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What is the total number of asymmetric carbon atoms present in a 2-ketohexose molecule?

1. 2
2. 3
3. 4
4. 5

Correct answer: 3

Solution

A 2-ketohexose (e.g. fructose) is CH2OH-CO-CHOH-CHOH-CHOH-CH2OH. C1 and C6 are CH2OH, C2 is the carbonyl; only C3, C4 and C5 bear four different groups. Hence there are 3 asymmetric carbons, not 4.

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