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

Disaccharides

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

Condensation reaction

O

C C

C C

C

CO

C C

C C

C

C

OH OH

O

C C

C C

C

CO

C C

C C

C

C

OH OH

Condensation reaction

O

C C

C C

C

CO

C C

C C

C

C

O

H2O

Condensation reaction

A disaccharide1,4 glycosidic bond

O

C C

C C

C

CO

C C

C C

C

C

O

41

Condensation reaction

& Isomerism

OH

OH

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

OH

OH

OH

CH2OH

O

O

OH

OH

OH

CH2OH

O

-1,4-glycosidic bond

- D-Maltose

Composed of -galactose and -glucose linked by a -1,4-glycosidic bond.

Lactose

Milk Sugar

-1,4-glycosidic bond

α-D-Lactose

Composed of glucose and fructose molecules joined by ,-1,2-glycosidic bond.

Has no isomers because mutarotation is blocked.

Table Sugar

Sucrose

O

CH2OH

OH

OH

CH2OH

oCH2OH

OHOH

OH

O

α, -1,2-glycosidic bond

Sucrose

Disaccharide SynthesisEnergy +

Disaccharide hydrolyzed to its corresponding of monosaccharides.

Maltose + H2O Glucose + Glucose

Lactose + H2O Glucose + Galactose

Sucrose + H2O Glucose + Fructose

Disaccharide Hydrolysis