The Haworth structures of pentose and hexose sugars are five- or six-membered rings. These cyclic structures are produced through an intramolecular reaction between a carbonyl group and hydroxyl group on the same monosaccharide molecule. Common examples are the α- and β-anomers of D-glucose. Monosaccharides can be oxidized to form sugar acids by oxidizing the aldehyde group. They can also be reduced to form sugar alcohols by reducing the carbonyl group. Fructose can rearrange and oxidize to form glucose. Sugar testing uses Benedict's reagent to detect glucose in the urine, indicating diabetes.

1. • The most stable forms
of pentose and hexose
sugars are five- or six-
atom rings.
• These rings, known as
Haworth structures, are
produced from the
reaction of a carbonyl
group and a hydroxyl
group in the same
molecule.
Haworth Structures of Monosaccharides
Learning Goal Draw and identify the Haworth structures of
monosaccharides.

2. Guide for Drawing Hayworth
Structures

3. Cyclic Structure for Glucose
STEP 1 Turn the Fischer projection clockwise by
90°.

4. STEP 2 Fold clockwise to make a hexagon and bond the O on
carbon 5 to carbon 1.
• Place the carbon 6 group in the — CH2OH group above carbon 5.
• Draw a bond between the oxygen of the — OH group on carbon 5 to
the carbonyl carbon.

5. Cyclic Structure for Glucose
STEP 3 Draw the new — OH group on carbon 1
below the ring to give the α isomer or above the
ring to give the β isomer.

6. Mutarotation - and β-D-Glucose
When placed in solution,
• cyclic structures open and close
• -D-glucose converts to β-D-glucose and vice versa
• at any time, only a small amount of open chain
forms

7. Haworth Structures of Fructose
Fructose
• is a ketohexose
• forms a five-atom ring structure with carbon 2 at the
right corner of the ring
• forms when the — OH group on carbon 5 reacts with
carbon 2 in the carbonyl group

8. Study Check
Write the cyclic forms of D-galactose.

9. Solution
STEP 1 Turn the Fischer projection clockwise by 90°.
1
2
3
4
5
6
6 5 4 3 2 1
C
OH
H
H
OH
H
OH
OH
H
HOH2C
O
H

10. STEP 2 Fold clockwise to make a hexagon and bond
the O on carbon 5 to the carbonyl group.
• Place the carbon 6 group above the ring.
• Write the — OH group on carbon 2
below the ring and the — OH groups
on carbon 3 and carbon 4 above
the ring.
C
HO
H
H
OH
H
OH OH
H
CH2OH
O
H

11. Solution
-D-Galactose -D-Galactose


STEP 3 Draw the new — OH group on carbon 1 below the
ring to give the α isomer or above the ring to give the 
isomer.

12. 13.5 Chemical Properties of
Monosaccharides
Sugar alcohols such as D-
sorbitol, D-xylitol from D-xylose,
and
D-mannitol from D-mannose are
used as sweeteners in many
sugar-free products such as diet
drinks and sugarless gum as well
as products for people with
diabetes.
Learning Goal Identify the products of oxidation or reduction
of monosaccharides; determine whether a carbohydrate is a
reducing sugar.

13. Oxidation of Monosaccharides
Monosaccharides in solution
• have small amounts of the open-chain form present
• have an aldehyde group with an adjacent hydroxyl
group that can be oxidized to carboxylic acid by an
oxidizing agent such as Benedict’s

14. Oxidation of Monosaccharides
Sugar acids
• are produced from the oxidation of the aldehyde
form as Cu2+ is reduced to Cu+
• are named by replacing the ose ending of the
monosaccharide with onic acid
A carbohydrate that reduces another substance (such
as the open chain form of D-glucose) is called a
reducing sugar.

15. Oxidation: Fructose to Glucose
Fructose, a ketohexose,
• contains a ketone group, which usually can’t be
oxidized
• can be oxidized in a basic Benedict’s solution when a
rearrangement occurs between the ketone group on
carbon 2 and the hydroxyl group on carbon 1
• is then converted to glucose, which produces an
aldehyde group with an adjacent hydroxyl that can be
oxidized

16. Oxidation: Fructose to Glucose
Fructose, a ketohexose, rearranges to form glucose and
is then oxidized in Benedict’s.

17. Reduction of Monosaccharides
The reduction of the carbonyl group in
monosaccharides
• produces sugar alcohols, which are also called alditols
• converts D-glucose to the sugar alcohol D-sorbitol

18. Reducing Sugars
The sugar alcohols
• are named by replacing the
ose ending of the
monosaccharide with itol
• include D-sorbitol, D-xylitol
from D-xylose, and D-
mannitol from D-mannose
• are used as sweeteners in
many sugar-free products
such as diet drinks and
sugarless gum

19. Study Check
Write the product of the reduction of D-mannose.
D-Mannose

20. Solution
Write the product of the reduction of D-mannose.
D-Mannose D-Mannitol
Reduction

21. Chemistry Link to Health:
Glucose Testing
• Normally, blood glucose
flows through the kidneys
and is reabsorbed into the
bloodstream.
• When the blood level
exceeds about 160 mg of
glucose/dL of blood, the
kidneys cannot reabsorb all
of the glucose, and it spills
over into the urine, a
condition known as
glucosuria.

22. Chemistry Link to Health:
Glucose Testing
• A symptom of diabetes mellitus is a high level of
glucose in the urine, which can be identified using
Benedict’s test.