Abstract on Sugars

Carbohydrates are polyhydroxy aldehydes or ketones. Therefore they undergo the same type of
reactions that alcohols, aldehydes or ketones undergo.

Many stereocenters exist in the typical monosaccharide. The stereocenter furthest from the
aldehyde or the ketone functionality determines if it is a D or L sugar.

Glucose is the only sugar whose structure you will be responsible for. The Fischer projection
shown below is the typical way it is represented when it is in it’s straight chain open form.

Glucose

H

CHO

OH

HO

H

H

OH

H

OH

CH2OH

Carbon whose stereochemistry determines type of sugar
if the OH is to the right it is a D sugar, if it is to the left it
is an L sugar.

Glucose is composed of 6 carbons and an aldehyde functionality and therefore is called an
aldohexose. The rules for naming and reactions it undergoes are typical for other aldoses.

Glucose, as do many other sugars, exists in a cyclic hemiacetal form. Since the ring that forms is a
six-membered oxygen containing ring it is called a pyranose. The carbon which has two oxygens
attached to it is called the anomeric carbon, if the anomeric C-O bond is down when drawn in the
conventional form then it is the a (alpha) anomer, if it is up then it is the ß (Beta) anomer.
(Alternatively, if the anomeric C-O bond is trans to the CH2OH it is the a anomer, if it is cis it is the
ß anomer.)

H

CHO

OH

HO

H

H

OH

H

OH

CH2OH

H

O

OH

OH

H

H

OH

H

OH

CH2OH

H

H

O

OH

H

OH

H

OH

H

OH

CH2OH

H

αanomer

βanomer

anomeric
carbon

(hemiacetal carbon)

If given the Fischer projection for any sugar and the fact that it forms a pyranose or furanose ring,
one should be able to draw the cyclic form (in the Haworth form or in a chair conformation).

Practice on the following:
D-Mannose exists primarily in the pyranose form, draw the ß isomer in both the Haworth form and
the chair conformation.

HO

CHO

H

HO

H

H

OH

H

OH

CH2OH
Mannose

Reactions of Sugars

The reactions that sugars undergo are typical of aldehydes and ketones.

Know how to convert sugars to:

Glycosides
React sugar with an alcohol (typically CH3OH) and an acid catalyst.
(NOTE:Glycosides can easily be converted back to the free sugar by hydrolysis, i.e. acid
and water)
Ethers
React sugar with OH-and alkylating agent (typically dimethylsulfate)
Ester
React sugar with acetic anhydride

Acids
Oxidize with Br2to give monoacid
Oxidize with HNO3to give diacid
Alcohols
Reduce with NaBH4

Reducing Sugars:
If there is a hemiacetal or hemiketal carbon present in a molecule it is a reducing sugar. Be able to
determine if a sugar is reducing or non-reducing. Reducing sugars react with basic Ag+solutions
to give a silver metal precipitate (siliver mirror).

Disaccharides

You do not have to know the structures for any disaccharides. What is important to know is how
monosaccharides are connectedto give disaccharides, i.e. they are connected with glycoside
linkages. Be able to determine if it is an a or ß linkage,and if it is a reducing or a nonreducing
sugar. There are enzymes which can break specifically an alpha or betaglycosidic linkage.

Disaccharides can be hydrolyzed back to monosaccharides with water and acid.

Polysaccharides

The three most important naturally occurring polysaccharides are:
Starch
Glycogen
Cellulose