CHEM 109C – Zhang
CLAS, F10
Carbohydrates I: Classification and Stereochemistry - KEY
Memorize structures for: fructose, ribose, glucose, galactose, mannose
1. Given the following Fischer projection for galactose, answer the following
questions
a. Describe/classify this sugar
It has an aldehyde functional group at C-1 and a total of 6 Cs,
so it is an aldohexose.
b. How many stereocenters does it contain?
There are 4 asymmetric Cs (Cs connected to 4 different
groups), so there are 4 stereocenters.
c. What is the total number of stereoisomers? Can you draw some of them?
With n stereocenters, there are 2n stereoisomers unless the
compound is a mesocompound (contains an internal plane of
symmetry) in which case there are 2n -1 stereoisomers.
There should be 24 or 16 stereoisomers.
etc.
d. Are the chiral C’s R or S? Assign R or S to each of them.
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CHEM 109C – Zhang
CLAS, F10
Carbohydrates I: Classification and Stereochemistry - KEY
e. Is this the D or L form?
The OH group on C-5 is on the right hand side, so this is the D
form.
2. Given the carbohydrates below, answer the following questions
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CHEM 109C – Zhang
CLAS, F10
Carbohydrates I: Classification and Stereochemistry - KEY
A.
B.
C.
D –glucose D-fructose
D-talose
a. Which is a ketose sugar?
b. Which is L-galactose?
D.
E.
L-galactose L-altrose
B
D (enantiomer of D-galactose)
The C-5 epimer of D-galactose is L-altrose (E)
c. Which is a C-4 epimer of D-galactose?
d. Is D a C-2 epimer of D-galactose?
A
T or F F, it is an enantiomer
The C-2 epimer of D-galactose is D-talose (C)
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