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| Names | |||
|---|---|---|---|
| Preferred IUPAC name Cyclopropene[1] | |||
| Identifiers | |||
3D model (JSmol) | |||
| ChemSpider |
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| MeSH | cyclopropene | ||
| UNII | |||
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| Properties | |||
| C3H4 | |||
| Molar mass | 40.065 g·mol−1 | ||
| Boiling point | −36 °C (−33 °F; 237 K) | ||
| Thermochemistry | |||
| 51.9–53.9 J K−1 mol−1 | |||
Std enthalpy of combustion(ΔcH⦵298) | −2032 – −2026 kJ mol−1 | ||
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |||
Cyclopropene is anorganic compound with theformulaC3H4. It is the simplestcycloalkene. Because thering is highly strained, cyclopropene is difficult to prepare and highly reactive. This colorless gas has been the subject for many fundamental studies of bonding and reactivity.[2] It does not occur naturally, but derivatives are known in somefatty acids. Derivatives of cyclopropene are used commercially to control ripening of some fruit.
The molecule has atriangular structure. The reduced length of thedouble bond compared to asingle bond causes the angle opposite the double bond to narrow to about 51° from the 60° angle found incyclopropane.[3] As with cyclopropane, the carbon–carbon bonding in the ring has increasedp character: the alkene carbon atoms use sp2.68hybridization for the ring.[4]
The first confirmed synthesis of cyclopropene, carried out by Dem'yanov and Doyarenko, involved thethermal decomposition of trimethylcyclopropylammonium hydroxide over platinized clay at approximately 300 °C.[5] This reaction produces mainlytrimethylamine and dimethylcyclopropyl amine, together with about 5% of cyclopropene. Later Schlatter improved the pyrolytic reaction conditions using platinizedasbestos as acatalyst at 320–330 °C and obtained cyclopropene in 45%yield.[6]
Cyclopropene can also be obtained in about 1% yield by thermolysis of the adduct ofcycloheptatriene anddimethyl acetylenedicarboxylate.[7]
Allyl chloride undergoesdehydrohalogenation upon treatment with the basesodium amide at 80 °C to produce cyclopropene in about 10% yield.[8]
The major byproduct of the reaction isallylamine. Adding allyl chloride tosodium bis(trimethylsilyl)amide in boilingtoluene over a period of 45–60 minutes produces the targeted compound in about 40% yield with an improvement in purity:[9]
1-Methylcyclopropene, used to slow the ripening in fruits,[10][11] is synthesized similarly but at room temperature from methallylchloride usingphenyllithium as the base:[12]
Treatment of nitrocyclopropanes withsodium methoxide eliminates the nitrite, giving the respective cyclopropene derivative.
The synthesis of purely aliphatic cyclopropenes was first illustrated by the copper-catalyzed additions of carbenes to alkynes. In the presence of a copper catalyst,ethyl diazoacetate reacts with acetylenes to give cyclopropenes. 1,2-Dimethylcyclopropene-3-carboxylate arises via this method from2-butyne. Copper, as copper sulfate and copper dust, are among the more popular forms of copper used to promote such reactions.Rhodium acetate has also been used.
Cyclopropene derivatives are generally too unstable for storage, unless they are disubstituted at the tetrahedral carbon atom. In some cases, monosubstitution with a bulky substituent provides adequate stabilization.[13]
Due to ring strain, cyclopropenes are highly activedienophiles.[13]
Studies on cyclopropene mainly focus on the consequences of its high ring strain. At 425 °C, cyclopropene isomerizes tomethylacetylene (propyne).
Attempted fractional distillation of cyclopropene at –36 °C (its predicted boiling point) results in polymerization. The mechanism is assumed to be a free-radical chain reaction, and the product, based on NMR spectra, is thought to be polycyclopropane.
Cyclopropene undergoes theDiels–Alder reaction withcyclopentadiene to give endo-tricyclo[3.2.1.02,4]oct-6-ene. This reaction is commonly used to check for the presence of cyclopropene, following its synthesis.[9]
Media related toCyclopropene at Wikimedia Commons
