3.1.5 Introduction to Organic Chemistry - Nomenclature
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Systematic nomenclature
There are millions of different organic molecules. To give every one of them a different name would involve a huge amount of learning, were no logical pattern to be adopted.
The IUPAC developed the systematic naming system (nomenclature) to allow many of the organic compounds to be named in a logical fashion by considering the structural features of the molecule. This systematic nomenclature arrives at the compound name in the following order:
- Step 1 - Longest carbon chain
- Step 2 - Unsaturation
- Step 3 - Functional groups
- Step 4 - Position of any functional groups
The first step is to identify the longest unbroken chain of carbon atoms. This gives the root of the name:
- 1 carbon = meth-
- 2 carbons = eth-
- 3 carbons = prop-
- 4 carbons = but-
- 5 carbons = pent-
- 6 carbons = hex-
The next step is to look for unsaturation (multiple bonds between carbon atoms) in the chain. If there are only single bonds then the suffix (the bit at the end) is -ane. If there is a doble bond, then the suffix is -ene, and for a triple bond the suffix is -yne.
The folowing step is to identify any functional groups. This means atoms that are NOT hydrogen. Each functional group gives rise to either a different suffix or, if not important enough, to a pre- prefix. At this stage priorities come into play.
- -OH is an alcohol group and takes the suffix -anol, UNLESS there is a more important group on the chain, in which case it becomes a hydroxy- prefix.
- -Cl is a group (along with other halogens) that always becomes a prefix. It is the lowest priority. When there are other halogens they are named in alphabetical order.
- -CHO is the aldehyde group. It becomes the -anal suffix. It is a high priority group.
- -COOH is the carboxylic acid group. It has the ending -anoic acid. it is also high priority (more so, even, than aldehydes).
Branching
This means that there are other hydrocarbon (alkyl) groups attached to the main chain. Each is named according to the number of carbons in the side chain adding the suffix yl. The whole part goes as a prefix.
- 1 carbon side chain = methyl-
- 2 carbon2 side chain = ethyl-
- 3 carbon side chain = propyl-
- etc
Finally, the position of the heteroatoms, or groups on the chain, or sidechains are numbered (counting from the end of the chain that gives rise to the smallest numbers) to avoid ambiguity. They are named alphabetically.
