Ketal

Ketal is the term for a class of acetal derivatives formed from ketones. In a typical ketal, the carbonyl carbon of a ketone is converted into a sp3 center bearing two alkoxy groups, giving the general structure RR'C(OR'')2, where the R groups are the substituents that were on the ketone and OR'' are two oxygen substituents from the diol. Ketals are the acetal products of ketones, in contrast to acetals derived from aldehydes.

Formation and structure: Ketals are prepared by reacting a ketone with one or more diols (or glycol) in the presence of an acid catalyst. The reaction is typically performed under dehydrating conditions to drive the equilibrium toward ketal formation, and cyclic ketals are common when a vicinal diol such as ethylene glycol or 1,3-propanediol is used. The resulting product often features a five- or six-membered ring in the case of cyclic ketals, such as dioxolane derivatives.

Hydrolysis and applications: Ketals are generally stable under neutral and basic conditions but can be hydrolyzed back to the original ketone and diol under acidic aqueous conditions, restoring the carbonyl functionality. Because they are reversible and can be selectively removed, ketals are widely used as protecting groups for ketones in multi-step synthesis. Cyclic ketals, known as acetonides when formed from acetone, are especially common for protecting vicinal diols in carbohydrate chemistry and other substrates.

Examples and notes: A classic example is the ketal formed from acetone and ethylene glycol, yielding a 1,3-dioxolane ring. Ketals formed from aldehydes are called acetals, highlighting the distinction between ketones and aldehydes as the carbonyl sources.