Homologous Series (thing)

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.

• The base for alkanes is C_nH_2n+2, ie. butane, fourth in the series is C₄H_2x4+2, or C₄H₁₀.
• Alkenes have one double covalent bond between two C atoms, thus their base formula is C_nH_2n.
• Alkynes, having a triple bond instead of a double bond, has the base C_nH_2n-2.
• Alcohols have the base formula C_nH_2n-1OH. I realise I have mentioned H twice. This is for a reason - the lone H does not bond to the carbons, but to the oxygen atom.
• Carboxylic acids have the base formula C_nH_2n-1OOH. One O double bonds to one carbon, the other O single bonds to the same C and also to the lone H.

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 C_nH_2n-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.