Cyclopropene

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Cyclopropene is the simplest cycloalkene, with the formula C3H4. It has a triangular ring of three carbon atoms with one double bond. The ring is highly strained, making cyclopropene hard to make and very reactive. It is a colorless gas at room temperature and does not occur naturally, though some derivatives appear in fatty acids. Some cyclopropene derivatives are used to help control the ripening of certain fruits.

Structure and properties:
- The short, strained ring makes the bond angles unusual; the angle opposite the double bond drops to about 51°, instead of the 60° seen in a normal triangle.
- The ring carbons use sp2-like hybridization due to the double bond.
- Molar mass: about 40 g/mol; boiling point around −36 °C.

Synthesis (overview):
- Early synthesis came from heating certain cyclopropylamine derivatives on catalysts, yielding small amounts of cyclopropene.
- Improvements use different catalytic pyrolysis methods, or dehydrohalogenation of allyl compounds, to boost yield.
- Modern routes also include copper- or other metal-catalyzed reactions of carbenes with alkynes to form cyclopropenes, including various substituted versions.

Reactivity and reactions:
- Because of its high strain, cyclopropene can polymerize if not carefully handled.
- It can isomerize to methylacetylene (propyne) at higher temperatures.
- It participates in cycloadditions, such as reacting with cyclopentadiene in a Diels–Alder type reaction to give bicyclic products. This reaction is sometimes used to confirm the presence of cyclopropene in a sample.