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.
This system ensures clarity and uniformity in the communication of chemical information among scientists.
Below, we provide a detailed guide based on the general rules of IUPAC nomenclature, illustrated with examples
for each rule, focusing on open-chain (aliphatic) compounds, carbocyclic compounds, and compounds with functional groups, accommodating structures with up to 10 carbon atoms.
1. Identifying the Longest Carbon Chain or 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 molecule with a seven-carbon chain is named based on "heptane" as its base name.
2. Numbering the Carbon Atoms in the Parent Chain
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: For a molecule named 4-ethyl-2,3-dimethylheptane, numbering starts from the end nearest to the substituents to give the lowest locants.
3. Identifying and Naming Substituents and Functional Groups
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 and Functional Groups
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" demonstrates the correct use of commas and hyphens.
7. Determining the Principal Functional Group
The principal functional group, which determines the suffix of the compound's name, takes priority in numbering.
Order of Priority for Common Functional Groups:
1. Carboxylic acid (-COOH): -oic acid
2. Ester (-COOR): -oate
3. Acid halide (-COX): -oyl halide
4. Amide (-CONH₂, -CONHR, or -CONR₂): -amide
5. Nitrile (-CN): -nitrile
6. Aldehyde (-CHO): -al
7. Ketone (>C=O): -one
8. Alcohol (-OH): -ol
9. Thiol (-SH): -thiol
10. Amine (-NH₂, -NHR, or -NR₂): -amine
11. Ether (-OR): alkoxy- or aryloxy-
12. Halides (-F, -Cl, -Br, -I): fluoro-, chloro-, bromo-, or iodo-
8. Naming and Numbering 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. Assembling the Name with Substituents in Alphabetical Order
Arrange substituents alphabetically, ignoring numerical prefixes, to form the complete IUPAC name.
Example: "4-chloro-3-hydroxybutan-2-one" is correctly assembled.
10. Naming Compounds with Multiple Identical Substituents
Use numerical prefixes (di-, tri-, tetra-, etc.) to indicate the quantity of identical substituents.
Example: "2,3-dimethyl-4-bromopentane" for a compound with two methyl groups and one bromo group.
11. Naming Compounds with Complex Substituents
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. Choosing Among Multiple Naming Conventions
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.
13. Naming Compounds with Cis/Trans or E/Z Isomers
Use cis/trans or E/Z notation for compounds with geometric isomers.
Example: "(Z)-4-chlorohex-3-ene" if substituents are on the same side; "(E)-4-chlorohex-3-ene" if on opposite sides.
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.