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690 Chapter 20
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48.
There are two possible structures for an optically active five-carbon alditol formed from a Wohl degrada-
tion of a d-hexose:
CH
2
OH
HO H
H OH
H OH
CH
2
OH
CH
2
OH
HO H
*HO H
H OH
CH
2
OH
CH
2
OH
HO H
O
H OH
CH
2
OH
CH
2
OH
HO H
*HO H
H OH
CH
2
OH
All OH groups are in equatorial positions.
Only the starred OH group is in an axial position.
The unknown aldohexose has only one OH group in an axial position. Therefore, the aldohexose that gives
the alditol on the left needs to have the OH on C-2 pointing to the left (so it will be axial). The other aldo-
hexose has an axial hydrogen, so it needs to have the OH on C-2 pointing to the right (so it will not be axial).
The two possible aldoses are d-mannose and d-galactose:
D
-mannose
C
H O
C
H O
HO H
H OH
HO H
H OH
CH
2
OH
D
-galactose
HO H
*HO H
H OH
H OH
CH
2
OH
If the two aldoses are reduced to alditols, the alditol of d-mannose will be optically active and the alditol of
d-galactose will be optically inactive.
alditol of -mannose
CH
2
OH
HO H
H OH
HO H
H OH
CH
2
OH
alditol of -galactose
CH
2
OH
HO H
*HO H
H OH
H OH
CH
2
OH
49.
The hydrogen that is bonded to the anomeric carbon is the hydrogen that has its signal at the highest fre-
quency, because it is the only hydrogen that is bonded to a carbon that is bonded to two oxygens. So the
two anomeric hydrogen (one from the
a
-anomer and one from the
b
-anomer) are responsible for the two
high-frequency doublets.