrolyzed into smaller carbohydrates.
Monosaccharides are carbohydrates with:
• 3-9 carbon atoms
• A carbonyl group (aldehyde or ketone)
• Several hydroxyl groups
O
║
C─H
│
H─ C ─ OH
│
H─ C ─ OH
│
CH2OH
Cn(H2O)n
CnH2nOn
Monosaccharides - Aldose
Aldose is monosaccharide:
• With an aldehyde group and many
hydroxyl (-OH) groups.
•
•
•
•
triose (3C atoms)
tetrose (4C atoms)
pentose (5 C atoms)
hexose (6 C atoms)
“Aldo-” + suffix
O
║
C ─ H aldose
│
H─ C ─ OH
│
H─ C ─ OH
│
CH2OH
an aldotetrose
(Erythose)
Monosaccharides - Ketose
Ketose is monosaccharide:
• With a ketone group and many
hydroxyl (-OH) groups.
•
•
•
•
triose (3C atoms)
tetrose (4C atoms)
pentose (5 C atoms)
hexose (6 C atoms)
CH2OH
│
C=O
ketose
│
H─ C ─ OH
│
H─ C ─ OH
│
H─ C ─ OH
│
CH2OH
“Keto-” + suffix
a ketohexose
(Fructose)
Some important Monosaccharides
Glucose
(C6H12O6, aldohexose) – blood sugar
• Is found in fruits, vegetables,
corn syrup, and honey.
H O
C
H C OH
HO C H
• Is found in disaccharides such as
sucrose, lactose, and maltose.
H C OH
H C OH
• Makes up polysaccharides such as
starch, cellulose, and glycogen.
CH2OH
Some important Monosaccharides
Fructose
(C6H12O6, ketohexose),
• Is the sweetest of the carbohydrates.
• Is found in fruit juices and honey (fruit sugar).
• In bloodstream, it is converted to its isomer,
glucose.
• Is bonded to glucose in sucrose (a
disaccharide known as table sugar).
CH2OH
C O
HO C H
H C OH
H C OH
CH2OH
Some important Monosaccharides
Galactose
(C6H12O6, aldohexose),
• Has a similar structure to glucose
except for the –OH on Carbon 4.
• Cannot find in the free form in nature.
• Exist in the cellular membranes of the
brain and nervous system.
H O
C
H C OH
HO C H
HO C H
H C OH
CH2OH
• Combines with glucose in lactose (a
disaccharide and a sugar in milk).
Disease - Galactosemia
Galactosemia
missing the enzyme that convert galactose to glucose.
Accumulation of galactose in the blood and tissues.
Mental retardation and cataract
Solution: removing the galactose from food: no milk.
Fischer Projections
- Horizontal lines represent bonds projecting forward from the stereocenter.
- Vertical lines represent bonds projecting to the rear.
- Only the stereocenter is in the plane.
CHO
H
C
Convert to Fischer
Projection
OH
CH2 OH
3D
CHO
H
OH
CH2 OH
2D
Fischer Projections
1. Carbon with four different groups bonded to it.
2. The chiral carbon furthest from the carbonyl group (-CHO).
H
HO
H
H
CHO
OH
H
OH
OH
CH2 OH
D-Glucose
D - glucose
More common in the nature
CHO
CHO
CHO
CH2H
OH
CHO
H CHO
OH
O
H HO
OH
C=HO
OH CHO
H CHO
OH
O
CHO
CHO
H
H
H
OH
O
HO HO
H
HH
H
HO
H
OHHO
H HH
O
H
OH
HO HO
HH
H OH
H
HO
H
OH
HO
H
HO
H
H H
OH
H CHOH
OH
HO
H
H HO
OH
H OH
H
H
H
OH
HO
H2
OH
CH
2
CH
CH
OH
H
D-Glucose
D-Galactose
CHOH
CHOH
2
2H
HOH
OH
H
OH
2 OH
2
D-Galactose
D-Fructose
CH
OH
CH
D-Glucose
D-Galactose
2
CH
OH
CH22
2
L - glucose
D-Glucose
D-Glucose
D-Galactose
D-Galactose
Amino Sugars
- Amino sugars contain an -NH2 group in place of an -OH group.
- Only three amino sugars are common in nature:
D-glucosamine, D-mannosamine, and D-galactosamine.
CHO
H N H2
HO H
H OH
H OH
CH2 OH
CHO
H 2N 2 H
HO H
H OH
H OH
CH2 OH
D-Glucosamine
D-Mannosamine
(C-2 stereois omer
of D-glucosamine
CHO
H N H2
HO H
HO 4 H
H OH
CH2 OH
D-Galactosamine
(C-4 stereois omer
of D-glucosamine)
H
HO
H
H
CHO O
N HCC
H
OH
OH
CH2 OH
N-Acetyl-Dglucos amine
Cyclic Structure – Haworth Structure
- Aldehydes and ketones react with alcohols to form hemiacetals.
- Cyclic hemiacetals form readily when the hydroxyl and carbonyl groups
are part of the same molecule.
O
4
1
H
red raw to show
-OH an d -CHO
clos e to each oth er
O-H
4-Hyd roxypentanal
1
4
O
H
C
H
O
H
O-H
O
A cyclic hemiacetal
Cyclic Structure – Haworth Structure
1
1
Anomeric carbon
1
1
Alpha (α)
More stable form
1
Beta ()
Anomers
Cyclic Structure – Haworth Structure
CH2OH
CH2OH
O
O
OH 
1
1
OH
OH
OH 
OH
OH
OH
OH
-Glucose
-Glucose
CH2OH
O
OH
O
OH
1
OH
CH2OH
CH2OH
OH
OH
-Galactose
1
OH
O
OH
OH
OH
-Galactose
OH
OH
OH
Cyclic Structure – Haworth Structure
O
O
HOCH2
CH2OH
O
HOCH2
OH
2
H
HO
H
OH
HOCH2
2
OH
H
H
OH
1
CH2 OH
O
2
H -fructose
HO
5
H
OH( )
H
HO
Anomeric carbon
 -D -Fructofuranose
( - D -Fructos e)
HO
H
CH2OH
H
-fructose
1
2
CH2 OH
C=O
HO
H
H
OH
H 5 OH
CH2 OH
HOCH2
5
O
H HO
H
OH ( )
2
CH2 OH
HO
H
1
Cyclic Structure – Haworth Structure
O
O
1
OH

CH2OH
CH2OH
OH
1
OH

OH
OH
OH
-Glucose
OH
OH
-Glucose
Humans have -amylase (an enzyme) and they can digest starch
products such as pasta (contain -glucose)
Humans do not have β-amylase (an enzyme) and they cannot digest
cellulose such as wood or paper (contain β-glucose)
Chair Conformation
6
CH2 OH
5
O OH()
H
H
4 OH
1
H
HO
H
3
2
H OH
-D -Glucop yranose
(Haw orth p rojection)
-D-Glucose
(Haworth projection)
6
CH2 OH
4
HO
HO
O
5
3
2
OH 1
OH( )
 - D -Glucopyranose
(ch air con formation)
-D-Glucose
(Chair conformation)
Mutarotation
Change in specific rotation that accompanies the equilibration
of α and  anomers in aqueous solution.
-D-glucose
64%
Open-chain form
< 0.02%
α-D-glucose
36%
Physical properties of Monosaccharides
- Colorless
- Crystalline solids
- Soluble in water (H-bond because of OH groups)
- Insoluble in nonpolar solvents
Chemical properties of Monosaccharides
1. Formation of Glycosides (acetals)
2. Oxidation
3. Reduction
Formation of Glycosides (acetals)
- Exist almost exclusively in cyclic hemiacetal forms.
- They react with an alcohol to give acetals.
- Acetals are stable in water and bases but they are hydrolyzed in acids.
anomeric
carbon
CH2 OH
O OH
H
+
H
H
+ CH3 OH
OH H
-H2 O
HO
H
glycos idic
H OH
CH2 OH
bond
-D -Glu copyran os e
O OCH3
H
(-D
-Glu cose)
-D-Glucose
H
+
OH H
H
HO
CH2 OH
OH
H
H
OH H
HO
OCH3
H OH
H OH
Methyl -D -glu copyran os ide Methyl -D -glu copyran os ide
Methyl α-D-Glucoside
Methyl -D-Glucoside
(Methyl
-D -glu coside)
(Methyl -D -glucos ide)
Oxidation of Monosaccharides
OH
H O
C
H O
C
H C OH
HO C H
H C OH
H C OH
CH2OH
D - glucose
Aldonic acids
H C OH
+
2Cu2+
Benedict’s
Reagent (blue)
Oxidation
HO C H
H C OH
+ Cu2O(s)
(Brike red)
H C OH
CH2OH
D – gluconic acid
Reducing sugars: reduce another substance.
Oxidation of Monosaccharides
O
H
CH2OH
C
Rearrangement
(Tautomerism)
C=O
D-fructose
(ketose)
H
C
OH
D-glucose
(aldose)
Oxidation of Monosaccharides
primary alcohol at C-6 of a hexose is oxidized to uronic acid
by an enzyme (catalyst).
CHO
enzymeH
OH
catalyzed
HO
H
Enzyme
oxidation
H
OH
H
OH
CH2 OH
D-Glucose
D-glucose
CHO
H
OH
COO
HO
H
HO
H
OH
HO
H
OH
COOH
acid
D-glucuronic D-Glucuronic
acid
(a uronic acid)(a uronic acid)
Exist in connective tissue
Detoxifies foreign phenols and alcohols
Reduction of Monosaccharides
H O
C
H O
CH2OH
C
H C OH
H C OH
HO C H
H2
Transition metals
H C OH
HO C H
H C OH
H C OH
H C OH
CH2OH
CH2OH
D - glucose
Alditols
D – Sorbitol (D – glucitol)
Sugars alcohols: sweetners in many sugar-free (diet drinks & sugarless gum).
Problem: diarrhea and cataract
Disaccharides
A disaccharide:
• Consists of two monosaccharides linked by a glycosidic
bond (when one –OH group reacts with another –OH group).
Glucose + glucose
maltose + H2O
Glucose + galactose
lactose + H2O
Glucose + fructose
sucrose + H2O
Disaccharides
Maltose:
•
•
•
•
•
Is a disaccharide of two glucose molecules.
Has a α -1,4-glycosidic bond (between two α-glucoses).
Is obtained from the breakdown of starches.
Is used in cereals and candies.
Is a reducing sugar (carbon 1 can open to give a free aldehyde to oxidize).
CH2OH
CH2OH
O
OH
OH
CH2OH
O
O
+
1
OH
OH
α-glucose
4
OH
OH
OH
OH
CH2OH
 -1,4-glycosidic
bond
1
OH
O
O
4
OH
OH
α-glucose
OH
OH
- maltose
OH 
Disaccharides
Lactose:
•
•
•
•
Is a disaccharide of galactose and glucose.
Has a β -1,4-glycosidic bond (between β-galactose and α-gulcose).
Is found in milk and milk products (almost no sweet).
Is a reducing sugar (carbon 1 can open to give a free aldehyde to oxidize).

-lactose
Disaccharides
Sucrose:
•
•
•
•
Is found in table sugar (obtained from sugar cane and sugar beets).
Consists of glucose and fructose.
Has an α,β-1,2-glycosidic bond (between α-glucose and -fructose).
Is not a reducing sugar (carbon 1 cannot open to give a free aldehyde
to oxidize).
Polysaccharides
• Polymers of many monosaccharides units.
(starch that stores glucose in plants such
as rice, potatoes, beans, and wheat - energy storage).
Amylose (20%)
• Starch
Amylopectin (80%)
• Glycogen (animal starch in muscle and liver. It is
hydrolyzed in our cells and provides energy ).
• Cellulose (plant and wood structures).
Polysaccharides
Amylose:
• Is a polysaccharide of α-glucose in a
continuous (unbranched) chain (helical or coil
form).
• Has α-1,4-glycosidic bonds between the
α-glucose units (250 to 4000 units).
α-1,4-glycosidic bond
Polysaccharides
Amylopectin:
• Is a polysaccharide of glucose units in branched chains.
• Has α-1,4-glycosidic bonds between the α-glucose units.
• Has α-1,6 bonds to branches of glucose units.
(at about every 25 glucose units, there is a branch).
• Glycogen has same structure (more highly branched-every 10-15 units).
Polysaccharides
Amlose, Amylopectin (starch)
H+ or amylase (enzyme in saliva)
Dextrins (6-8 glucose units)
H+ or amylase (enzyme in saliva)
Maltose (2 glucose units)
H+ or maltase (enzyme)
Many α-D-glucose units
Digestion process
Respiration
C6H12O6 + 6O2
6CO2 + 6H2O + energy
glucose
Fermentation
C6H12O6
Yeast
2C2H5OH + CO2 + energy
Ethanol
Polysaccharides
Cellulose:
• Is a polysaccharide of glucose units in unbranched chains with
ß-1,4-glycosidic bonds (2200 glucose units).
• Has rigid structure (H-bond) and insoluble in water.
• Is the major structural material of wood & plants (cotton: 100%).
• Cannot be digested by humans because of the
ß-1,4-glycosidic bonds (needs a special enzyme).