# Cyclopropene

Cyclopropene is an organic compound with the formula C$_3$H$_4$. It is the simplest cycloalkene. Because the ring 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. It does not occur naturally, but derivatives are known in some fatty acids. Derivatives of cyclopropene are used commercially to control ripening of some fruit.

## Structure and bonding

The molecule has a triangular structure. The reduced length of the double bond compared to a single bond causes the angle opposite the double bond to narrow to about 51° from the 60° angle found in cyclopropane. As with cyclopropane, the carbon–carbon bonding in the ring has increased p character: the alkene carbon atoms use sp$^{2.68}$ hybridization for the ring.

## Synthesis of cyclopropene and derivatives
### Early syntheses

The first confirmed synthesis of cyclopropene, carried out by Dem'yanov and Doyarenko, involved the thermal decomposition of trimethylcyclopropylammonium hydroxide over platinized clay at approximately 300 °C. This reaction produces mainly trimethylamine and dimethylcyclopropyl amine, together with about 5% of cyclopropene. Later Schlatter improved the pyrolytic reaction conditions using platinized asbestos as a catalyst at 320–330 °C and obtained cyclopropene in 45% yield.
Cyclopropene can also be obtained in about 1% yield by thermolysis of the adduct of cycloheptatriene and dimethyl acetylenedicarboxylate.

### Modern syntheses from allyl chlorides

Allyl chloride undergoes dehydrohalogenation upon treatment with the base sodium amide at 80 °C to produce cyclopropene in about 10% yield.
    CH$_2$=CHCH$_2$Cl + NaNH$_2$ → C$_3$H$_4$ + NaCl + NH$_3$
The major byproduct of the reaction is allylamine. Adding allyl chloride to sodium bis(trimethylsilyl)amide in boiling toluene over a period of 45–60 minutes produces the targeted compound in about 40% yield with an improvement in purity:
    CH$_2$=CHCH$_2$Cl + NaN(TMS)$_2$ → C$_3$H$_4$ + NaCl + NH(TMS)$_2$
1-Methylcyclopropene is synthesized similarly but at room temperature from methallylchloride using phenyllithium as the base:
    CH$_2$=C(CH$_3$)CH$_2$Cl + LiC$_6$H$_5$ → CH$_3$−C$_3$H$_3$ + LiCl + C$_6$H$_6$

### Syntheses of derivatives

Treatment of nitrocyclopropanes with sodium 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 from 2-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.

## Reactions of cyclopropene

Studies on cyclopropene mainly focus on the consequences of its high ring strain. At 425 °C, cyclopropene isomerizes to methylacetylene (propyne).
    C$_3$H$_4$ → H$_3$CC≡CH
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 the Diels–Alder reaction with cyclopentadiene to give endo-tricyclo[3.2.1.0$^{2,4}$]oct-6-ene. This reaction is commonly used to check for the presence of cyclopropene, following its synthesis.

## Related compounds

* Malvalic acid is a toxic cyclopropene fatty acid that occurs in cottonseed oil.
* 1-Methylcyclopropene (1-MCP) is used to slow the ripening in fruits.
* Borirenes, phosphirenes, and silirenes are boron-, phosphorus-, and silicon-substituted cyclopropenes, with the formula RBC$_2$R'$_2$, RPC$_2$R'$_2$, and R$_2$SiC$_2$R'$_2$.
* Cyclopropene fatty acids a class of naturally occurring cyclopropenes.