IUPAC, an international body engaged in developing standards for naming chemical elements and compounds, has laid several rules known as 'IUPAC rules' to identify a molecule by its parent alkane chain and the functional group. It has assigned various suffixes to identify organic chemistry's commonly seen functional groups.
For example, molecules with an alkyl chain and functional group carboxylic acid (-COOH) are called alkanoic acid. Butanoic acid (4CH3-3CH2-2CH2-1COOH) is a four-carbon alkyl chain consisting of carboxyl functional group -COOH. (Refer to IUPAC rules for naming a compound in each category).
Compounds with two or more functional groups are called polyfunctional compounds, and their naming follows a priority order.
1) The functional group with the highest priority order is the principal functional group.
The order of preference is-
Carboxylic acid> Sulfonic acid> Anhydrides> Ester> Acid halide> Amide> Nitrile> Aldehyde> Ketone> Alcohol> Phenols> Thiols> Amines
(Learning tip- functional groups with more bonds to the heteroatom will have higher priority than the ones with the least)
The other functional groups in the molecule; other than the principle functional group are treated as substituents and assigned as a prefix.
Some functional groups that are always substituent groups are-
Halides, alkoxides, azides, alkyl (R-), C6H5, nitroso (N=O), and nitro.
2) The parent alkane chain must be selected in such a way that it must contain the principal functional group and the maximum number of secondary functional groups and multiple bonds (double or triple), if any.
3) While numbering the parent carbon chain for priority, the principal functional group must get the lowest number, followed by double bonds, triple bonds, and substituents.
Principal functional group > double bond > triple bond > substituents
Therefore, the principal functional group always gets numbered 1.
4) The secondary functional groups and the substituent groups are always written as prefixes and written in alphabetical order.
Some of the prefixes used for secondary functional groups are-
If the molecule has multiple halogens, in that case, they are ordered alphabetically. Therefore, bromo comes before chloro, fluoro and iodo.
Example of molecules with multiple functional groups named according to the priority order
1)
The carbon of the -COOH functional group has maximum bonds to the heteroatom Oxygen than carbon of carbonyl (C=O); therefore, it gets the highest priority and the lowest number 1.
The -COOH is the principal functional group, a suffix, and -C=O is the secondary functional group, a prefix.
Prefix- oxo
Suffix- oic acid
Parent six carbon alkane chain- hexane
The name of the compound is- 5-oxohexanoic acid.