Organic Chemistry

Section 22.6 Osazone Formation 929

PROBLEM 11

Write the mechanism for the base-catalyzed conversion of D-fructose into D-glucose and

D-mannose.

PROBLEM 12

When D-tagatose is added to a basic aqueous solution, an equilibrium mixture of three

monosaccharides is obtained. What are these monosaccharides?

If an oxidizing agent stronger than those discussed previously is used (such as
), one or more of the alcohol groups can be oxidized in addition to the aldehyde
group. A primary alcohol is the one most easily oxidized. The product that is obtained
when both the aldehyde and the primary alcohol groups of an aldose are oxidized is
called an aldaric acid. (In an acid, end is oxidized. In an acid,
both ends oxidized.)

PROBLEM 13

a. Name an aldohexose other than D-glucose that is oxidized to D-glucaric acid by nitric acid.

b. What is another name for D-glucaric acid?

c. Name another pair of aldohexoses that are oxidized to identical aldaric acids.

22.6 Osazone Formation

The tendency of monosaccharides to form syrups that do not crystallize made the pu-
rification and isolation of monosaccharides difficult. Emil Fischer found that when
phenylhydrazine is added to an aldose or a ketose, a yellow crystalline solid that is in-
soluble in water is formed. He called this derivative an osazone (“ose”for sugar;
“azone”for hydrazone). Osazones are easily isolated and purified and were once used
extensively to identify monosaccharides.

Aldehydes and ketones react with one equivalent of phenylhydrazine, forming
phenylhydrazones (Section 18.6). Aldoses and ketoses, in contrast, react with three
equivalents of phenylhydrazine, forming osazones. One equivalent functions as an ox-
idizing agent and is reduced to aniline and ammonia. Two equivalents form imines
with carbonyl groups. The reaction stops at this point, regardless of how much phenyl-
hydrazine is present. (Recall that the pH at which imine formation is carried out must
be carefully controlled; Section 18.6.)

D-glucose

HO H HNO^3

OH

HOH

HOH

CH 2 OH

H

HC O

HO H

OH

HOH

HOH

COOH

H

COOH

D-glucaric acid

an aldaric acid

∆

are

aldonic one aldaric

HNO 3

HO H + 3 NH 2 NH

OH

HOH

HOH

CH 2 OH

H

HC O

HOHOHH ++NH 3 + 2 H 2 O

HOH

CH 2 OH

NH 2

C NNHC 6 H 5

HC NNHC 6 H 5

D-glucose the osazone of D-glucose

catalytic

H+