The Basics[]
An Homologous Series is a load of compounds that react in similar ways because they have the same reactive bits. All that varies between members of an homologous series is the length of the chain.
Translated into language that will pick up marks in an AS exam:
An homologous series is a family of compounds with the same functional group, varying in length by CH2
The naming system (nomenclature) of all homologous series are based on that of Alkanes, so remember what you learned at GCSE.
| Carbon Atoms | Prefix | Mnemonic |
|---|---|---|
| 1 | Meth- | My |
| 2 | Eth- |
Evil |
| 3 | Prop- | Plane |
| 4 | But- | Bombed |
| 5 | Pent- | Paris |
| 6 | Hex- | Ha |
| 7 | Hept- | Ha |
| 8 | Oct- | Oh |
| 9 | Non- | No |
| 10 | Dec- | Damn it! |
Exam Hint[]
The homologous series studied at AS are:
Alkenes
Alcohols
Halogenoalkanes
but the syllabus also touches on some series studied at more depth at A2, such as Amines, Ketones, Aldehydes, Carboxyllic Acids and Nitriles.
The naming for all of the above is based on that of Alkanes. So a saturated, straight-chain compound with five Carbons will always start with Pentane and end with the appropriate suffix. If it were a Nitrile, etc. If the suffix starts with a vowel then the e on Pentane would be dropped. An Amine would be Pentanamine, an Aldehyde is Pentanal etc.
Side-chains are named as prefixes - with their position noted as the distance from the functional group.
So, this aldehyde is 3-methylbutanal because the Methyl group is on Carbon-3, assuming that the Aldehyde group is on Carbon-1