Chemistry Reference
In-Depth Information
Of course, we can use the same strategy to protect
a diol. We would convert this into an acetal or ketal
using a suitable aldehyde or ketone.
OH
CH
3
O
H
+
CH
3
+
O
+
H
2
O
HO
CH
3
HO
OH
CH
3
O
acetone
glycerol
cyclic ketal
(acetonide)
Acetal and ketal linkages are widely found in nat-
ural sugars and polysaccharides. The structure of
sucrose is a splendid example.
Sucrose
is a dis-
accharide, composed of two linked monosaccharide
units, glucose in pyranose ring form and fructose
in
glucopyranose is a hemiacetal derived from the
aldehyde-containing sugar glucose. In sucrose, it is
present as its
α
anomer. On the other hand, fructose
is a ketone-containing sugar, and it forms a hemiketal
furanose ring by reaction of the C-5 hydroxyl with
the ketone group.
furanose
ring
form.
As
we
have
seen
above,
H
O
hemiacetal
O
C
R
R
H
OH
H
CH
2
OH
HO
H
O
HO
OH
H
H
OH
HO
acetal
CH
2
OH
O
pyranose ring
acetal
HO
R
OH
CH
2
OH
O
OH
R
HO
H
α
-
D
-glucopyranose
HO
D
-glucose
hemiacetal
HO
RO
aldose
O
HOH
2
C
HO
O
OR
CH
2
OH
O
H
RO
CH
2
OH
2
HOH
2
C
OH
R
HO
HO
R
O
furanose ring
ketal
HO
sucrose
ketal
H
H
OH
CH
2
OH
5
HO
OH
CH
2
OH
OH
β-
D
-fructofuranose
RO
D
-fructose
hemiketal
hemiketal
ketose
R
R
In sucrose, fructose is present as the
β
anomer.
Now, one of these sugars has acted as an alcohol
to make a bond to the other sugar. We can look at
this in two ways. Either fructose acts as an alcohol
to react with the hemiacetal glucose to form an
acetal, or alternatively, glucose is the alcohol that
reacts with the hemiketal fructose to form a ketal.
In sucrose, the pyranose ring is an acetal, whilst
the furanose ring is a ketal. This all seems rather
complicated at first - look carefully at the structures
whilst considering the text.
In aqueous solution, both glucose (hemiacetal) and
fructose (hemiketal) exist as equilibrium mixtures
of cyclic and open-chain carbonyl forms. Sucrose,
however, is a single stable substance (acetal and
ketal), and conversion back to glucose and fructose
requires more rigorous hydrolytic conditions, such as
heating with aqueous acid.
Box 7.3
Invert sugar
Invert sugar
is the name given to an equimolar mixture of glucose and fructose, obtained from sucrose by
hydrolysis with acid or alternatively using the enzyme
invertase
. During the process, the optical activity changes
from that of sucrose, [
α
]
D
+
66
.
5
◦
, to that resulting from an equal mixture of glucose and fructose.
