Chemistry:Hexane

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Hexane (/ˈhɛkseɪn/) or n-hexane is an organic compound, a straight-chain alkane with six carbon atoms and the molecular formula C6H14.[1]

Hexane is a colorless liquid, odorless when pure, and with a boiling point of approximately 69 °C (156 °F). It is widely used as a cheap, relatively safe, largely unreactive, and easily evaporated non-polar solvent, and modern gasoline blends contain about 3% hexane.[2]

The term hexanes refers to a mixture, composed largely (>60%) of n-hexane, with varying amounts of the isomeric compounds 2-methylpentane and 3-methylpentane, and possibly, smaller amounts of nonisomeric C5, C6, and C7 (cyclo)alkanes. These "hexanes" mixtures are cheaper than pure hexane and are often used in large-scale operations not requiring a single isomer (e.g., as cleaning solvent or for chromatography).

Isomers

Main article: Chemistry:C6H14
Common name IUPAC name Text formula Skeletal formula
Normal hexane,
n-Hexane 		Hexane CH3(CH2)4CH3 150px
Isohexane 2-Methylpentane (CH3)2CH(CH2)2CH3 110px
3-Methylpentane CH3CH2CH(CH3)CH2CH3 110px
2,3-Dimethylbutane (CH3)2CHCH(CH3)2 90px
Neohexane 2,2-Dimethylbutane (CH3)3CCH2CH3 90px

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 soybean oil) from seeds, for cleansing and degreasing a variety of items, and in textile manufacturing.

A typical laboratory use of hexanes is to extract oil and grease contaminants from water and soil for analysis.[3] 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.[4]

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.

As an internal combustion engine fuel, n-hexane has low research and motor octane numbers of 25 and 26 respectively.[5] In 1983 its share in Japanese gasoline varied around 6%,[6] in 1992 it was present in American gas between 1 and 3%,[7] and in Swedish automobile fuel in the same year the share was consistently under 2%, often below 1%.[8] By 2011 its share in US gas stood between 1 and 7%.[9]

Production

Hexane is 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.[10] 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.[11][12] 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.[13]

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, hexanes characteristically exhibit 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.[14]

Safety

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.[15] Chronic occupational exposure to elevated levels of n-hexane has been demonstrated to be associated with peripheral neuropathy in auto mechanics in the US, and neurotoxicity in workers in printing presses, and shoe and furniture factories in Asia, Europe, and North America.[16]

The US 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.[17] However, for n-hexane, the current NIOSH REL is 50 ppm (180 mg/m3 (0.079 gr/cu ft)) over an 8-hour workday.[18] This limit was proposed as a permissible exposure limit (PEL) by the Occupational Safety and Health Administration in 1989; however, this PEL was overruled in US courts in 1992.[19] The current n-hexane PEL in the US is 500 ppm (1,800 mg/m3 (0.79 gr/cu ft)).[18]

Hexane and other volatile hydrocarbons (petroleum ether) present an aspiration risk.[20] 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.[21]

Like gasoline, hexane is highly volatile and is an explosion risk.

Incidents

The 1981 Louisville sewer explosions, which destroyed over 13 mi (21 km) of sewer lines and streets in the Kentucky city, were caused by ignition of hexane vapors which had been illegally discharged from a soybean processing plant owned by Ralston-Purina.

Hexane was attributed as the cause of an explosion that occurred in the National University of Río Cuarto, Argentina on 5 December 2007, due to a hexane spill near a heat-producing machine that exploded, producing a fire that killed one student and injured 24 more.

Occupational hexane poisoning has occurred with Japanese sandal workers, Italian shoe workers,[22] Taiwan press proofing workers, and others.[23] Analysis of Taiwanese workers has shown occupational exposure to substances including n-hexane.[24] In 2010–2011, Chinese workers manufacturing iPhones were reported to have suffered hexane poisoning.[25][26]

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. 2,5-Hexanediol may be further oxidized to 2,5-hexanedione, which is neurotoxic and produces a polyneuropathy.[21] In view of this behavior, replacement of n-hexane as a solvent has been discussed. n-Heptane is a possible alternative.[27]

See also

References

  1. ↑ PubChem. "n-HEXANE" (in en). https://pubchem.ncbi.nlm.nih.gov/compound/8058. 
  2. ↑ "n-Hexane - Hazardous Agents". https://haz-map.com/Agents/105. 
  3. ↑ Use of ozone depleting substances in laboratories. Kbh: Nordisk Ministerråd. 2003. ISBN 92-893-0884-2. OCLC 474188215. http://www.norden.org/en/publications/publikationer/2003-516. 
  4. ↑ Schwindeman, James A.; Woltermann, Chris J.; Letchford, Robert J. (2002-05-01). "Safe handling of organolithium compounds in the laboratory". Chemical Health & Safety 9 (3): 6–11. doi:10.1016/s1074-9098(02)00295-2. ISSN 1074-9098. https://pubs.acs.org/doi/full/10.1016/S1074-9098(02)00295-2. 
  5. ↑ Al Ibrahim, Emad; Farooq, Aamir (2020-01-16). "Octane Prediction from Infrared Spectroscopic Data". Energy & Fuels 34 (1): 817–826. doi:10.1021/acs.energyfuels.9b02816. ISSN 0887-0624. Bibcode: 2020EnFue..34..817A. https://repository.kaust.edu.sa/handle/10754/660233. 
  6. ↑ Ikeda, Masayuki; Kumai, Miho; Watanabe, Takao; Fujita, Hiroyoshi (1984). "Aromatic and Other Contents in Automobile Gasoline in Japan". Industrial Health 22 (4): 235–241. doi:10.2486/indhealth.22.235. PMID 6526699. Bibcode: 1984IndHe..22..235I. https://www.jstage.jst.go.jp/article/indhealth1963/22/4/22_4_235/_article/-char/ja/. 
  7. ↑ Doskey, Paul V.; Porter, Joseph A.; Scheff, Peter A. (November 1992). "Source Fingerprints for Volatile Non-Methane Hydrocarbons" (in en). Journal of the Air & Waste Management Association 42 (11): 1437–1445. doi:10.1080/10473289.1992.10467090. ISSN 1047-3289. Bibcode: 1992JAWMA..42.1437D. https://www.tandfonline.com/doi/full/10.1080/10473289.1992.10467090. 
  8. ↑ Östermark, Ulf; Petersson, Göran (1992-09-01). "Assessment of hydrocarbons in vapours of conventional and alkylate-based petrol". Chemosphere 25 (6): 763–768. doi:10.1016/0045-6535(92)90066-Z. ISSN 0045-6535. Bibcode: 1992Chmsp..25..763O. https://publications.lib.chalmers.se/records/fulltext/local_72590.pdf. 
  9. ↑ "Hydrocarbon Composition of Gasoline Vapor Emissions from Enclosed Fuel Tanks". United States Environmental Protection Agency. 2011. https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=P100GPED.TXT. 
  10. ↑ Le Van Mao, R.; Melancon, S.; Gauthier-Campbell, C.; Kletnieks, P. (2001-05-01). "Selective deep catalytic cracking process (SDCC) of petroleum feedstocks for the production of light olefins. I. The Catlever effect obtained with a two reaction-zones system on the conversion of n-hexane" (in en). Catalysis Letters 73 (2): 181–186. doi:10.1023/A:1016685523095. ISSN 1572-879X. https://doi.org/10.1023/A:1016685523095. 
  11. ↑ "Organic Chemistry-I". Nsdl.niscair.res.in. http://nsdl.niscair.res.in/bitstream/123456789/777/1/Revised+organic+chemistry.pdf. 
  12. ↑ "13. Hydrocarbons | Textbooks". Textbook.s-anand.net. http://textbook.s-anand.net/ncert/class-11/chemistry/13-hydrocarbons. 
  13. ↑ William D. McCain (1990). The properties of petroleum fluids. PennWell. ISBN 978-0-87814-335-1. https://books.google.com/books?id=1TJQ64JN2ZUC. 
  14. ↑ Koch, H.; Haaf, W. (1973). "1-Adamantanecarboxylic Acid". Organic Syntheses. http://www.orgsyn.org/demo.aspx?prep=cv5p0020. ; Collective Volume, 5, pp. 20 
  15. ↑ "N-HEXANE". Toxicology data network Hazardous Substances Data Bank. National Library of Medicine. https://www.nlm.nih.gov/toxnet/index.html. 
  16. ↑ Centers for Disease Control and Prevention (CDC) (2001-11-16). "n-Hexane-related peripheral neuropathy among automotive technicians--California, 1999-2000". MMWR. Morbidity and Mortality Weekly Report 50 (45): 1011–1013. ISSN 0149-2195. PMID 11724159. 
  17. ↑ "CDC – NIOSH Pocket Guide to Chemical Hazards – Hexane isomers (excluding n-Hexane)". cdc.gov. https://www.cdc.gov/niosh/npg/npgd0323.html. 
  18. ↑ 18.0 18.1 CDC (2018-03-28). "n-Hexane" (in en-us). https://www.cdc.gov/niosh/npg/npgd0322.html. 
  19. ↑ "Appendix G: 1989 Air Contaminants Update Project - Exposure Limits NOT in Effect" (in en-us). 2020-02-20. https://www.cdc.gov/niosh/npg/nengapdxg.html. 
  20. ↑ Gad, Shayne C (2005), "Petroleum Hydrocarbons", Encyclopedia of Toxicology, 3 (2nd ed.), Elsevier, pp. 377–379 
  21. ↑ 21.0 21.1 Clough, Stephen R; Mulholland, Leyna (2005). "Hexane". Encyclopedia of Toxicology. 2 (2nd ed.). Elsevier. pp. 522–525. 
  22. ↑ 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. doi:10.1159/000115166. PMID 6249607. 
  23. ↑ n-Hexane, Environmental Health Criteria, World Health Organization, 1991, http://www.inchem.org/documents/ehc/ehc/ehc122.htm 
  24. ↑ 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. PMID 23251841. 
  25. ↑ "Workers poisoned while making iPhones – ABC News (Australian Broadcasting Corporation)". Australian Broadcasting Corporation. 2010-10-26. http://www.abc.net.au/news/stories/2010/10/26/3048024.htm. 
  26. ↑ David Barboza (22 February 2011). "Workers Sickened at Apple Supplier in China". The New York Times. https://www.nytimes.com/2011/02/23/technology/23apple.html?pagewanted=all. 
  27. ↑ "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". Biological Reactive Intermediates V. Advances in Experimental Medicine and Biology. 387. 1996. pp. 411–427. doi:10.1007/978-1-4757-9480-9_50. ISBN 978-1-4757-9482-3. 



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