4.1 IUPAC naming

The variety of carbon chains and functional groups mean a theoretically unlimited number of molecules are possible. The International Union of Pure and Applied Chemistry (IUPAC) has developed a system for naming any organic molecule.

We can use the following molecule as an example:

We can start by finding the longest straight chain of carbon atoms. There are eight of them, so it will be an octyl compound. One of the terminal (i.e. on the end) carbons has a hydroxy group, so we can call that carbon 1 or C1. We will be counting from 1 to 8 along the carbon chain. (Typically, carbon 1 would be whichever one gives the substituents the lowest numbers; in this case the lowest number is conveniently 1.) Therefore, this is a derivative of octanol, specifically 1-octanol or, more precisely, octan-1-ol. Counting atoms, we find that C3 has an amino (i.e. -NH₂) group, so we will have 3-amino- in the name. The prefix for the -SH group on C4 is mercapto-, while the CH₃O- group on C6 is a methoxy- group. Therefore, this compound is 6-methoxy-4-mercapto-3-amino-octan-1-ol.

To be fair, these IUPAC names can be unweildly, but they do desribe very well the structure of the compound's molecule. Most examples are simpler than this, for example octanol which can be octan-1-ol, octan-2-ol, octan-3-ol, or octan-4-ol, differing by the location of the hydroxy group along the carbon chain.

If the molecule has a double bond, then we also indicate its position by counting the bonds from C1. There are many compounds that can be called hexenol, such as 1-hexen-1-ol ,
2-hexen-1-ol , all the way up to 5-hexen-1-ol , and so on.

As we will see, some of these compounds consist of further isomers.

If the carbon chain is cyclic, then one of the substituents is automatically on C1 and the ring atoms are counted in the direction that results in the lowest numbers of the other substituents.

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