Pentane
Alkane with 5 carbon atoms C5H12
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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.
History
Normal pentane was discovered in 1862 by Carl Schorlemmer, who, while analyzing pyrolysis products of the cannel coal mined in Wigan, identified, separated by fractional distillation and studied a series of liquid hydrocarbons inert to nitric and sulfuric acids. The lightest of them, which he called hydride of amyl, had an empirical formula of C5H12, density of 0.636 at 17 °C and boiled between 39 and 40 °C. In the next year he identified the same compound in the Pennsylvanian oil. By 1872 he switched his nomenclature to the modern one, leading to it being called Pentane.
Beyond Schorlemmer's initial work, scientists discovered that the molecular formula C5H12 could represent different structural arrangements, leading to the identification of isopentane and neopentane. This discovery contributed significantly to the understanding of isomerism and hydrocarbons in the 19th century. The high volatility and low boiling point of pentane made it useful as a solvent and in fuels. Its use expanded in the 1970s as a blowing agent for foams, replacing CFCs. The petroleum refining industry utilizes pentanes, particularly isopentane, to produce high-octane fuels.
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Isomers
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.
Because of their low boiling points, low cost, and relative safety, pentanes are used as a working medium in geothermal power stations and organic Rankine cycles. It is also used in some blended refrigerants.
Pentanes are solvents in many ordinary products, e.g. in some pesticides.
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; this explanation is contradicted by the fact that neopentane has a lower density than the other two isomers, and the high melting point is actually caused by neopentane's significantly lower entropy of fusion.
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.
Rotation about two central single C-C bonds of n-pentane produces four different conformations.
Reactions
Like other alkanes, pentanes are largely unreactive at standard room temperature and conditions - however, with sufficient activation energy (e.g., 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
Without zeolite catalysts, such reactions are unselective, so 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.
Production and occurrence
Pentane is produced by fractional distillation of petroleum and purified by rectification (successive distillations).
It occurs in alcoholic beverages and in hop oil. It is a component of exhaled breath for some individuals. A degradation product of unsaturated fatty acids, its presence is associated with some diseases and cancers.
Pentane is a relatively minor component of automobile gasoline, with its share varying within 1–6% in 1990s Sweden, 2–13% in 1990s US and 1–3% in the US in 2011. At 62, its octane number (both RON and MON) is quite low.