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. Page 1 of 3 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 Page 2 of 3 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) Page 3 of 3

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