isopropylidene

Isopropylidene is a chemical term used to denote a divalent substituent derived from acetone, commonly represented as (CH3)2C–. The group is derived from the structural unit of propan-2-one and is used to connect two parts of a molecule, often by forming a bridging carbon atom that carries two methyl groups. In many contexts, the isopropylidene fragment is written as isopropylidene- or as C(CH3)2–.

In organic synthesis, isopropylidene is best known for its role in acetal or ketal protection chemistry. When two hydroxyl groups (a diol) react with acetone under acid catalysis, an isopropylidene acetal is formed, also called an acetonide; this protects adjacent hydroxyl groups temporarily. The same concept applies in carbohydrate chemistry to protect sugar rings, enhancing selectivity in subsequent steps.

Isopropylidene derivatives appear in various contexts, including ligands in organometallic complexes and building blocks in polymer chemistry. The term is often encountered in older literature as 'isopropylidene' attached to other groups (for example, isopropylidene derivatives of halides or carbonyl compounds), indicating the use of the acetone-derived fragment to link two points or to mask functionality.

Related concepts include acetonide, acetals and ketals, and protecting groups.