The enantiopure aldehyde shown is reduced with sodium borohydride, followed by work-up. Which structure is the product? (Wedge/hash bonds indicate absolute configuration.)

AStructure A
BStructure B
CStructure C
DStructure D

Answer: D

2-Methylbutanal has its only stereocentre at the alpha carbon, which is not touched by hydride attack at the carbonyl carbon. No bond to that carbon is broken, so the product 2-methylbutan-1-ol keeps the same spatial arrangement and remains a single enantiomer; the CH3 and ethyl groups are in the same relative positions.

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