Cyclopentadiene

Part of speech: noun

Definitions

1. An organic compound consisting of a five-membered carbon ring with two non-adjacent double bonds and four hydrogen atoms
2. A hydrocarbon with a cyclic structure of five carbons bearing two conjugated double bonds and saturating hydrogens
3. A five-carbon ring molecule featuring two separated double bonds and commonly used as a chemical precursor in organic synthesis

Etymology: Cyclopentadiene is a chemist's coinage rather than a word with a folk history, and its name is built almost entirely from parts that were fixed in the 19th century. "Cyclo-" comes from the Greek "kyklos," meaning circle or ring, a prefix chemists adopted for molecules that close into rings. "Penta-" is the Greek numeral for five, referring to the five carbon atoms in the ring. "Diene" is the standard chemical suffix for a molecule containing two carbon-carbon double bonds, formed from "di-" (two) and the "-ene" ending used for unsaturated hydrocarbons such as ethene. The compound itself was first isolated from coal tar, and chemists later recognized its unusual tendency to react with itself. At room temperature, two molecules join in a Diels-Alder reaction to form dicyclopentadiene, a dimer that cracks back into the monomer when heated. Because of this behaviour, cyclopentadiene is usually stored cold and used freshly prepared from its dimer.