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Alkanes: role and usage

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What do the alkanes do?

Origin of life

Alkanes are generally considered unreactive as there are no functional groups present. However, alkanes or other hydrocarbons are probably the reasons why we have life on earth. As suggested by Nobel laureate Harold Urey and Stanley Miller in their famous Urey-Miller experiment in 1953, methane, ammonia, water and hydrogen could have produced amino acids which are the building blocks of life on earth.1,2 During the experiment, Miller created electrical spark in the mixture of gaseous molecules and later obtained minute amounts of amino acids.3 Although, later this hypothesis of origin of life was challenged by other hypotheses, the experiment itself was inspiration for further research on origin of life.4,5,6,7,8,9,10 Watch Jack Szostak describing the Urey-Miller experiment in the video below.

Try this tutorial from NOVA on detecting alien life.

Role in various organisms

Pristane and Undecane
Pristane and Undecane

Alkanes are found in various organisms. Ants and cockroaches have n-undecane which is a pheromone in their bodies11. It helps cockroaches aggregate. It is postulated that when ants sting, the alkanes help the formic acid spread through the victim’s contact point12. Pristane is found in shark liver oil and also in sperm whale, lobster, zooplankton, etc. It’s apparent stability under anaerobic condition makes it a good biological marker and used for many medical studies13.

Usage of alkanes in daily life

Usage of alkanes in petroleum

Earlier in this website, we have seen the various fractions from crude oil and their usage. Let us see couple of important aspects of petroleum in our daily life. An interactive program on the usage of petroleum can be found here.

Refining crude oil

Crude oil needs to be refined not only to remove the dark color and bad odor but also to remove sulfur compounds which lower the response of gasoline to anti-knock agents like tetraethyl lead (now not recommended). Crude oil is refined after fractional distillation. In these videos (below), you can get an idea how crude oil is refined.

Octane number

Octane number of a given fuel is a measure of its performance as a motor/ aviation fuel. The higher the octane number of the fuel, the more compression it can withstand. A fuel with lower octane number can cause problems related to knocking. Knocking can cause severe damage to the engine and may also be fatal when all the fuel instead of burning smoothly, explodes at a certain portion.

Historically it was found that iso-octane can increase the ability of a fuel to withstand compression and hence arbitrarily given an octane number of 100. n-heptane on the other hand was found to detonate easily and given an octane number of zero. Octane number does not necessarily mean that the fuel should contain iso-octane. It is measured in a test engine where the octane number of a fuel under consideration is assigned by comparison to the same anti-knocking properties of a mixture of iso-octane and n-heptane. [stextbox id=”info” caption=”Sample problem”]A fuel with octane number of 95 is used in a motor. What does that tell us about the performance of the fuel?

A fuel with octane number of 95 tells us it has the same anti-knocking ability as the mixture of 95% iso-octane and 5% n-heptane (percentage calculated by volume). The performance of the fuel can now be compared with other fuels with various octane numbers and can be considered high performance fuel. [/stextbox]

Common usage of alkanes in natural gas

Natural gas contains lighter alkanes (mainly methane, ethane, propane and butane) and produces less pollutants compared to petroleum derived fuels. Some of the usage are listed below14:

  • heating/cooling house (air conditioning)

  • ovens

  • fireplaces

  • outdoor lights

  • Fuel cells

  • plastic products (from cracking of ethane, propane)

  • fertilizer (in Haber-Bosch process for production of ammonia, hydrogen comes from methane)

  • fabric

  • anti-freeze

  • Motor fuel [used as compressed natural gas (CNG) or liquefied natural gas (LNG) less pollution and cheaper]

Biogas

Biogas is a non-petroleum fuel produced via anaerobic digestion or fermentation of renewable materials such as manure, crops, sewage, farm waste, municipal waste, etc. Biogas mainly comprises of methane and carbon dioxide and also contains hydrogen, nitrogen, oxygen and hydrogen sulfide15. In India, biogas fuel is being used in many rural households16.

Renewable diesel

Renewable diesel as opposed to petrodiesel is produced from renewable resources such as vegetable oil, fat under elevated temperature, pressure and in the presence of catalyst.17 It is also known as green diesel because it does not contain sulfur and other toxic materials. Renewable diesel can contain alkanes of 9-18 carbon atoms.18

References

References for this article
  1. Miller’s hypothesis [↩]
  2. Miller’s observation [↩]
  3. Miller-Urey hypothesis [↩]
  4. Earth’s early atmosphere [↩]
  5. Bada’s experiment [↩]
  6. Commentary on Bada’s experiment [↩]
  7. NASA’s research [↩]
  8. Commentary on Bada’s experiment in Astrobiology magazine [↩]
  9. Bada’s experiment details in Science [↩]
  10. Planning Urey-Miller experiment on Mars [↩]
  11. Alkanes in ants [↩]
  12. Ant stinging [↩]
  13. Matabolism of pristane [↩]
  14. Usage of Natural Gas [↩]
  15. Biogas composition [↩]
  16. Indian Biogas Association website [↩]
  17. Comparison between renewable diesel and biodiesel [↩]
  18. Importance of renewable diesel [↩]