Simple organic compounds: alkanes

Methane (1 carbon), ethane (2 carbons)

In the previous post, I casually mentioned methane and ethane in the atmosphere of Saturn’s moon Titan. I’ve started with the terms in that post and worked back to the supporting concepts so you can read until you get to things you already know.

  • Methane and ethane are the simplest hydrocarbons.
  • Hydrocarbons are molecules that are comprised entirely of atoms of hydrogen and carbon. Specifically, they have no oxygen, which is the commonest other component of organic molecules.
    • Incidentally, a molecule that contains hydrogen, carbon, and oxygen is a carbohydrate, a completely different kind of molecule. In his book Chariots of the Gods, Emmanuel Velikovsky casually changed hydrocarbons (fuel) into carbohydrates (food) with a twist of the typewriter, demonstrating that he knew nothing about chemistry.
  • Organic molecules contain carbon and are more complex than carbon dioxide (CO2). They’re called organic because a hundred years ago, we thought that only biological processes could form them: that’s all that scientists had ever observed. Since then, we have learned to synthesize them step by step, observed them forming from inorganic compounds, and detected them in interstellar clouds. The chemistry of carbon is called organic chemistry.
  • The smallest alkane is methane, with one carbon. Ethane has two, propane has three, butane has four. After that they use Greek numeric prefixes pentane, hexane, heptane, octane, and so on.
  • Alkanes are saturated hydrocarbons: saturated means that every available bond is filled: the molecule is saturated with hydrogen and cannot take up any more. There are no double bonds on any of the carbon atoms in an alkane. They are sometimes said to have a carbon “backbone.” You can read more about alkanes. Hydrogen atoms tend to clutter up chemical structure diagrams, so they are often left out: just assume that a hydrogen atom fills every unspecified bond.
  • Carbon atoms can form four covalent (electron-sharing) bonds, so each carbon atom can connect to four other atoms. This is the secret to its success in building large, even gigantic molecules and why we think that other life might be “carbon-based.”
Tetrahedral structure of methane


  • The carbon bonds naturally radiate to the four tips of a triangular pyramid with the nucleus in the centre: carbon molecules are not flat. Methane is a tetrahedral molecule. molecules with carbon chains are, at the very least, crinkled. Thus, even the simple structure of a three-carbon molecule has two conformations, with the ends of the molecule bent towards or away from each other.

Carbon molecules showing "handedness"

  • If a carbon atom has different atoms attached to its bonds, it can form a molecule with mirror image versions like left-handed or right-handed mittens. No matter which way you turn them, you can’t superimpose them. This is called chirality (handedness) and the molecules are called enantiomers of each other.
  • Carbon has 6 protons in its nucleus; otherwise, it would be something else: the number of protons defines the element. If the nucleus has more or fewer neutrons, it’s an isotope of the element. A carbon atom usually has 6 neutrons but there’s an isotope with 8 neutrons, which is slightly less stable and decays over time. [Corrected as noted below.]
  • An atom has a nucleus surrounded by electrons. A nucleus is built of protons (positive charge) and neutrons (no charge) surrounded at some distance by electrons with negative charge.

2 Responses to “Simple organic compounds: alkanes”

  1. OrchidGrowinMan Says:

    Big “Oops” on the nuclear configuration paragraph:
    s/b 6p +, 6(8)n


  2. monado Says:

    Sorry, I meant to correct that before I hit “Publish”. You’re right of course. Six protons, 6 or 8 neutrons, total mass 12 or 14.

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