Cyclopropane is the cycloalkane 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 resulting in D3h molecular symmetry. The small size of the ring creates substantial ring strain in the structure. Cyclopropane is an anaesthetic. In modern anaesthetic practice, it has been superseded by other agents. Due to its extreme reactivity, cyclopropane-oxygen mixtures may explode.
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.
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 coloured orange.
The triangular structure of cyclopropane requires the bond angles between carbon-carbon covalent bonds to be 60°. This is far less than the thermodynamically most stable angle of 109.5° and leads to significant ring strain. The molecule also has torsional strain due to the eclipsed conformation of its hydrogen atoms. As such, the bonds between the carbon atoms are considerably weaker than in a typical alkane, resulting in much higher reactivity. Bonding between the carbon centres is generally described in terms of bent bonds. In this model the carbon-carbon bonds are bent outwards so that the inter-orbital angle is 104°. This reduces the level of bond strain and is achieved by distorting the sp3 hybridisation of carbon atoms to technically sp5 hybridisation so that the C-C bonds have more π character than normal. One unusual consequence of bent bonding is that while the C-C bonds in cyclopropane are weaker than normal, the carbon atoms are also closer together than in a regular alkane bond: 151 pm versus 153 pm. Stability due to cyclic delocalization of the six electrons of cyclopropane's three C-C σ bonds was given by Michael J. S. Dewar as an explanation of the only slightly greater strain of cyclopropane as compared to cyclobutane with cyclohexane as reference with Estr=0 kcal/mol. This stabilization is referred to as σ aromaticity, in contrast to the usual π aromaticity, that, for example, is a highly stabilizing effect in benzene. Other studies do not support the role of σ-aromaticity in cyclopropane and the existence of an induced ring current; such studies provide an alternative explanation for the energetic stabilization and abnormal magnetic behaviour of cyclopropane.
Synthesis
Cyclopropane was first produced via a Wurtz coupling, in which 1,3-dibromopropane was cyclised 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.
Cyclopropanation
Cyclopropane rings are found in numerous biomolecules and pharmaceutical drugs. As such the formation of cyclopropane rings, generally referred to as cyclopropanation, is an active area of chemical research.
