An ketal is a functional group or molecule containing the functional group of a carbon bonded to two -OR groups. It is essentially equivalent to an acetal,and often the term acetal is used instead. The difference derives from the reaction which created the molecule. Acetals traditionally derive from the product of the reaction of an aldehyde with excess alcohol, whereas the name ketal derives from the product of the reaction of a ketone with excess alcohol.

Ketals/Acetals can be used as carbonyl-protecting groups.

Reactions

Synthesis

1. Nucleophilic addition of an alcohol to a resonance stabilized cation
   • ketone + acid + alcohol ↔ hemiacetal + alcohol + acid (catalyst) ↔ ketal + water

Ketals/Acetals can, under certain conditions, be converted back into a hemiacetal and an alcohol.