Organic Chemistry

928 CHAPTER 22 Carbohydrates

PROBLEM 9

What products are obtained from the reduction of

a.D-idose? b.D-sorbose?

PROBLEM 10

a. What other monosaccharide is reduced only to the alditol obtained from the reduction of

1.D-talose?

2.D-galactose?

b. What monosaccharide is reduced to two alditols, one of which is the alditol obtained

from the reduction of

1.D-talose?

2.D-mannose?

Oxidation

Aldoses can be distinguished from ketoses by observing what happens to the color of an

aqueous solution of bromine when it is added to the sugar. is a mild oxidizing agent

and easily oxidizes the aldehyde group, but it cannot oxidize ketones or alcohols. Conse-

quently, if a small amount of an aqueous solution of is added to an unknown mono-

saccharide, the reddish-brown color of will disappear if the monosaccharide is an

aldose, but will persist if the monosaccharide is a ketose. The product of the oxidation re-

action is an aldonic acid.

Both aldoses and ketoses are oxidized to aldonic acids by Tollens reagent

so that reagent cannot be used to distinguish between aldoses and ke-

toses. Recall from Section 20.3, however, that Tollens reagent oxidizes aldehydes

but not ketones. Why, then, are ketoses oxidized by Tollens reagent, while ketones

are not? Ketoses are oxidized because the reaction is carried out under basic condi-

tions, and in a basic solution, ketoses are converted into aldoses by enolization

(Section 19.2). For example, the ketose D-fructose is in equilibrium with its enol.

However, the enol of D-fructose is also the enol of D-glucose, as well as the enol of

D-mannose. Therefore, when the enol reketonizes, all three carbonyl compounds

are formed.

NH 3 ,HO-),

(Ag+,

+

H 2 O

red

HO H

OH

HOH

HOH

CH 2 OH

Br 2 +

colorless

2 Br−

H

HC O

D-glucose

HO H

OH

HOH

HOH

CH 2 OH

H

COOH

D-gluconic acid

an aldonic acid

Br 2

Br 2

Br 2

HO H

HOH

HOH

CH 2 OH

CH 2 OH

CO

D-fructose

a ketose

HO H

HOH

HOH

CH 2 OH

HC OH

COH

enol of D-fructose

enol of D-glucose

enol of D-mannose

+

HO−, H 2 O

HO−, H 2 O

HO−, H 2 O

HO−, H 2 O

HO H

OH

HOH

HOH

CH 2 OH

H

HC O

D-glucose

an aldose

HO H

H

HOH

HOH

CH 2 OH

HO

HC O

D-mannose

an aldose