cyclopropane n : a colorless flammable gas sometimes used as an inhalation anesthetic
the simplest alicyclic hydrocarbon, C3H6
- Finnish: syklopropaani
- Greek: κυκλοπροπάνιο
- Italian: ciclopropano
Cyclopropane is a cycloalkane molecule with the molecular formula C3H6, consisting of three carbon atoms linked to each other to form a ring, with each carbon atom bearing two hydrogen atoms. The bonds between the carbon atoms are a great deal weaker than in a typical carbon-carbon bond. This is the result of the 60° angle between the carbon atoms, which is far less than the normal angle of 109.5° for bonds between atoms with sp3 hybridised orbitals. This angle strain has to be subtracted from the normal C-C bond energy, making the resultant compound more reactive than acyclic alkanes and other cycloalkanes such as cyclohexane and cyclopentane. This is the banana bond description of cycloalkanes.
However, cyclopropanes are more stable than a simple angle strain analysis would suggest. Cyclopropane can also be modeled as a three-center-bonded orbital combination of methylene carbenes. This results in the Walsh orbital description of cyclopropane, where the C-C bonds have mostly pi character. This is also why cyclopropanes often have reactivity similar to alkenes. This is also why carbenes can easily add into alkenes to produce cyclopropanes. Cyclopropanes taken to the extreme are tetrahedranes and propellanes.
Cyclopropane is an anaesthetic when inhaled, but has been superseded by other agents in modern anaesthetic practice. This is due to its extreme reactivity under normal conditions: when the gas is mixed with oxygen there is a significant risk of explosion.
HistoryCyclopropane was discovered in 1881 by August Freund, who also proposed the right structure for the new substance in his first paper. Freund reacted 1,3-dibromopropane with sodium, the reaction is a intramolecular Wurtz reaction leading directly to cyclopropane. The yield of the reaction can be improved by the use of zinc instead of sodium. Cyclopropane had no commercial application until Henderson and Lucas discovered its anaesthetic properties in 1929. The industrial scale production started not before 1930.
SafetyBecause of the strain in the carbon-carbon bonds of cyclopropane, the molecule has an enormous amount of potential energy. In pure form, it will break down to form linear hydrocarbons, including "normal", non-cyclic propene. This decomposition is potentially explosive, especially if the cyclopropane is liquified, pressurized, or contained within tanks. Explosions of cyclopropane and oxygen are even more powerful, because the energy released by the formation of normal propane is compounded by the energy released via the oxidation of the carbon and hydrogen present. At room temperature, sufficient volumes of liquified cyclopropane will self-detonate. To guard against this, the liquid is shipped in cylinders filled with tungsten wool, which prevents high-speed collisions between molecules and vastly improves stability. Pipes to carry cyclopropane must likewise be of small diameter, or else filled with unreactive metal or glass wool, to prevent explosions. Even if these precautions are followed, cyclopropane is dangerous to handle and manufacture, and is no longer used for anaesthesia.
CyclopropanesCyclopropanes are a class of organic compounds sharing the common cyclopropane ring, in which one or more hydrogens may be substituted. These compounds are found in biomolecules; for instance, the pyrethrum insecticides (found in certain Chrysanthemum species) contain a cyclopropane ring.
Organic synthesisCyclopropanes can be prepared in the laboratory by organic synthesis in various ways and many methods are simply called cyclopropanation:
- addition of sodium to 1,3-dibromopropane in the Freund reaction (1881)
- addition to an alkene of a zinc carbenoid in the Simmons-Smith reaction (1958) for example to cinnamyl alcohol. In one adaptation an amide is reacted with two equivalents of dichloromethane aided by titanium tetrachloride and magnesium:
- a possible reaction mechanism for this cyclopropanation was proposed:
- nucleophilic displacement of a leaving group by a carbon nucleophile in a 1,3 relationship, for example the synthesis of cyclopropylacetylene from 5-chloro-1-pentyne. Another example can be found in the Bingel reaction. An asymmetric reaction creating three stereocenters is demonstrated in a reaction of cyclohexenone with bromonitromethane assisted by trans-2,5-dimethylpiperazine as a base and a pyrrolidine based tetrazole organocatalyst:
- an intramolecular Wurtz coupling for example in the synthesis of bicyclo[1.1.0]butane
- Rearrangement reaction of certain cyclobutane compounds for instance the conversion of 1,2-cyclobutanediol to cyclopropanecarboxaldehyde
- photochemical rearrangement reaction of 1,4-dienes to vinylcyclopropanes in the di-pi-methane rearrangement
- reaction of esters with Grignards in presence of a titanium alkoxide in the Kulinkovich reaction
- extrusion of nitrogen in certain pyrazolines in the so called Kishner cyclopropane synthesis
Organic reactionsAlthough cyclopropanes are formally cycloalkanes, they are very reactive due to considerable strain energy and due to double bond character.
- Cyclopropyl groups participate in cycloaddition reaction such as the formal [5+2]cycloaddition shown below:
- Cyclopropyl groups also engage in many rearrangement reactions. An extreme example is found in the compound bullvalene. A cyclopropane ring is an intermediate in the Favorskii rearrangement. Certain methylenecyclopropanes are found to convert to cyclobutenes:
cyclopropane in German: Cyclopropan
cyclopropane in Modern Greek (1453-): Κυκλοπροπάνιο
cyclopropane in Spanish: Ciclopropano
cyclopropane in French: Cyclopropane
cyclopropane in Italian: Ciclopropano
cyclopropane in Hebrew: ציקלופרופאן
cyclopropane in Hungarian: Ciklopropán
cyclopropane in Dutch: Cyclopropaan
cyclopropane in Japanese: シクロプロパン
cyclopropane in Polish: Cyklopropan
cyclopropane in Portuguese: Ciclopropano
cyclopropane in Russian: Циклопропан
cyclopropane in Finnish: Syklopropaani
cyclopropane in Swedish: Cyklopropan
cyclopropane in Vietnamese: Cycloprôpan
cyclopropane in Turkish: Siklopropan
cyclopropane in Chinese: 環丙烷