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Skeletal formula of cyclopropene
Skeletal formula of cyclopropene with implicit hydrogens shown
Ball and stick model of cyclopropene
Systematic IUPAC name
2781-85-3 YesY
ChemSpider 109788 N
Jmol-3D images Image
MeSH cyclopropene
PubChem 123173
Molar mass 40.06 g·mol−1
Boiling point −36 °C (−33 °F; 237 K)
51.9-53.9 J K−1 mol−1
-2032--2026 kJ mol−1
Except where noted otherwise, data is given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
 N verify (what isYesY/N?)
Infobox references

Cyclopropene is an organic compound with the formula C3H4. It is the simplest cycloalkene. It has a triangular structure. Because the ring is highly strained, cyclopropene is both difficult to prepare and interesting to study.[2] Like cyclopropane, the carbon ring of cyclopropene is planar. The reduced length of the double bond compared to a single bond causes the angle opposite the double bond to narrow to about 51°[3] from the 60° angle found in cyclopropane. As with cyclopropane, the carbon–carbon bonding in the ring has increased p character: the alkene carbons use sp2.68 hybridization for the ring.[4]

Synthesis of cyclopropene and derivatives[edit]

Early syntheses[edit]

The first confirmed synthesis of cyclopropene, carried out by Dem'yanov and Doyarenko, involved the thermal decomposition of trimethylcyclopropylammonium hydroxide over platinized clay at 320–330 °C under a CO2 atmosphere. This reaction produces mainly trimethylamine and dimethylcyclopropyl amine, together with about 5% of cyclopropene. Cyclopropene can also be obtained in about 1% yield by thermolysis of the adduct of cycloheptatriene and dimethyl acetylenedicarboxylate.

Modern syntheses from allyl chlorides[edit]

Allyl chloride undergoes dehydrohalogenation upon treatment with the base sodium amide at 80 °C to produce cyclopropene in about 10% yield.[5]

CH2=CHCH2Cl + NaNH2 → C3H4 (cyclopropene) + NaCl + NH3

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:[6]

CH2=CHCH2Cl + NaN(TMS)2 → C3H4 (cyclopropene) + NaCl + NH(TMS)2

1-Methylcyclopropene is synthesized similarly but at room temperature from methallylchloride using phenyllithium as the base:[7]

CH2=C(CH3)CH2Cl + LiC6H5 → CH3C3H3 (1-methylcylopropene) + LiCl + C6H6

Syntheses of derivatives[edit]

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 sulfate catalyst, ethyl diazoacetate reacts with acetylenes to give cyclopropenes. 1,2-Dimethylcyclopropene-3-carboxylate arises via this method from 2-butyne. Copper has proved to be useful as a catalyst in a variety of cyclopropene syntheses. Copper sulfate and copper dust are among the more popular forms of copper used.

Reactions of cyclopropene[edit]

Studies on cyclopropene mainly focus on the consequences of its high ring strain. At 425 °C, cyclopropene isomerizes to methylacetylene (propyne).

C3H4 → H3CC≡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[,4]oct-6-ene. This reaction is commonly used to check for the presence of cyclopropene, following its synthesis.[6]


Related compounds[edit]

  • 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.[8][9]
  • Borirenes, phosphirenes, and silirenes are boron-, phosphorus-, and silicon-substituted cyclopropenes, with the formula RBC2R'2, RPC2R'2, and R2SiC2R'2.

External links[edit]


  1. ^ "cyclopropene - Compound Summary". PubChem Compound. USA: National Center for Biotechnology Information. 27 March 2005. Identification and Related Records. Retrieved 9 October 2011. 
  2. ^ Carter, F. L.; Frampton, V. L. (1964). "Review of the Chemistry of Cyclopropene Compounds". Chemical Reviews 64: 497–525. doi:10.1021/cr60231a001. 
  3. ^ Staley, S. W.; Norden, T. D.; Su, C.-F.; Rall, M.; Harmony, M. D. (1987). "Structure of 3-cyanocyclopropene by microwave spectroscopy and ab initio molecular orbital calculations. Evidence for substituent-ring double bond interactions". J. Am. Chem. Soc. 109 (10): 2880–2884. doi:10.1021/ja00244a004. 
  4. ^ Allen, F. H. (1982). "The geometry of small rings: Molecular geometry of cyclopropene and its derivatives". Tetrahedron 38 (5): 645–655. doi:10.1016/0040-4020(82)80206-8. 
  5. ^ Closs, G.L.; Krantz, K.D. (1966). "A Simple Synthesis of Cyclopropene". Journal of Organic Chemistry 31: 638. doi:10.1021/jo01340a534. 
  6. ^ a b Binger, P.; Wedermann, P.; Brinker, U. H. (2000). "Cyclopropene: A New Simple Synthesis and Its Diels-Alder reaction with Cyclopentadiene". Org. Synth. 77: 254. ; Coll. Vol. 10, p. 231 
  7. ^ Clarke, T. C.; Duncan, C. D.; Magid, R. M. (1971). "An Efficient and Convenient Synthesis of 1-Methylcyclopropene". J. Org. Chem 36: 1320. doi:10.1021/jo00808a041. 
  8. ^ Beaudry, R.; Watkins, C. (2001). "Use of 1-MCP on Apples". Perishable Handling Quarterly (University of California) (108): 12. 
  9. ^ Trinchero, G. D.; Sozzi, G. O.; Covatta, F.; Fraschina, A. A. (May 2004). "Inhibition of ethylene action by 1-methylcyclopropene extends postharvest life of "Bartlett" pears". Postharvest Biology and Technology 32 (2): 193–204. doi:10.1016/j.postharvbio.2003.11.009.