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Hexane /ˈhɛkseɪn/ is an alkane of six carbon atoms, with the chemical formula C6H14.

The term may refer to any of the five structural isomers with that formula, or to a mixture of them.[5] In IUPAC nomenclature, however, hexane is the unbranched isomer (n-hexane); the other four isomers are named as methylated derivatives of pentane and butane. IUPAC also uses the term as the root of many compounds with a linear six-carbon backbone, such as 2-methylhexane.

Hexanes are significant constituents of gasoline. They are all colorless liquids, odorless when pure, with boiling points between 50 and 70 °C (122 and 158 °F). They are widely used as cheap, relatively safe, largely unreactive, and easily evaporated non-polar solvents.

Isomers

Common name IUPAC name Text formula Skeletal formula
normal hexane
n-hexane		 hexane CH3(CH2)4CH3 Hexane-2D-Skeletal.svg
isohexane 2-methylpentane (CH3)2CH(CH2)2CH3 2-metilpentāns.svg
3-methylpentane CH3CH2CH(CH3)CH2CH3 3-metilpentāns.svg
2,3-dimethylbutane (CH3)2CHCH(CH3)2 2,3-dimetilbutāns.svg
neohexane 2,2-dimethylbutane (CH3)3CCH2CH3 2,2-dimetilbutāns.svg

Uses

In industry, hexanes are used in the formulation of glues for shoes, leather products, and roofing. They are also used to extract cooking oils (such as canola oil or soy oil) from seeds, for cleansing and degreasing a variety of items, and in textile manufacturing. They are commonly used in food based soybean oil extraction in the United States, and are potentially present as contaminants in all soy food products in which the technique is used; the lack of regulation by the FDA of this contaminant is a matter of some controversy.[6][7]

A typical laboratory use of hexanes is to extract oil and grease contaminants from water and soil for analysis.[8] Since hexane cannot be easily deprotonated, it is used in the laboratory for reactions that involve very strong bases, such as the preparation of organolithiums. For example, butyllithiums are typically supplied as a hexane solution.[citation needed]

Hexanes are commonly used in chromatography as a non-polar solvent. Higher alkanes present as impurities in hexanes have similar retention times as the solvent, meaning that fractions containing hexane will also contain these impurities. In preparative chromatography, concentration of a large volume of hexanes can result in a sample that is appreciably contaminated by alkanes. This may result in a solid compound being obtained as an oil and the alkanes may interfere with analysis.[citation needed]

Production

Hexanes are chiefly obtained by refining crude oil. The exact composition of the fraction depends largely on the source of the oil (crude or reformed) and the constraints of the refining. The industrial product (usually around 50% by weight of the straight-chain isomer) is the fraction boiling at 65–70 °C (149–158 °F).

Physical properties

All alkanes are colorless.[9][10] The boiling points of the various hexanes are somewhat similar and, as for other alkanes, are generally lower for the more branched forms. The melting points are quite different and the trend is not apparent.[11]

Isomer M.P. (°C) M.P. (°F) B.P. (°C) B.P. (°F)
n-hexane −95.3 −139.5 68.7 155.7
3-methylpentane −118.0 −180.4 63.3 145.9
2-methylpentane (isohexane) −153.7 −244.7 60.3 140.5
2,3-dimethylbutane −128.6 −199.5 58.0 136.4
2,2-dimethylbutane (neohexane) −99.8 −147.6 49.7 121.5

Hexane has considerable vapor pressure at room temperature:

Temperature (°C) Temperature (°F) Vapor pressure (mmHg) Vapor pressure (kPa)
−40 −40 3.36 0.448
−30 −22 7.12 0.949
−20 −4 14.01 1.868
−10 14 25.91 3.454
0 32 45.37 6.049
10 50 75.74 10.098
20 68 121.26 16.167
25 77 151.28 20.169
30 86 187.11 24.946
40 104 279.42 37.253
50 122 405.31 54.037
60 140 572.76 76.362

Reactivity

Like most alkanes, hexane characteristically exhibits low reactivity and are suitable solvents for reactive compounds. Commercial samples of n-hexane however often contains methylcyclopentane, which features tertiary C-H bonds, which are incompatible with some radical reactions.[12]

Safety

The acute toxicity of n-hexane is rather low, requiring grams per kilogram. Inhalation of n-hexane at 5000 ppm for 10 minutes produces marked vertigo; 2500-1000 ppm for 12 hours produces drowsiness, fatigue, loss of appetite, and paresthesia in the distal extremities; 2500–5000 ppm produces muscle weakness, cold pulsation in the extremities, blurred vision, headache and anorexia.[13] The National Institute for Occupational Safety and Health (NIOSH) has set a recommended exposure limit (REL) for hexane isomers (not n-hexane) of 100 ppm (350 mg/m3 (0.15 gr/cu ft)) over an 8-hour workday.[14]

Hexane and other volatile hydrocarbons (petroleum ether) present an aspiration risk.[15] n-Hexane is sometimes used as a denaturant for alcohol, and as a cleaning agent in the textile, furniture, and leather industries. It is slowly being replaced with other solvents.[16]

Incidents

Occupational hexane poisoning has occurred with Japanese sandal workers, Italian shoe workers,[17] Taiwan press proofing workers, and others.[18] Analysis of Taiwanese workers has shown occupational exposure to substances including n-hexane.[19] In 2010–2011, Chinese workers manufacturing iPhones were reported to have suffered hexane poisoning.[20][21]

Biotransformation

n-Hexane is biotransformed to 2-hexanol and further to 2,5-hexanediol in the body. The conversion is catalyzed by the enzyme cytochrome P450 utilizing oxygen from air. The pathway is called omega oxidation. 2,5-Hexanediol may be further oxidized to 2,5-hexanedione, which is neurotoxic and produces a polyneuropathy.[16] In view of this behavior, replacement of n-hexane as a solvent has been discussed. n-Heptane is a possible alternative.[22]

See also

References

  1. ^ http://rspl.royalsocietypublishing.org/content/15/54.full.pdf+html
  2. ^ "n-hexane – Compound Summary". PubChem Compound. USA: National Center for Biotechnology Information. 16 September 2004. Identification and Related Records. Retrieved 31 December 2011. 
  3. ^ a b c "NIOSH Pocket Guide to Chemical Hazards #0322". National Institute for Occupational Safety and Health (NIOSH). 
  4. ^ a b "n-Hexane". Immediately Dangerous to Life and Health Concentrations (IDLH). National Institute for Occupational Safety and Health (NIOSH). 
  5. ^ "C5 and C6 alkanes". A and B Scott Organic Chemistry. Retrieved 30 October 2007. 
  6. ^ "The Tofurky Company : Our Ingredients". Tofurky.com. Retrieved 2015-03-17. 
  7. ^ Palmer, Brian (2010-04-26). "A study found hexane in soy protein. Should you stop eating veggie burgers?". Slate.com. Retrieved 2015-03-17. 
  8. ^ Use of ozone depleting substances in laboratories. The Nordic Council (2003). ISBN 92-893-0884-2
  9. ^ "Organic Chemistry-I" (PDF). Nsdl.niscair.res.in. Retrieved 2014-02-17. 
  10. ^ "13. Hydrocarbons Textbooks". Textbook.s-anand.net. Archived from the original on 6 October 2014. Retrieved 17 February 2014. 
  11. ^ William D. McCain (1990). The properties of petroleum fluids. PennWell. ISBN 0-87814-335-1. 
  12. ^ Koch, H.; Haaf, W. (1973). "1-Adamantanecarboxylic Acid". Organic Syntheses. ; Collective Volume, 5, p. 20 
  13. ^ "N-HEXANE". Toxicology data network Hazardous Substances Data Bank. National Library of Medicine. 
  14. ^ "CDC – NIOSH Pocket Guide to Chemical Hazards – Hexane isomers (excluding n-Hexane)". www.cdc.gov. Retrieved 2015-11-03. 
  15. ^ Gad, Shayne C (2005), "Petroleum Hydrocarbons", Encyclopedia of Toxicology, 3 (2nd ed.), Elsevier, pp. 377–379 
  16. ^ a b Clough, Stephen R; Mulholland, Leyna (2005). "Hexane". Encyclopedia of Toxicology. 2 (2nd ed.). Elsevier. pp. 522–525. 
  17. ^ Rizzuto, N; De Grandis, D; Di Trapani, G; Pasinato, E (1980). "N-hexane polyneuropathy. An occupational disease of shoemakers". European neurology. 19 (5): 308–15. PMID 6249607. 
  18. ^ n-Hexane, Environmental Health Criteria (122), World Health Organization, 1991 
  19. ^ Liu, C. H.; Huang, C. Y.; Huang, C. C. (2012). "Occupational Neurotoxic Diseases in Taiwan". Safety and Health at Work. 3 (4): 257–67. doi:10.5491/SHAW.2012.3.4.257. PMC 3521924 Freely accessible. PMID 23251841. 
  20. ^ "Workers poisoned while making iPhones – ABC News (Australian Broadcasting Corporation)". Abc.net.au. 2010-10-26. Retrieved 2015-03-17. 
  21. ^ David Barboza (February 22, 2011). "Workers Sickened at Apple Supplier in China". The New York Times. Retrieved 2015-03-17. 
  22. ^ Filser JG, Csanády GA, Dietz W, Kessler W, Kreuzer PE, Richter M, Störmer A (1996). "Comparative estimation of the neurotoxic risks of n-hexane and n-heptane in rats and humans based on the formation of the metabolites 2,5-hexanedione and 2,5-heptanedione". Adv Exp Med Biol. 387: 411–427. PMID 8794236. 

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