ketal
Summary
A ketal is a carbon bonded to two OR groups and two carbon groups, R₂C(OR′)₂, made by reacting a ketone with two alcohols (or one diol) under acid catalysis. The most testable point: ketals resist base, nucleophiles and hydrides, so they protect ketones, and aqueous acid removes them.
Detail
Meaning: A ketal has a central carbon bonded to two alkoxy groups (OR′) and two carbon groups. It forms from a ketone and two alcohols, or one diol such as ethylene glycol (giving a cyclic ketal), with an acid catalyst: R₂C=O + 2 R′OH ⇌ R₂C(OR′)₂ + H₂O. The first intermediate is a hemiketal (one OR, one OH). Acid protonates its OH, water leaves, and a second alcohol adds. Every step is reversible. Removing water favors the ketal, and adding aqueous acid hydrolyzes it back to the ketone.
On the MCAT: Ketals appear as protecting groups. A typical passage has a molecule with both a ketone and an ester. To reduce only the ester with LiAlH4, you first convert the ketone to a ketal, which does not react with hydrides, Grignard reagents or other bases and nucleophiles. After the reduction, H₃O⁺ restores the ketone. Ketals also appear in carbohydrates: a cyclic ketose (such as fructose) is a cyclic hemiketal, and linking it to another alcohol gives a ketal-type glycosidic bond.
Common confusion: Students mix up ketal and hemiketal. A hemiketal has one OR and one OH, and the ketal has two OR groups. Ketals are stable in base but hydrolyze in acid. A ketal comes from a ketone, and an acetal from an aldehyde, so its carbon carries at least one H. Some texts now call ketals a subclass of acetals.
Sources
- Organic Chemistry by Paula Yurkanis Bruice
- Organic Chemistry by John McMurry
- Organic Chemistry by Francis Carey
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.