The structures shown are four tautomers/isomers of the diketone drawn. Which one is the most stable?

AStructure A
BStructure B
CStructure C
DStructure D

Answer: B

The diketone is pentane-2,4-dione. Its Z-enol (OH and C=O cis) is conjugated (C=C-C=O) and has an intramolecular hydrogen bond forming a six-membered chelate ring; the E-enol cannot chelate, the terminal enol is not conjugated, and the dienol loses a C=O bond.

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