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684 Chapter 20
Copyright © 2017 Pearson Education, Inc.
22.
D
-erythrose
H OH
H OH
CH
2
OH
C
H O
O
OH
H
OH OH
-
D
-erythrofuranose
O
H
OH
OH OH
-
D
-erythrofuranose
23.
First recall that in the chair conformation, an
a
-anomer has the anomeric OH group in the axial position
and a
b
-anomer has the anomeric OH group in the equatorial position. Then recall that glucose has all of
its OH groups in equatorial positions. Now this question can be answered easily.
All the OH groups in
b
-d-glucose are in equatorial positions. Because
b
-d-mannose is a C-2 epimer of
b
-d-glucose, the C-2 OH group of
b
-d-mannose is the only one in the axial position.
24.
a.
d-Idose differs in configuration from d-glucose at C-2, C-3, and C-4. Therefore, the OH groups at C-2,
C-3, and C-4 in
b
-d-idose are in axial positions.
b.
d-Allose is a C-3 epimer of d-glucose. Therefore, the OH group at C-3 is in the axial position and
because it is the
a
-anomer, the OH group at C-1 (the anomeric carbon) is also in the axial position.
25.
O
HO
OH
H
CH
2
OH
HO
OH
O
HO
OH
H
CH
2
OH
HO
OCH
2
CH
3
O
HO
OH
OCH
2
CH
3
CH
2
OH
HO
H
CH
3
CH
2
OH
HCl
+
-
D
-galactose
-
D
-galactoside
ethyl
-
D
-galactoside
ethyl
26.
If more than a trace amount of acid is used, the amine that acts as a nucleophile when it forms the
N
-glycoside becomes protonated, and a protonated amine is not a nucleophile.
27.
Solved in the text.
28.
a.
a
-d-talose (or
a
-d-talopyranose) (reducing; it is a hemiacetal)
b.
methyl
a
-d-galactoside (or methyl
a
-d-galactopyranoside) (nonreducing; it is an acetal)
c.
ethyl
b
-d-psicoside (or ethyl
b
-d-psicofuranoside) (nonreducing; it is an acetal)
29.
specific rotation of glucose
+
specific rotation of fructose
=
-
22.0
+
52.7
+
specific rotation of fructose
=
-
22.0
specific rotation of fructose
=
-
22.0
+
1
-
52.7
2
specific rotation of fructose
=
-
74.7
30.
a.
Both sugars in the disaccharide are glucose.
b.
It is a 1,6 -glycosidic linkage.