Ketal

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Structure of a generic ketal

A ketal is a functional group or molecule containing a carbon bonded to two -OR groups, where O is oxygen and R represents any alkyl group. 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 of alcohol, whereas the name ketal derives from the product of the reaction of a ketone with excess alcohol. Although this term had been abandoned, it has been reinstated by IUPAC as a subclass of acetals. Ketals have the general form shown on the right, where R, R', R'', and R''' are carbon backbones. If R = H, the structure is a traditional acetal.

Ketals and acetals can be used as carbonyl-protecting groups.

Reactions

Synthesis

Although there are plenty ways to synthesize a ketal, they are generally made using an alcohol, an acid catalysis and any way to remove water from the reaction (e.g. a Dean-Stark apparatus), driving the equilibrium of the reaction to the completion.

Acetals formed from ketones were originally called ketals. The name was dropped recently from IUPAC system of naming compounds, so the products of addition of two equivalents of alcohol to both aldehydes and ketones are called acetals.

Ketals and acetals can, under acidic conditions, be converted back into a hemiacetal and an alcohol, as the entire reaction is an equilibrium.

See also

• Functional group
• Fructone, an ethylene glycol ketal and aroma compound