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12
1.1.3. Ring
formation
(alcohol + aldehyde = hemiacetal):
α
-‐
and
β
-‐forms and
the Haworth
formula
The reactivity of the aldehyde in aldoses (or in the case of ketoses
1
– the keto
group) favours ring formation.
Most hexoses (6-carbon sugars) such as glucose form predominantly 6-
membered pyranose rings. A new asymmetric carbon is formed, giving rise to
α
- and
β
-forms. The stereochemistry of sugar rings is normally represented by
the famous Haworth formula.
1
In this course we do not meet ketoses, but they are important carbohydrates. In ketoses the
carbonyl function is not terminal, making them ketones. One example is fructose, where
carbon number 2 is the carbonyl (>C=O)
C
C OH
H
C H
HO
C OH
H
C
OH
H
CH
2
OH
O H
C
C OH
H
C H
HO
C OH
H
CH
CH
2
OH
OH
H
O
C
C OH
H
C H
HO
C OH
H
CH
CH
2
OH
H
HO
O
α
-D-glucose
β
-D-glucose
O
OH
OH
CH
2
OH
HO
OH
O
OH
CH
2
OH
HO
OH
OH
Abbreviated Haworth
(-H not shown)
Fisher-Haworth interconversion rule
for
D
-sugars:
Right
in Fisher
=
Down
in Haworth