IUPAC Nomenclature
The universal language of Organic Chemistry. Master the selection of the principal chain, functional group priorities, and the naming of complex cyclic and bicyclic structures.
Module Focus
With millions of organic compounds in existence, memorizing common names is impossible. The International Union of Pure and Applied Chemistry (IUPAC) established a systematic set of rules. In NEET, you must flawlessly apply the hierarchy: Principal Functional Group > Multiple Bonds > Number of Carbon Atoms > Substituents. Let's break down the rules visually.
1. Selection of Principal Carbon Chain (PCC)
The very first step is to identify the longest continuous carbon chain. However, length isn't everything. The chain MUST include the highest priority functional group and maximum multiple bonds.
Rules for Selecting the PCC
- Select the chain containing the Principal Functional Group (PFG).
- If there's no PFG or it's tied, select the chain containing the maximum number of multiple bonds (C=C or C≡C).
- If tied, select the chain with the maximum number of carbon atoms.
- If still tied, select the chain that gives the maximum number of substituents (side chains).
The continuous chain has 7 carbons (Heptane). Numbering from the left gives substituents at 3, 4, 5. Numbering from the right gives 3, 4, 5. Wait, structure shows: 1, 2, 3(Me), 4(Me), 5(Me), 6, 7. Correct IUPAC: 3,4,5-trimethylheptane.
2. The Rule of Lowest Locant
Once the PCC is selected, you must number it. Numbering begins from the end that gives the lowest number to the principal functional group, then to multiple bonds, and finally to substituents.
Many students use the "lowest sum rule" to decide numbering. IUPAC officially discarded this. You MUST use the First Point of Difference Rule.
Compare the locant sets term by term. The set that has the lower number at the FIRST point of difference is correct, regardless of the total sum.
Right-to-Left: 3, 4, 9 (Sum = 16)
First Point of Difference: Compare (2) vs (3). Since 2 is lower than 3, the Left-to-Right numbering is CORRECT, even though its sum (17) is higher!
Name: 2,7,8-trimethyldecane
3. Priority Order of Functional Groups
When a molecule has more than one functional group, one is chosen as the Principal Functional Group (PFG) and acts as the secondary suffix. All other groups are treated as substituents (prefixes).
-COOH > -SO₃H > -COOCO- > -COOR > -COX > -CONH₂ > -CN > -NC > -CHO > >C=O > -OH > -SH > -NH₂
(Carboxylic Acid > Sulphonic Acid > Anhydride > Ester > Acid Halide > Amide > Nitrile > Isocyanide > Aldehyde > Ketone > Alcohol > Thiol > Amine)
| Functional Group | Prefix (If Substituent) | Suffix (If Principal) |
|---|---|---|
| -COOH | carboxy | -oic acid / -carboxylic acid |
| -COOR | alkoxycarbonyl | alkyl ... -oate |
| -CONH₂ | carbamoyl | -amide / -carboxamide |
| -CN | cyano | -nitrile / -carbonitrile |
| -CHO | formyl / oxo | -al / -carbaldehyde |
| >C=O | oxo | -one |
| -OH | hydroxy | -ol |
| -NH₂ | amino | -amine |
PCC: 6 carbons (hexanoic acid).
Numbering: Right to left (COOH gets 1).
Substituents: -OH at C4 (hydroxy), =O at C5 (oxo). Alphabetical order: hydroxy before oxo.
Name: 4-hydroxy-5-oxohexanoic acid
4. Alicyclic, Bicyclo, and Spiro Compounds
Two rings sharing two or more atoms. The shared atoms are "bridgehead" carbons.
- Format: Bicyclo[x.y.z]alkane
- x, y, z are the number of carbons in each path between bridgeheads, listed in DESCENDING order (largest to smallest).
- Numbering starts at a bridgehead, goes along the longest path, then medium, then shortest.
Bicyclo[2.2.1]heptane
Two rings sharing exactly ONE carbon atom (the spiro carbon).
- Format: Spiro[x.y]alkane
- x, y are the number of carbons in each ring (excluding the spiro carbon), listed in ASCENDING order (smallest to largest).
- Numbering starts in the smaller ring, adjacent to the spiro carbon, goes around, through the spiro, into the larger ring.
Spiro[3.4]octane
5. Aromatic Compounds (Benzene Derivatives)
Benzene acts as the parent chain. If the principal functional group is directly attached to the benzene ring, special suffixes are used.
NEET Grand Test: Nomenclature
15 High-Order Thinking Questions testing functional group priorities, complex branching, and strict IUPAC rules.
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