C5H6

C5H6 is the molecular formula that can describe several five‑carbon hydrocarbon isomers. The most well known member is cyclopentadiene, a conjugated diene that plays a central role in organic and organometallic chemistry. Because two cyclopentadiene units readily form the dimer dicyclopentadiene, commercial samples are typically stabilized with inhibitors to prevent premature polymerization.

Cyclopentadiene structure and properties: In cyclopentadiene, a five‑membered ring contains two conjugated double bonds, giving a 1,3‑diene system within a planar ring. The formula arises from three ring carbons bearing one hydrogen each and two methylene carbons contributing two hydrogens apiece, totaling C5H6. It is a colorless to pale yellow liquid at room temperature with a boiling point around 42 °C. The compound is highly reactive and tends to dimerize via a cycloaddition process to form dicyclopentadiene, a reaction that occurs readily at ambient conditions unless inhibitory additives are present.

Chemistry and applications: Cyclopentadiene is a key precursor to the cyclopentadienyl ligand (Cp−), obtained by deprotonation and used to form a wide range of organometallic complexes, including metallocenes such as iron and nickel derivatives. In synthetic organic chemistry, it serves as a dienophile or diene in Diels–Alder reactions, enabling rapid construction of cyclic frameworks. The Cp ligand class underpins many catalytic and materials applications.

Production and handling: In industry, cyclopentadiene is commonly generated from the dimeric DCPD through controlled cracking or distillation, and is stabilized with inhibitors to prevent polymerization during storage. It is flammable and irritant, reacts readily with air and moisture, and is handled under controlled conditions to minimize runaway polymerization and exposure risks.