Organic Chemistry

946 CHAPTER 22 Carbohydrates

The most common disaccharide is sucrose (table sugar). Sucrose is obtained from

sugar beets and sugarcane. About 90 million tons of sucrose are produced in the

world each year. Sucrose consists of a D-glucose subunit and a D-fructose subunit

linked by a glycosidic bond between C-1 of glucose (in the ) and C-2 of

fructose (in the ).

Unlike the other disaccharides that have been discussed, sucrose is not a reducing

sugar and does not exhibit mutarotation because the glycosidic bond is between the

anomeric carbon of glucose and the anomeric carbon of fructose. Sucrose, therefore,

does not have a hemiacetal or hemiketal group, so it is not in equilibrium with the

readily oxidized open-chain aldehyde or ketone form in aqueous solution.

Sucrose has a specific rotation of When it is hydrolyzed, the resulting

equimolar mixture of glucose and fructose has a specific rotation of Because

the sign of the rotation changes when sucrose is hydrolyzed, a mixture of glucose

and fructose is called invert sugar. The enzyme that catalyzes the hydrolysis of su-

crose is called invertase. Honeybees have invertase, so the honey they produce is a

mixture of sucrose, glucose, and fructose. Because fructose is sweeter than sucrose,

invert sugar is sweeter than sucrose. Some foods are advertised as containing fructose

instead of sucrose, which means that they achieve the same level of sweetness with a

lower sugar content.

22.18 Polysaccharides

Polysaccharides contain as few as 10 or as many as several thousand monosaccha-

ride units joined together by glycosidic linkages. The molecular weight of the indi-

vidual polysaccharide chains is variable. The most common polysaccharides are

starch and cellulose.

Starch is the major component of flour, potatoes, rice, beans, corn, and peas. Starch

is a mixture of two different polysaccharides: amylose (about 20%) and amylopectin

(about 80%). Amylose is composed of unbranched chains of D-glucose units joined by

linkages.

CH 2 OH

HO

O

HO

O

CH 2 OH

HO

O

HO

O

O

CH 2 OH

HO

O

HO

O

an -1,4′-glycosidic linkage

three subunits of amylose

a-1,4¿-glycosidic

1:1

• 22.0°.

+66.5°.

O

HO

O

HO

HO

HO

HO

sucrose

linkage at glucose

linkage at fructose

CH 2 OH

HOCH 2

CH 2 OH

O

b-position

a-position

3-D Molecule:

Sucrose