The International Union of Pure and Applied Chemistry (IUPAC) nomenclature system is a comprehensive set of rules for naming organic compounds in a systematic and standardized manner.
Below is a concise guide to the key rules for naming organic compounds, focusing on aliphatic and carbocyclic compounds, along with functional groups.
1) Identifying the Longest Carbon Chain (Principal Chain)
The first step is to find the longest continuous chain of carbon atoms, which serves as the parent chain or principal chain. This determines the base name of the compound.
Example: A seven-carbon chain is named "heptane."
2) Numbering the Carbon Atoms
Number the carbon atoms in the parent chain starting from the end closest to the first substituent or functional group to give the lowest numbers to substituents.
Example: 4-ethyl-2,3-dimethylheptane, numbering starts from the end nearest to the substituents to give the lowest locants.
3) Identifying and Naming Substituents
Identify all substituents and functional groups attached to the parent chain and use appropriate prefixes (di-, tri-, tetra-, etc.) for identical groups.
Example: In 4-ethyl-2,3-dimethylheptane, the substituents are one ethyl group and two methyl groups.
4) Assigning Locants to Substituents
Specify the position of each substituent or functional group on the parent chain with a locant.
Example: In 4-ethyl-2,3-dimethylheptane, ethyl is at position 4, and methyl groups are at positions 2 and 3.
5) Assembling the Name in Alphabetical Order
Arrange the substituents and functional groups alphabetically, ignoring numerical prefixes, and combine them with the parent chain name.
Example: "4-ethyl-2,3-dimethylheptane" is assembled alphabetically by substituent names.
6) Separating Numbers and Words
Use commas to separate numbers and hyphens to separate numbers from words in the compound name.
Example: "4-ethyl-2,3-dimethylheptane".
7) Determining the Principal Functional Group
The principal functional group, which determines the suffix of the compound's name, takes priority in numbering.
Priority order:
Carboxylic acid (-oic acid)
Ester (-oate)
Aldehyde (-al)
Ketone (-one)
Alcohol (-ol)
Amine (-amine)
Example: In "4-chloro-3-hydroxybutan-2-one", the ketone (-one) is the principal functional group.
8) Naming Substituent Functional Groups
Name other functional groups as substituents, giving the principal functional group the lowest locant.
Example: "4-chloro-3-hydroxybutan-2-one" prioritizes the ketone functional group.
9) Using Numerical Prefixes for Identical Substituents
Arrange substituents alphabetically, ignoring numerical prefixes, to form the complete IUPAC name.
Example: "2,3-dimethyl-4-bromopentane" (two methyl groups and one bromo group).
10) Naming Complex Substituents
For complex substituents, use parentheses and number the substituent relative to its attachment point.
Example: "3-(1-methylethyl) pentane" for a chain with an isopropyl group.
11) Choosing Among Naming Options
Use parentheses for complex substituents and number the substituent’s chain relative to its attachment point.
Example: "3-(1-methylethyl) pentane" for a compound with an isopropyl group attached to a pentane chain.
12) Naming Geometric Isomers (Cis/Trans or E/Z)
Select the naming convention that results in the lowest set of locants for substituents.
Example: "4-chlorohex-3-ene" is preferred over "3-chlorohex-4-ene" for a compound with a double bond and a chlorine substituent
Here is a table of common functional groups with their names, formulas, prefixes, and suffixes:
Functional Group | Name | Formula | Prefix | Suffix |
Alkane | (saturated) | -C-C- | -ane | |
Alkene | (unsaturated) | -C=C- | -ene | |
Alkyne | (unsaturated) | -C≡C- | -yne | |
Alcohol | Hydroxyl | -OH | hydroxy- | -ol |
Ether | Ether | -O- | alkoxy- | |
Amine | Amine | -NH2 | amino- | -amine |
-NHR | ||||
-NR2 | ||||
Aldehyde | Aldehyde | -CHO | formyl- | -al |
Ketone | Ketone | >C=O | oxo- | -one |
Carboxylic Acid | Carboxyl | -COOH | carboxy- | -oic acid |
Ester | Ester | -COOR | alkoxycarbonyl- | -oate |
Amide | Amide | -CONH2 | carbamoyl- | -amide |
-CONHR | ||||
-CONR2 | ||||
Nitrile | Nitrile | -CN | cyano- | -nitrile |
Acid Halide | Acid Halide | -COX | halocarbonyl- | -oyl halide |
Isocyanate | Isocyanate | -NCO | isocyanato- | |
Isothiocyanate | Isothiocyanate | -NCS | isothiocyanato- | |
Thiol | Thiol | -SH | mercapto- | -thiol |
Sulfide | Sulfide | -S- | thio- | |
Disulfide | Disulfide | -S-S- | ||
Thioester | Thioester | -COSR | thio- | |
Haloalkane (alkyl halide) | Halide | -X | fluoro-, chloro-, bromo-, iodo- |
Note that "X" in the acid halide and haloalkane rows represents a halogen atom (F, Cl, Br, or I). "R" in the ester, amide, and thioester rows represents an alkyl or aryl group.