Cyclopropane is the cycloalkane with the molecular formula (CH2)3, consisting of three (CH2) linked to each other to form a triangular ring. The small size of the ring creates substantial ring strain in the structure. Cyclopropane itself is mainly of theoretical interest, but many Cyclopropanes are of commercial or biological significance.
Cyclopropane was used as a clinical inhalational anesthetic from the 1930s through the 1980s. The substance's high flammability poses a risk of fire and explosions in operating rooms due to its tendency to accumulate in confined spaces, as its density is higher than that of air.
History
Cyclopropane was discovered in 1881 by
August Freund, who also proposed the correct structure for the substance in his first paper.
Freund treated 1,3-dibromopropane with
sodium, causing an intramolecular
Wurtz reaction leading directly to cyclopropane.
The yield of the reaction was improved by Gustavson in 1887 with 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.
In modern anaesthetic practice, it has been superseded by other agents.
Anaesthesia
Cyclopropane was introduced into clinical use by the American anaesthetist Ralph Waters who used a closed system with carbon dioxide absorption to conserve this then-costly agent.
Cyclopropane is a relatively potent, non-irritating and sweet smelling agent with a minimum alveolar concentration of 17.5%
and a of 0.55. This meant induction of anaesthesia by inhalation of cyclopropane and oxygen was rapid and not unpleasant. However at the conclusion of prolonged anaesthesia patients could suffer a sudden decrease in blood pressure, potentially leading to cardiac dysrhythmia: a reaction known as "cyclopropane shock".
For this reason, as well as its high cost and its explosive nature,
it was latterly used only for the induction of anaesthesia, and has not been available for clinical use since the mid-1980s.
Cylinders and flow meters were colored orange (now orange is used for the anesthetic gas
enflurane).
Pharmacology
Cyclopropane is inactive at the
GABAA receptor and
, and instead acts as an NMDA receptor antagonist.
It also inhibits the
AMPA receptor and nicotinic acetylcholine receptors, and activates certain K
2P channels.
Structure and bonding
The triangular structure of cyclopropane requires the
between carbon-carbon covalent bonds to be 60°. The molecule has D
3h molecular symmetry. The C-C distances are 151
picometer versus 153-155 pm.
Despite their shortness, the C-C bonds in cyclopropane are weakened by 34 kcal/mol vs ordinary C-C bonds. In addition to ring strain, the molecule also has torsional strain due to the eclipsed conformation of its hydrogen atoms. The C-H bonds in cyclopropane are stronger than ordinary C-H bonds as reflected by NMR coupling constants.
Bonding between the carbon centres is generally described in terms of .[Eric V. Anslyn and Dennis A. Dougherty. Modern Physical Organic Chemistry. 2006. pages 850-852] In this model the carbon-carbon bonds are bent outwards so that the inter-orbital angle is 104°.
The unusual structural properties of cyclopropane have spawned many theoretical discussions. One theory invokes σ-aromaticity: the stabilization afforded by delocalization of the six electrons of cyclopropane's three C-C σ bonds to explain why the strain of cyclopropane is "only" 27.6 kcal/mol as compared to cyclobutane (26.2 kcal/mol) with cyclohexane as reference with Estr=0 kcal/mol,[S. W. Benson, Thermochemical Kinetics, S. 273, J. Wiley & Sons, New York, London, Sydney, Toronto 1976]
Synthesis
Cyclopropane was first produced via a
Wurtz coupling, in which 1,3-dibromopropane was
cyclisation using
sodium.
[ The yield of this reaction can be improved by the use of zinc as the dehalogenating agent and sodium iodide as a catalyst.]
- BrCH2CH2CH2Br + 2 Na → (CH2)3 + 2 NaBr
Reactions
Owing to the increased π-character of its C-C bonds, cyclopropane is often assumed to add bromine to give 1,3-dibromopropane, but this reaction proceeds poorly.[Advanced organic Chemistry, Reactions, mechanisms and structure 3ed. Jerry March ]
Cyclopropane and its derivatives can oxidatively add to transition metals, in a process referred to as C–C activation.
Safety
Cyclopropane is highly flammable. However, despite its strain energy it does not exhibit explosive behavior substantially different from other alkanes.
See also
-
Tetrahedrane contains four fused cyclopropane rings that form the faces of a tetrahedron
-
Propellane contains three cyclopropane rings that share a single central carbon-carbon bond.
-
Spiropentane is two cyclopropane rings fused at a vertex
-
Cyclopropene
-
Methylenecyclopropane
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