The aldehyde shown is treated with excess methanol in the presence of dry hydrogen chloride. Which structure is the stable final product?

AStructure A
BStructure B
CStructure C
DStructure D

Answer: A

Ethanal first forms a hemiacetal, which is unstable and is converted in dry acid by a second molecule of methanol into the acetal 1,1-dimethoxyethane. The ester and ether are not formed under these conditions.

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