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Organic chemistry for medicine and biology
students
Chem 2311
Chapter 16
Carbohydrates
By Prof. Dr.
Adel M. Awadallah
Islamic University of Gaza
Carbohydrates
• Carbohydrates are the most abundant organic molecules in
nature
• The empiric formula for many of the simpler carbohydrates is
(CH2O)n, hence the name “hydrate of carbon”
• Carbohydrates are classified as:
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Polyhydroxyaldehydes (aldoses)
Polyhydroxyketones (Ketoses)
Monosaccharides
D,L-Sugars
Epimers are diastereomers that differ only in one stereogenic center.
D-(-)-Erythrose and D-(-)-threose
D-glucose , D-manose
D-glucose, D-galactose
Anomeric carbons and the cyclic hemiacetal structure of Glucose
Anomeric carbon is the carbon that forms the newstereogenic center
D-glucose is an aldohexose with
the formula (C·H2O .6)The red
atoms highlight the aldehyde group,
and the blue atoms highlight the
asymmetric center furthest from the
aldehyde; because this -OH is on
the right of the Fischer projection ,
this is a D sugar.
The α and β anomers of glucose.
Note the position of the anomeric
carbon (red or green) relative to the
CH2OH group bound to carbon 5:
they are either on the opposite
sides (α), or the same side (β.)
Mutarotation
Mutarotation: is the change in optical rotation due to
interconversion of anomers in solution. It is explained by
the equilibrium
Pyranose and Furanose Structures
Glucofuranoses are present to less than 1 % in glucose solutions
O
O
Pyran
furan
The Ketose D-fructose exists in solution mainly in two
furanose forms
Problem: D-Erythrose cannot exist in pyranose forms, but furanose
forms are possible. Explain. Draw the structure for α-D-erythrofuranose
CHO
H
H
OH
OH
CH2OH
O
HO
O
OH
OH
OH
OH
OH
Conformations of Pyranoses
In β-D-Glucose all the larger substituents are at each ring carbon
are equatorial, hence this form is preffered at equilibrium
Esters and Ethers from Monosaccharides
Reduction of Monosaccharides
Aldoses are reduced to alditols
Sorbitol is used commercially as a sweetener and sugar substituent
Oxidation of Monosaccharides
Aloses are oxidized to aldonic acids
A carbohydrate that reduces Ag+ or Cu2+ is called a reducing sugar
Aldoses are oxidized by aqueous nitric acid to dicarboxylic
acids called aldaric acids
Formation of Glycosides from Monosaccharides
Disaccharides (Maltose)
Cellobiose
Lactose
Sucrose
Sweeteners
Polysaccharides
Starch and Glycogen Starch is the energy storing carbohydrate of plants
Hydrolysis of starch
H2O/H+
Starch
maltose
H2O/H+
Glucose
Starch is composed of amylose (20%) and amylopectin
In amylose, the glucose units (50 -300) are in continuous chain with 1,4 linkages
Amylopectin (300 – 5000 glucose units) is highly branched molecule
Chains with consecutive 1,4 links average 25-30 units in length. These
chains are connected by 1,6 linkages.
Glycogen
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•
•
•
•
Energy storing carbohydrate of anilmals
1,4 and 1,6 linked glucose units
about 100,000 glucose units
A branch every 8-12 glucose units
Produced from glucose that is absorbed from the intestines into the
blood
• Glycogen helps maintain the glucose balance in the body, by
removing and storing excess glucose from ingested food and later
supplying it to the blood
Cellulose
Cellulose is an unbranched polymer of glucose joined by 1,4-βglycosidic bonds (average 5000 units)
Humans and other animals digest starch and glycogen
Many bacteria contain β-glucosidase and can hydrolyze cellulose
(found in Termites (‫ )النمل األبيض‬and Ruminants ( ‫))الحيوانات المجترة‬