L3 - Chemical Properties of Monosaccharide

Reactions Con’t.
Formation of a
Ketal The addition of
alcohols to ketones
produces an unstable
hemiketal intermediate
that in-turn, produces an
ketal.
The alcohol here is a
reactant not a catalyst
• The hemiketal is formed
by the addition of an alcohol
(R-O-H) by breaking the C = O
carbonyl double bond and
attaching the ether group
(-O-R) to the carbonyl carbon
and the hydrogen to the
carbonyl oxygen.
• The unstable hemiketal is
then converted into a ketal by
the substitution of the newly
formed hydroxyl (-O-H) group
by a new ether from a second
alcohol (R-O-H), also
producing water (H 2 O).
Even though hemiacetals and
hemiketals are relatively unstable,
cyclical hemiacetals and hemiketals are
commonly found because they are more
stable than open chains.
Cyclic Structures of Monosaccharides
All monosaccharides with at least five carbon
atoms exist predominantly as cyclic
hemiacetals and hemiketals.
A Haworth structure can be used to depict
the α and β anomers of a monosaccharide.
Anomers are stereoisomers that differ in the
3-D arrangement of groups at the anomeric
carbon of an acetal, ketal, hemiacetal, or
hemiketal group.
• Sixmembered
pyranose
ring system
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