The aldehyde shown is treated with dilute NaOH at low temperature. Which structure is the aldol addition product (no dehydration)?

AStructure A
BStructure B
CStructure C
DStructure D

Answer: B

The drawn compound is ethanal. The enolate of one molecule adds to the C=O of another to give 3-hydroxybutanal (aldol). Heating would dehydrate it to but-2-enal (Structure A); the other options have the wrong skeleton or the wrong alpha carbon.

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