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Cyclopropane

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Title: Cyclopropane  
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Cyclopropane

Cyclopropane[1]
Cyclopropane - displayed formula
Cyclopropane - skeletal formula
Names
IUPAC name
Cyclopropane
Identifiers
 Y
ChEBI  Y
ChEMBL  N
ChemSpider  Y
Jmol-3D images Image
KEGG  Y
PubChem
UNII  Y
Properties
C3H6
Molar mass 42.08 g/mol
Density 1.879 g/L (1 atm, 0 °C)
Melting point −128 °C (−198 °F; 145 K)
Boiling point −33 °C (−27 °F; 240 K)
Acidity (pKa) ~46
Hazards
Main hazards Highly flammable
Asphyxiant
Safety data sheet External MSDS
NFPA 704
4
1
0
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
 N  (: Y/N?)

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 resulting in D3h molecular symmetry. Cyclopropane and propene have the same molecular formula but have different structures, making them structural isomers.

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 In modern anaesthetic practice, it has been superseded by other agents, due to its extreme reactivity under normal conditions: when the gas is mixed with oxygen, there is a significant risk of explosion.

Contents

  • History 1
  • Anaesthesia 2
    • Pharmacology 2.1
  • Structure and bonding 3
  • Synthesis 4
    • Cyclopropanation 4.1
  • Reactions 5
  • Safety 6
  • See also 7
  • References 8
  • External links 9

History

Cyclopropane was discovered in 1881 by August Freund, who also proposed the correct structure for the new substance in his first paper.[2] Freund treated 1,3-dibromopropane with sodium, causing an intramolecular Wurtz reaction leading directly to cyclopropane.[3] The yield of the reaction was improved by Gustavson in 1887 with the use of zinc instead of sodium.[4] Cyclopropane had no commercial application until Henderson and Lucas discovered its anaesthetic properties in 1929;[5] industrial production had begun by 1936.[6]

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%[7] 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".[8] For this reason, as well as its high cost and its explosive nature,[9] 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.

Pharmacology

Cyclopropane is inactive at the GABAA and glycine receptors, and instead acts as an NMDA receptor antagonist.[10][11] It also inhibits the AMPA receptor and nicotinic acetylcholine receptors, and activates certain K2P channels.[10][11][12]

Structure and bonding

Orbital overlap in the bent bonding model of cyclopropane

The triangular structure of cyclopropane requires the bond angles between carbon-carbon bonds to be 60°. This is far less than the thermodynamically most stable angle of 109.5° (for bonds between atoms with sp3 hybridised orbitals) 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.[13] 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[14],[15] (i.e. 1/6 s density and 5/6 p density) so that the C-C bonds have more π character than normal[16] (at the same time the carbon-to-hydrogen bonds gain more s-character). 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 (average alkene bond: 146 pm).[17]

An alternative model for describing the bonding in cyclopropane involves Walsh diagrams and aims to do a better job fitting molecular orbital theory in light of spectroscopic evidence and group symmetry arguments. In this model cyclopropane is described as a three-center bonded orbital combination of methylene carbenes.

Cyclic delocalization of the six electrons of cyclopropane's three CC σ bonds was given by Michael J. S. Dewar as explanation of the - compared to cyclobutane - relatively low strain energy of cyclopropane ("only" 27.6 vs. 26.2 kcal mol−1, cyclohexane as reference with Estr = 0 kcal mol−1 [18]). This stabilization is referred to as σ-aromaticity,[19][20] cf. the cyclic delocalization of the six π electrons in benzene as the archetypical example of aromaticity. The assumption of a diamagnetic ring current in cyclopropane is in line with the shielding of its protons in nmr spectra and with its unusual magnetic properties (high diamagnetic susceptibility, high anisotropy of the magnetic susceptibility). More recent studies of the extent to which cyclopropane is stabilized by σ-aromaticity do attribute a stabilization of 11.3 kcal mol−1 to this effect.[21]

Synthesis

Cyclopropane was first produced via a Wurtz coupling, in which 1,3-dibromopropane was cyclised using sodium.[2] The yield of this reaction can be improved by exchanging the metal for zinc.[4]

BrCH2CH2CH2Br + 2 Na → (CH2)3 + 2 NaBr

Cyclopropanation

Cyclopropane rings are found in numerous biomolecules (e.g., pyrethrins, a group of natural insecticides) and pharmaceutical drugs. As such the formation of cyclopropane rings, generally referred to as cyclopropanation, is an active area of chemical research.

Reactions

Owing to the increased π-character of its C-C bonds, cyclopropane can react like an alkene in certain cases. For instance it undergoes hydrohalogenation with mineral acids to give linear alkyl halides. Substituted cyclopropanes also react, following Markovnikov's rule.[22]

Electrophilic addition of HBr to cyclopropane

Safety

Cyclopropane is highly flammable. However, despite its strain energy it is not substantially more explosive than other alkanes.

See also

References

  1. ^ Merck Index, 11th Edition, 2755.
  2. ^ a b August Freund (1881). "Über Trimethylen" [On trimethylene]. Journal für Praktische Chemie 26 (1): 367–377.  
  3. ^ August Freund (1882). "Über Trimethylen" [On trimethylene]. Monatshefte für Chemie … 3 (1): 625–635.  
  4. ^ a b G. Gustavson (1887). "Ueber eine neue Darstellungsmethode des Trimethylens" [On a new method of preparing trimethylene]. Journal für Praktische Chemie 36: 300–305.  
  5. ^ G. H. W. Lucas and V. E. Henderson (1 August 1929). "A New Anesthetic: Cyclopropane : A Preliminary Report". Can Med Assoc J. 21 (2): 173–5.  
  6. ^ H. B. Hass, E. T. McBee, and G. E. Hinds (1936). "Synthesis of Cyclopropane". Industrial & Engineering Chemistry 28 (10): 1178–81.  
  7. ^ Eger, Edmond I.; Brandstater, Bernard; Saidman, Lawrence J.; Regan, Michael J.; Severinghaus, John W.; Munson, Edwin S. (1965). "Equipotent Alveolar Concentrations of Methoxyflurane, Halothane, Diethyl Ether, Fluroxene, Cyclopropane, Xenon and Nitrous Oxide in the Dog". Anesthesiology 26 (6): 771–777.  
  8. ^ JOHNSTONE, M; Alberts, JR (July 1950). "Cyclopropane anesthesia and ventricular arrhythmias.". British heart journal 12 (3): 239–44.  
  9. ^ MacDonald, AG (June 1994). "A short history of fires and explosions caused by anaesthetic agents.". British journal of anaesthesia 72 (6): 710–22.  
  10. ^ a b Hugh C. Hemmings; Philip M. Hopkins (2006). Foundations of Anesthesia: Basic Sciences for Clinical Practice. Elsevier Health Sciences. pp. 292–.  
  11. ^ a b Hemmings, Hugh C. (2009). "Molecular Targets of General Anesthetics in the Nervous System". pp. 11–31.  
  12. ^ Hara K, Eger EI, Laster MJ, Harris RA (December 2002). "Nonhalogenated alkanes cyclopropane and butane affect neurotransmitter-gated ion channel and G-protein-coupled receptors: differential actions on GABAA and glycine receptors". Anesthesiology 97 (6): 1512–20.  
  13. ^ Eric V. Anslyn and Dennis A. Dougherty. Modern Physical Organic Chemistry. 2006. pages 850-852
  14. ^ http://isites.harvard.edu/fs/docs/icb.topic93502.files/Lectures_and_Handouts/06-Handouts/deMeijere.pdf
  15. ^ http://isites.harvard.edu/fs/docs/icb.topic1032290.files/lecture%203.pdf
  16. ^ Knipe, edited by A.C. (2007). March's advanced organic chemistry reactions, mechanisms, and structure. (6th ed.). Hoboken, N.J.: Wiley-Interscience. p. 219.  
  17. ^ Allen, Frank H.; Kennard, Olga; Watson, David G.; Brammer, Lee; Orpen, A. Guy; Taylor, Robin (1987). "Tables of bond lengths determined by X-ray and neutron diffraction. Part 1. Bond lengths in organic compounds". Journal of the Chemical Society, Perkin Transactions 2 (12): S1–S19.  
  18. ^ S. W. Benson, Themochemical Kinetics, S. 273, J. Wiley & Sons, New York, London, Sydney, Toronto 1976
  19. ^ Dewar, M. J. (1984). "Chemical Implicatons of σ Conjugation". J. Am. Chem. Soc. 106: 669–682.  
  20. ^ Cremer, D. (1988). "Pros and Cons of σ-Aromaticity". Tetrahedron 44 (2): 7427–7454.  
  21. ^ Exner, Kai; Paul; von Ragué, Schleyer (2001). "Theoretical Bond Energies:  A Critical Evaluation". J. Phys. Chem. A 105 (13): 3407–3416.  
  22. ^ Advanced organic Chemistry, Reactions, mechanisms and structure 3ed. Jerry March ISBN 0-471-85472-7

External links

  • Synthesis of Cyclopropanes and related compounds
  • carbon triangle
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