The diol shown is refluxed with cyclohexanone and a trace of toluene-4-sulphonic acid, with removal of water. Which structure is the product?

AStructure A
BStructure B
CStructure C
DStructure D

Answer: D

Ethane-1,2-diol reacts with cyclohexanone under acid catalysis with water removal to give a cyclic ketal, the spiro compound 1,4-dioxaspiro[4.5]decane. The five-membered dioxolane ring is favoured, the six-membered dioxane would need propane-1,3-diol, the hemiketal is only an intermediate and the open ether lacks the second ketal oxygen.

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