As featured on the Everything2 Podcast, season 1, episode 9.


A chemistry term referring to any given group of hydrocarbons that differ from the next one in the series by CH2. Alkanes, for example, start with the gas methane, with the molecular formula CH4. The next alkane is ethene, with the molecular formula C2H6. It continues, with propane being C3H8. It is easy to work out the molecular formula for any given hydrocarbon, assuming that you know the base of the homologous series.

To name the compounds, follow these guidelines:

  • If naming an alkane, make it end with -ane. If naming an alkene, make it end with -ene, and so on. Alcohols end with -anol and carboxylic acids end with -anoic acid.
  • Find the amount of carbons in the molecule and name it according to this table:
    • 1=meth-
    • 2=eth-
    • 3=prop-
    • 4=but-
    • 5=pent-
    • 6=hex-
    • 7=hept-
    • 8=oct-
    • 9=non-
    • 10=dec-
    For more complex hydrocarbons, search the Internet for a bigger table.
  • Some carbon molecules may be bonded with Cl or F atoms, in this case add either chloro or fluoro out the front.
  • Isomers of alkenes and alkynes should be named according to the lowest-numbered carbon atom the double-bond is located.
  • If you really want to be technical, sometimes (especially with higher hydrocarbons) chlorines and fluorines will be bonded at different carbons (making different isomers of that particular chloro/fluorocarbon), and there may even be multiple chlorines or fluorines. Name them according to the number of the carbon atom that they are bonded to. You can also get methyls, ethyls and higher, which have a base of CnH2n-1. Include them.
  • The result may or may not look something like this:

3,6-dimethyl 4-fluoro oct-2-ene

So what happens if you have two compounds with the same molecular formula? You're gonna have to look. These are also isomers on a more complex level, and can be a pain in the neck. Say you've got two C3H4 molecules that are chemically different. Is that possible? I hear you ask. The answer is yes.

In this example, I'm referring to an isomer of propene and one of propyne. If an alkane has more than one double bond, it is called a diene, triene etc. The propene I'm talking about is one with the middle C double-bonded to each of the other two Cs, and two hydrogens single bonded to each of the outside carbons. That satisfies C3H4, and the bonding requirements of each atom. The "normal" isomer of propyne is the other isomer.


If you have a headache by now, I don't blame you. This took me the best part of two weeks to learn.

Log in or register to write something here or to contact authors.