Reduction of Monosaccharides
The reduction of the carbonyl group produces sugar alcohols, or alditols.
D-Glucose is reduced to D-glucitol also called sorbitol.
REACTIONS OF MONOSACCHARIDES
Oxidation of Monosaccharides Monosaccharides are reducing sugars if their
carbonyl groups oxidize to give carboxylic acids. Benedict’s reagent (CuSO4) can oxidize aldehydes with adjacent
hydroxyl groups The blue Cu2+ ions in the Benedict’s reagent are reduced to form
a brick-red precipitate, Cu2O In the Benedict’s test, D-glucose is oxidized to D-
gluconic acid. Glucose is a reducing sugar.
C
C
C
C
C
CH2OH
HO
OHH
HO H
OHH
OHH
C
C
C
C
C
CH2OH
OHO
OHH
HO H
OHH
OHH
+ Cu2+
D-Glucose D-Gluconic acid
+ Cu2O(s)
monosaccharide
D-glucose
aldonic acid
gluconic a.
alduronic acidglucuronic a.
aldaric acid
glucaric a.
CH2OH
COH =
CH2OH
COHO =
COOH
COH =
COOH
COOH
Oxidation of Monosaccharides
A disaccharide is formed when a hydroxyl group on one monosaccharide reacts with the anomeric carbon (C-1) of another monosaccharide to form a glycosidic bond
Links may be α or β 1-4’ link: The anomeric carbon is bonded
to oxygen on C4 of second sugar. 1-2’ link: The anomeric carbons of the two sugars are bonded
through an oxygen. The free anomeric carbon is called reducing end According to the position of the linkage between the sugar units,
disaccharides are classified into non-reducing such as sucrose and reducing such as maltose and lactose.
Disaccharides
Disaccharides Formed from two monosaccharides Joined by a glycosidic bond A condensation reaction:
glucose + glucose maltose glucose + galactose lactose glucose + fructose sucrose
OCH2OH
OHOH
OH
OH
OCH2OH
OHOH
OH
OH
and Anomers for D-Glucose
The new –OH on C1 is drawn down for the anomer, and up for the anomer.
-D-Glucose -D-Glucose
D-(-)-Fructose
CH2OH
O
OH
OH
OH
CH2OH
..
..
..:
O
HOH
H
H
OH
CH2OH
OHCH2OH
b-D-(-)-Fructofuranose
cis = bup = D
anomericcarbon
FRUCTOSE
1
2
3
4
5
6
1
2
34
5
6
O
standard position
Cyclic Structure of Fructose
As a ketohexose, fructose forms a cyclic structure when the —OH on C-5 reacts with the C=O on C-2.
CH2OH
C
C
C
C
CH2OH
O
HO H
OHH
OHH
D-Fructose
CH2OH
OH
OH
OH
CH2OHO
CH2OH
OH
OH
CH2OH
OHO
-D-Fructose -D-Fructose
A disaccharide in which two -D-glucose molecules are joined by an -1,4-glycosidic bond.
Malt Sugar
Maltose
Composed of glucose and fructose molecules joined by ,-1,2-glycosidic bond.
Has no isomers because mutarotation is blocked.
Table Sugar
Sucrose