Cyclopropane

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.{{cite journal | trans-title = On a new method of preparing trimethylene

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 blood/gas partition coefficient 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 and glycine receptors, and instead acts as an NMDA receptor antagonist. It also inhibits the AMPA receptor and nicotinic acetylcholine receptors, and activates certain K2P channels.

Structure and bonding

The triangular structure of cyclopropane requires the bond angles between carbon-carbon covalent bonds to be 60°. The molecule has D3h molecular symmetry. The C-C distances are 151 pm versus 153-155 pm.{{cite book|title=Polymer Mechanochemistry

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 bent bonds. 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, in contrast to the usual π aromaticity, that, for example, has 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. :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. Hydrohalogenation with hydrohalic acids gives linear 1-halopropanes. Substituted cyclopropanes also react, following Markovnikov's rule. :[[Image:Addition of HBr to Cyclopropane.svg|Electrophilic addition of HBr to cyclopropane|class=skin-invert]]

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.

References

References

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