|
|
|
|
Cyclopropane
|
| |
|
| |
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 considerably weaker than in a typical carbon-carbon bond, yielding reactivity similar to or greater than alkenes. Baeyer strain theory explains why: the angle strain from the 60° angle between the carbon atoms (less than the normal angle of 109.5° for bonds between atoms with sp3 hybridised orbitals) reduces the compound's carbon-carbon bond energy, making it more reactive than other cycloalkanes such as cyclohexane and cyclopentane.
Discussion
Ask a question about 'Cyclopropane' Start a new discussion about 'Cyclopropane' Answer questions from other users |
Encyclopedia
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 considerably weaker than in a typical carbon-carbon bond, yielding reactivity similar to or greater than alkenes. Baeyer strain theory explains why: the angle strain from the 60° angle between the carbon atoms (less than the normal angle of 109.5° for bonds between atoms with sp3 hybridised orbitals) reduces the compound's carbon-carbon bond energy, making it more reactive than other cycloalkanes such as cyclohexane and cyclopentane. The molecule also has torsional strain due to the eclipsed conformation of its hydrogen atoms. It is somewhat stabilized by some pi character in its carbon-carbon bonds, indicated by the Walsh orbital description whereas it is modeled as a three-center-bonded orbital combination of methylene carbenes . Bent bonds also describe the bonding in cyclopropane.
The smallest polycyclic compounds contain multiple fused cyclopropane rings. Tetrahedrane contains four fused cyclopropane rings which form the faces of a tetrahedron. [1.1.1]Propellane contains three cyclopropane rings which share a single central carbon-carbon bond.
Cyclopropane is an anaesthetic when inhaled. In modern anaesthetic practice, it has been superseded by other agents, due to its extreme reactivity under normal conditions: when the gas is mixed with oxygen there is a significant risk of explosion.
History
Cyclopropane 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 an 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; industrial production had begun by 1936.
Safety
Because 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.
Cyclopropanes
Cyclopropanes 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 synthesis
Cyclopropanes can be prepared in the laboratory by organic synthesis in various ways and many methods are simply called cyclopropanation:
- :a possible reaction mechanism for this cyclopropanation was proposed:
- * addition to an alkene of a carbene such as dibromocarbene in the synthesis of propellane or methyl diazoacetate The Walsh orbital description of the molecule facilitates this reaction.
-
- 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 reactions
Although 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:
- : This asymmetric synthesis is catalyzed by a rhodium BINAP system with 96% enantiomeric excess.
- :This reaction is catalyzed by platinum(II) chloride in a carbon monoxide environment. The proposed reaction mechanism is supported by deuterium labeling.
- In another version of the same reaction the catalyst is PdBr2 is prepared in situ from palladium(II) acetate and copper(II) bromide and the solvent is toluene.
Molecular orbitals
Bonding between the carbon centers in cyclopropane is generally described by invoking bent bonds.
External links
|
| |
|
|
