6.3.4 Mutarotation
The term mutarotation means the variation of optical rotation with time,
observed in a solution of sugar on standing. Let us have a look at this
phenomenon in a glucose solution. The pure a anomer of glucose has an
m.p. of 146
C and a specific rotation [a] D þ 112.2
, and the specific
rotation on standing is þ 52.6
, while pure b anomer has an m.p. of
148–155
C and a specific rotation [a] D þ 18.7
, and the specific rotation
on standing is þ 52.6
. When a sample of either pure anomer is dissolved in
water, its optical rotation slowly changes and ultimately reaches a constant
value of þ 52.6
. Both anomers, in solution, reach an equilibrium with fixed
amounts of a (35 per cent), b (64 per cent) and open chain ($1 per cent)
forms.
6.3.5 Acetal and ketal formation in sugars
We have already learnt that hemiacetal or hemiketal exists in the cyclic
structure of a sugar. For example, the anomeric carbon (C-1) in glucose
is a hemiacetal, and that in fructose is a hemiketal. When the hydroxyl
group in a hemiacetal or hemiketal is replaced by a –OR group, acetal
or ketal, respectively, is formed. R can be an alkyl group or another
sugar. The following example shows the acetal formation in glucopyranose.
O OR
O
H
O
H O
H
OH
O
O
H
O
H O
H
OH
OR
β Anomer (β configuration)
α Anomer (α configuration)
1
1
Anomeric carbon
Acetal formation, −OR replaces −OH
Acetal formation in glucopyranose
Acetals and ketals are also called glycosides. Acetals and ketals (glycosides)
are not in equilibrium with any open chain form. Only hemi-acetals and
hemiketals can exist in equilibrium with an open chain form. Acetals and
ketals do not undergo mutarotation or show any of the reactions specific to
the aldehyde or ketone groups. For example, they cannot be oxidized easily
to form sugar acids. As an acetal, the carbonyl group is effectively
protected.
When glucose is treated with methanol containing hydrogen chloride, and
prolonged heat is applied, acetals are formed. In this reaction the hemiacetal function is converted to the monomethyl acetal.
6.3 CARBOHYDRATES
307
The term mutarotation means the variation of optical rotation with time,
observed in a solution of sugar on standing. Let us have a look at this
phenomenon in a glucose solution. The pure a anomer of glucose has an
m.p. of 146
C and a specific rotation [a] D þ 112.2
, and the specific
rotation on standing is þ 52.6
, while pure b anomer has an m.p. of
148–155
C and a specific rotation [a] D þ 18.7
, and the specific rotation
on standing is þ 52.6
. When a sample of either pure anomer is dissolved in
water, its optical rotation slowly changes and ultimately reaches a constant
value of þ 52.6
. Both anomers, in solution, reach an equilibrium with fixed
amounts of a (35 per cent), b (64 per cent) and open chain ($1 per cent)
forms.
6.3.5 Acetal and ketal formation in sugars
We have already learnt that hemiacetal or hemiketal exists in the cyclic
structure of a sugar. For example, the anomeric carbon (C-1) in glucose
is a hemiacetal, and that in fructose is a hemiketal. When the hydroxyl
group in a hemiacetal or hemiketal is replaced by a –OR group, acetal
or ketal, respectively, is formed. R can be an alkyl group or another
sugar. The following example shows the acetal formation in glucopyranose.
O OR
O
H
O
H O
H
OH
O
O
H
O
H O
H
OH
OR
β Anomer (β configuration)
α Anomer (α configuration)
1
1
Anomeric carbon
Acetal formation, −OR replaces −OH
Acetal formation in glucopyranose
Acetals and ketals are also called glycosides. Acetals and ketals (glycosides)
are not in equilibrium with any open chain form. Only hemi-acetals and
hemiketals can exist in equilibrium with an open chain form. Acetals and
ketals do not undergo mutarotation or show any of the reactions specific to
the aldehyde or ketone groups. For example, they cannot be oxidized easily
to form sugar acids. As an acetal, the carbonyl group is effectively
protected.
When glucose is treated with methanol containing hydrogen chloride, and
prolonged heat is applied, acetals are formed. In this reaction the hemiacetal function is converted to the monomethyl acetal.
6.3 CARBOHYDRATES
307