What is a ketal and how is it formed?

A ketal is a compound containing a central carbon atom bonded to two ether groups.

Ketals are formed by the reaction of a ketone or aldehyde with an alcohol in the presence of an acid catalyst. The acid catalyst protonates the carbonyl oxygen, making it more electrophilic and susceptible to attack by the alcohol. The alcohol then donates a pair of electrons to the carbonyl carbon, forming a tetrahedral intermediate. The intermediate then loses a water molecule to form the ketal.

Ketals are commonly used in organic synthesis as protecting groups for carbonyl compounds. By converting a carbonyl group into a ketal, the carbonyl group is shielded from unwanted reactions, allowing for selective functionalization of other parts of the molecule. Ketals can also be used as solvents, as they are often more stable and less reactive than the corresponding aldehydes or ketones.

In biological systems, ketals are involved in a variety of processes, including the biosynthesis of steroids and other natural products. For example, the biosynthesis of cholesterol involves the formation of a ketal intermediate. Ketals are also found in some drugs, such as the antihistamine ketotifen, which contains a ketal group in its structure.