Pentane

Pentane is an organic compound with the formula C5H12—that is, an alkane with five carbon atoms. The term may refer to any of three structural isomers, or to a mixture of them: in the IUPAC nomenclature, however, pentane means exclusively the n-pentane isomer; the other two are called isopentane (methylbutane) and neopentane (dimethylpropane). Cyclopentane is not an isomer of pentane because it has only 10 hydrogen atoms where pentane has 12.

Pentane

Pentane 3D ball.png
Pentane 3D spacefill.png
Names
Preferred IUPAC name
Pentane
Other names
Quintane;[1] Refrigerant-4-13-0
Identifiers
CAS Number
3D model (JSmol)
Beilstein Reference
 969132
ChEBI
ChEMBL
ChemSpider
DrugBank
ECHA InfoCard 100.003.358
EC Number
  • 203-692-4
Gmelin Reference
 1766
MeSH pentane
PubChem CID
RTECS number
  • RZ9450000
UNII
UN number 1265
Properties[2]
Chemical formula
 C5H12
Molar mass 72.151 g·mol−1
Appearance Colourless liquid
Odor Gasoline-like[3]
Density 0.626 g mL−1; 0.6262 g mL−1 (at 20 °C)
Melting point −130.5 to −129.1 °C; −202.8 to −200.3 °F; 142.7 to 144.1 K
Boiling point 35.9 to 36.3 °C; 96.5 to 97.3 °F; 309.0 to 309.4 K
Solubility in water
 40 mg L−1 (at 20 °C)
log P 3.255
Vapor pressure 57.90 kPa (at 20.0 °C)
Henry's law
constant (kH)
 7.8 nmol Pa−1 kg−1
Acidity (pKa) ~45
Basicity (pKb) ~59
UV-vis (λmax) 200 nm
Magnetic susceptibility (χ)
 -63.05·10−6 cm3/mol
Refractive index (nD)
 1.358
Viscosity 0.240 mPa·s (at 20 °C)
Thermochemistry
Heat capacity (C)
 167.19 J K−1 mol−1
Std molar
entropy (So298)
 263.47 J K−1 mol−1
Std enthalpy of
formation (ΔfH⦵298)
 −174.1–−172.9 kJ mol−1
Std enthalpy of
combustion (ΔcH⦵298)
 −3.5095–−3.5085 MJ mol−1
Hazards
Safety data sheet See: data page
GHS pictograms
GHS Signal word Danger
GHS hazard statements
 H225, H304, H336, H411
GHS precautionary statements
 P210, P261, P273, P301+310, P331
NFPA 704 (fire diamond)
Flammability code 4: Will rapidly or completely vaporize at normal atmospheric pressure and temperature, or is readily dispersed in air and will burn readily. Flash point below 23 °C (73 °F). E.g. propaneHealth code 1: Exposure would cause irritation but only minor residual injury. E.g. turpentineReactivity code 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g. liquid nitrogenSpecial hazards (white): no code
4
1
0
Flash point −49.0 °C (−56.2 °F; 224.2 K)
Autoignition
temperature
 260.0 °C (500.0 °F; 533.1 K)
Explosive limits 1.5–7.8%[3]
Lethal dose or concentration (LD, LC):
  • 3 g kg−1 (dermal, rabbit)
  • 5 g kg−1 (oral, mouse)
130,000 mg/m3 (mouse, 30 min)
128,200 ppm (mouse, 37 min)
325,000 mg/m3 (mouse, 2 hr)[4]
NIOSH (US health exposure limits):
PEL (Permissible)
 TWA 1000 ppm (2950 mg/m3)[3]
REL (Recommended)
 TWA 120 ppm (350 mg/m3) C 610 ppm (1800 mg/m3) [15-minute][3]
IDLH (Immediate danger)
 1500 ppm[3]
Related compounds
Related alkanes
Supplementary data page
Structure and
properties
 Refractive index (n),
Dielectric constant (εr), etc.
Thermodynamic
data
 Phase behaviour
solid–liquid–gas
Spectral data
 UV, IR, NMR, MS
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Pentanes are components of some fuels and are employed as specialty solvents in the laboratory. Their properties are very similar to those of butanes and hexanes.

Industrial uses

Pentanes are some of the primary blowing agents used in the production of polystyrene foam and other foams. Usually, a mixture of n-, i-, and increasingly cyclopentane is used for this purpose.

Acid-catalyzed isomerization gives isopentane, which is used in producing high-octane fuels.[5]

Because of their low boiling points, low cost, and relative safety, pentanes are used as a working medium in geothermal power stations in some blended refrigerants also used as a working medium in organic rankine cycles.

Pentanes are solvents in many ordinary products, e.g. in some pesticides.[6]

Laboratory use

Pentanes are relatively inexpensive and are the most volatile liquid alkanes at room temperature, so they are often used in the laboratory as solvents that can be conveniently and rapidly evaporated. However, because of their nonpolarity and lack of functionality, they dissolve only nonpolar and alkyl-rich compounds. Pentanes are miscible with most common nonpolar solvents such as chlorocarbons, aromatics, and ethers.

They are often used in liquid chromatography.

Physical properties

The boiling points of the pentane isomers range from about 9 to 36 °C. As is the case for other alkanes, the more thickly branched isomers tend to have lower boiling points.

The same tends to be true for the melting points of alkane isomers, and that of isopentane is 30 °C lower than that of n-pentane. However, the melting point of neopentane, the most heavily branched of the three, is 100 °C higher than that of isopentane. The anomalously high melting point of neopentane has been attributed to the tetrahedral molecules packing more closely in solid form. But this explanation is contradicted by the fact that neopentane has a lower density than the other two isomers.[7]

The branched isomers are more stable (have lower heat of formation and heat of combustion) than n-pentane. The difference is 1.8 kcal/mol for isopentane, and 5 kcal/mol for neopentane.[8]

Rotation about two central single C-C bonds of n-pentane produces four different conformations.[9]

Reactions

Like other alkanes, pentanes are largely unreactive at standard room temperature and conditions - however, with sufficient activation energy (i.e. an open flame), they readily oxidize to form carbon dioxide and water:

C5H12 + 8 O2 → 5 CO2 + 6 H2O + heat/energy

Like other alkanes, pentanes undergo free radical chlorination:

C5H12 + Cl2 → C5H11Cl + HCl

Such reactions are unselective; with n-pentane, the result is a mixture of the 1-, 2-, and 3-chloropentanes, as well as more highly chlorinated derivatives. Other radical halogenations can also occur.

References
  1. Hofmann, August Wilhelm Von (1 January 1867). "I. On the action of trichloride of phosphorus on the salts of the aromatic monamines". Proceedings of the Royal Society of London. 15: 54–62. doi:10.1098/rspl.1866.0018.
  2. Record of n-Pentane in the GESTIS Substance Database of the Institute for Occupational Safety and Health, accessed on 19 April 2011.
  3. NIOSH Pocket Guide to Chemical Hazards. "#0486". National Institute for Occupational Safety and Health (NIOSH).
  4. "n-Pentane". Immediately Dangerous to Life and Health Concentrations (IDLH). National Institute for Occupational Safety and Health (NIOSH).
  5. Karl Griesbaum, Arno Behr, Dieter Biedenkapp, Heinz-Werner Voges, Dorothea Garbe, Christian Paetz, Gerd Collin, Dieter Mayer, Hartmut Höke (2002). "Hydrocarbons". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a13_227. ISBN 978-3527306732.CS1 maint: uses authors parameter (link)
  6. Milne, ed., G.W.A. (2005). Gardner's Commercially Important Chemicals: Synonyms, Trade Names, and Properties. Hoboken, New Jersey: John Wiley & Sons, Inc. p. 477. ISBN 978-0-471-73518-2.CS1 maint: extra text: authors list (link)
  7. James Wei (1999), Molecular Symmetry, Rotational Entropy, and Elevated Melting Points. Ind. Eng. Chem. Res., volume 38 issue 12, pp. 5019–5027 doi:10.1021/ie990588m
  8. From the values listed at Standard enthalpy change of formation (data table).
  9. Roman M. Balabin (2009). "Enthalpy Difference between Conformations of Normal Alkanes: Raman Spectroscopy Study of n-Pentane and n-Butane". J. Phys. Chem. A. 113 (6): 1012–9. doi:10.1021/jp809639s. PMID 19152252.
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