Basic Organic Chemistry for Astrobiology – Hydrocarbons

  • Organic compounds include most carbon-containing compounds, with a few exceptions, the most notable exception being carbon dioxide (CO2).  Carbon is the “backbone” element for life on Earth, and organic compounds are key to life’s chemistry.
  • Organic compounds include polymers, which are large molecules composed of chains or networks of carbon-based sub-units (monomers), joined by carbon-carbon bonds.
  • Organic compounds that contain only carbon and hydrogen are called hydrocarbons.  Hydrocarbons include the simplest organic compounds; yet hydrocarbons also include some of the most complex and astrobiologically interesting molecules found in space to date.
  • Hydrocarbons are generally hydrophobic (“water repellent”).  Like oil, and are also generally flammable/combustible, and many hydrocarbons serve as fuels.  As a simple example of hydrocarbon combustion, methane (CH4) + elemental oxygen burns to form carbon dioxide + water.
  •  Most of the Earth’s hydrocarbons occur as crude oil, resulting from the decomposition of organic matter, which leads to an abundance of carbon and hydrogen.
  • Organic compounds, including hydrocarbons, when oxidized, yield substantial amounts of energy.  That is why organic compounds make good fuels – for machines (for example, in gasoline) as well as organisms and the cells that make them up (for example, glucose).
  • Very generally and conceptually, oxidization or “breakdown” of a complex organic compound liberates energy for the following reason.  The chemical bonds in the compound to be broken down are higher energy than the chemical bonds of the simpler product compounds.  As such, the bonding energy in compound to be broken down  (let’s call that “A”) is higher than the total bonding energy of the resultant compounds (let’s call that “B”).  Since overall energy must be conserved for the reaction, the difference in bonding energy (A minus B) accounts for the liberated energy.
  • Hydrocarbons are present in the atmospheres of Jupiter and Saturn, and surface hydrocarbon lakes are even believed to exist on Titan, moon of Saturn.
  •  Organic compounds that contain one or more heteroatoms (any atom other than carbon or hydrogen) are not hydrocarbons.  There are many types of non-hydrocarbon organic compounds.
  • Hydrocarbons include aliphatic hydrocarbons and aromatic hydrocarbons.
  • Aliphatic hydrocarbons are simply non-aromatic hydrocarbons; they do not contain an aromatic carbon ring (described further below).  Most aliphatic hydrocarbons are flammable, and many are used as fuel.  The simplest is methane (CH4).  “Saturated” and “unsaturated”, which can refer to hydrocarbons, are terms that relate to the degree of bonding between carbon atoms, which affects the amount of hydrogen that each carbon atom can bond with.  Aliphatic hydrocarbons include some varieties that are structured as rings, but they are non-aromatic rings; these are called alicyclic, and include, for example, cycloalkanes.
  • Aromatic hydrocarbons contain rings of carbon atoms with alternating single and double bonds in what is known as an aromatic configuration, which typically includes a benzene ring (a simple aromatic ring of 6 carbon atoms, each bonded with a hydrogen atom, having the formula (C6H6).  This particular bonding configuration causes aromatic hydrocarbons to be relatively stable; they form  easily but are difficult to break in chemical reactions.  The name “aromatic” historically stems from the fact that they typically have strong, pungent aromas.
  • Hydrocarbons (as well as other organic compounds) are cyclic if they include a series of atoms joined in a loop or ring (which may or may not be an aromatic ring), and are polycyclic if they include multiple rings in each molecule.
  • Aromatic hydrocarbons include polycyclic aromatic hydrocarbons (“PAHs”), composed of multiple aromatic rings.  PAHs are present in fossil fuels.  Some are carcinogenic, and PAHs are a very significant environmental pollutant.
  • However, PAHs have some intriguing astrobiological aspects.   PAHs appear to be abundant in the universe, including comets, meteorites, nebulae and the interstellar medium, and PAHs are some of the most complex molecules found in space to date.  Furthermore, PAHs are thought to have been abundant in the oceans of early Earth, and have also been found in the upper atmosphere of Titan, moon of Saturn.  It has been suggested that PAHs may potentially have provided starting molecules for the origin of life on Earth, and elsewhere in the universe (a similar suggestion has been made regarding”buckyballs”, which are roughly spherical molecules of only carbon).  More specifically, it has been suggested that PAHs may be involved in, and provide carbon for, reactions that lead to formation of more complex organic compounds, potentially even leading to RNA molecule synthesis (RNA being a key molecule in genetics).
  • Resources include:,,,
Representations of various PAH molecules – carbon-based aromatic rings fused together into various configurations. Could ubiquitous, carbon-rich PAHs have been starter molecules for the origin of life on Earth and elsewhere in the universe? Image: public domain, available at

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