IUPAC Nomenclature: Functional Groups
Polyfunctional Compounds & Priority Rules | Organic Chemistry
1. What is a Functional Group?
An atom or group of atoms present in a molecule that determines its specific chemical properties (e.g., $-OH$ for alcohols, $-COOH$ for carboxylic acids).
2. The Priority Table (Decreasing Order)
When multiple functional groups are present, the one higher in this list is the Principal Functional Group (PFG) and determines the suffix. All others are treated as Substituents (Prefixes).
| Class | Group | Prefix (Substituent) | Suffix (Principal) |
|---|---|---|---|
| Carboxylic Acid | $-COOH$ | Carboxy | -oic acid |
| Sulphonic Acid | $-SO_3H$ | Sulpho | -sulphonic acid |
| Ester | $-COOR$ | Alkoxycarbonyl | -oate |
| Acid Halide | $-COX$ | Haloformyl | -oyl halide |
| Amide | $-CONH_2$ | Carbamoyl | -amide |
| Nitrile | $-CN$ | Cyano | -nitrile |
| Aldehyde | $-CHO$ | Formyl / Oxo | -al |
| Ketone | $>C=O$ | Oxo | -one |
| Alcohol | $-OH$ | Hydroxy | -ol |
| Amine | $-NH_2$ | Amino | -amine |
| Alkene | $C=C$ | - | -ene |
| Alkyne | $C \equiv C$ | - | -yne |
Note: Groups like Halogens ($-X$), Nitro ($-NO_2$), Alkoxy ($-OR$), and Alkyl ($-R$) are always treated as substituents (Prefixes).
3. Rules for Polyfunctional Compounds
- Identify PFG: Select the group highest in the priority table. This gives the suffix.
- Select Parent Chain: The longest chain containing the Principal Functional Group (and double/triple bonds if possible).
- Numbering: Number the chain such that the PFG gets the lowest possible number.
- Naming: Arrange substituents (including non-principal functional groups) alphabetically as prefixes.
4. Worked Examples
A. Keto-Acid
Structure: $CH_3-CO-CH_2-COOH$
- Groups: Ketone ($-CO-$) and Carboxylic Acid ($-COOH$).
- Priority: Acid > Ketone. Acid is PFG (suffix: -oic acid). Ketone is substituent (prefix: oxo).
- Numbering: Start from COOH carbon (1). Ketone is at C3.
- Name: 3-Oxobutanoic acid.
B. Hydroxy-Aldehyde
Structure: $HO-CH_2-CH_2-CHO$
- Groups: Alcohol ($-OH$) and Aldehyde ($-CHO$).
- Priority: Aldehyde > Alcohol. Aldehyde is PFG (suffix: -al). Alcohol is substituent (prefix: hydroxy).
- Numbering: Start from CHO (1). OH is at C3.
- Name: 3-Hydroxypropanal.
5. Special Naming (Esters & Anhydrides)
- Esters ($R-COO-R'$): Named as Alkyl alkanoate. The alkyl group ($R'$) attached to Oxygen is written first as a separate word.
Ex: $CH_3-COO-CH_3$ is Methyl ethanoate. - Cyclic Compounds: If carbon-containing groups like $-CHO$ or $-COOH$ are directly attached to a ring, special suffixes are used:
$-CHO \to$ carbaldehyde (e.g., Cyclohexanecarbaldehyde).
$-COOH \to$ carboxylic acid (e.g., Cyclopentanecarboxylic acid).
Practice Quiz
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