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Question 21.2: Identifying D and L Isomers Identify the following monosacch......

Identifying D and L Isomers

Identify the following monosaccharides as (a) D-ribose or L-ribose, (b) D-mannose or L-mannose.

\begin{matrix}(a)  \underset{    \diagdown }{H}           \underset{\diagup\diagup                 }{O}\\ \underset{\mid}{C}\\ H— \underset{\mid}{C}— OH\\ H— \underset{\mid}{C}— OH \\ H— \underset{\mid}{C}— OH \\               CH_2OH\end{matrix}                \begin{matrix} (b)  \underset{    \diagdown }{H}          \underset{\diagup\diagup                  }{O}\\ \underset{\mid}{C}\\ H— \underset{\mid}{C}— OH\\ H— \underset{\mid}{C}— OH \\ HO — \underset{\mid}{C} — H       \\ HO — \underset{\mid}{C} — H       \\                   CH_2OH\end{matrix}

ANALYSIS To identify D or L isomers, you must check the location of the —OH group on the chiral carbon atom farthest from the carbonyl group. In a Fischer projection, this is the carbon atom above the bottom one. The —OH group points left in an L enantiomer and right in a D enantiomer.

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In (a) the —OH group on the chiral carbon above the bottom of the structure points to the right, so this is D-ribose. In (b) this —OH group points to the left, so this is L-mannose.

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