Choosing the Heading Parent

The first step in choosing the index heading parent is to identify the principal functional group (the term characteristic group is now preferred by lUPAC)  

Very occasional deviations from this lUPAC principle may sometimes be made for ease of nomenclature, e.g., in the Dictionary of Natural Products for compounds containing both lactone and carboxylic acid functions, e.g., 3,14-dihydroxycard-20(22)-enolid-19-oic acid. CAS does not do this. 

Only one type of function may be expressed as a suffix in a name. If more than one type of functional group is present, those of lower priority are expressed using substiment prefixes. 

The suffixes -oic acid, -oyl halide, -amide, -nitrile, and -al are used when the functional group is at the end of a carbon chain, as in pentanoic acid. The endings -carboxylic acid, etc., are used when the group is attached to a ring, as in 2-pyridinecarboxylic acid. 

If there are two or more different functional groups, consult Table 3.5, and the functional group highest in the list takes precedence. The other groups become substituents. 

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