stereoisomers carbohydrates

[echoyjeff222]

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I didn't learn carbs in school, so I'm trying to figure out why the answer is B and not C. I thought it was just 2^4, but we don't count eight of the 16 because L-forms aren't stereoisomers of D forms? ??

 

[TheMightyTexan]

@echoyjeff222 Tricky question.. D-aldohexose signifies that the bottom hydroxyl group is on the right. If there are 16 forms of aldohexose with its 4 chiral carbons, since D-aldohexose has to have the bottom chiral carbon oriented right, that would exclude the possibility of this being rearranged leaving only 3 chiral carbon variations 2^3.

Essentially, the question is asking about D-aldohexose sterioisomers and not just aldohexose isomers. If the bottom chiral carbon was changed it would be longer a form of D-alsohexose.