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The term epimer is used to describe diastereomers that differ in the configuration about a single carbon atom. Which pairs of the eight possible aldopentoses are epimers?

Answer 1

Jacob Anderson

The eight aldopentoses are shown below.

Note: The configurations at C-4 of L-xylose and L-lyxose should be HO-C-H.

Each aldopentoses has three stereocentres, so each will have three epimers.

For each aldopentose, the epimers at C-2, C-3, and C-4 are:

D-ribose: D-arabinose, D-xylose, and L-lyxose
D-arabinose: D-ribose, D-lyxose, and L-xylose
D-xylose: D-lyxose, D-ribose, and L-arabinose
D-lyxose: D-xylose, D-arabinose, and L-ribose
L-ribose: L-arabinose, L-xylose, and D-lyxose
L-arabinose: L-ribose, L-lyxose, and D-xylose
L-xylose: L-lyxose, L-ribose, and D-arabinose
L-lyxose: L-xylose, L-arabinose, and D-ribose

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Answer 2

Isaac Aronson

Ribose and arabinose are epimers among the eight possible aldopentoses.

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Answer from HIX Tutor

When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.