§ 8.2NCERT Class 11 · Chemistry · Chapter 8
IUPAC Systematic Nomenclature of Organic Compounds
To eliminate ambiguity in naming the millions of existing organic molecules, the International Union of Pure and Applied Chemistry (IUPAC) devised a standardized logical framework. Every organic structure maps to a unique systematic name composed of five elemental building blocks.
1. Anatomy of an IUPAC Name
IUPAC Name = [Secondary Prefix] + [Primary Prefix] + [Root Word] + [Primary Suffix] + [Secondary Suffix]
- Root Word: Indicates the number of carbon atoms in the principal continuous parent carbon chain (C1: Meth-, C2: Eth-, C3: Prop-, C4: But-, C5: Pent-, C6: Hex-, C7: Hept-, C8: Oct-, C9: Non-, C10: Dec-).
- Primary Suffix: Denotes the saturation state of the parent chain (−ane for all C−C single bonds, −ene for double bonds, −yne for triple bonds).
- Secondary Suffix: Denotes the highest-ranking principal functional group (e.g. −oic acid, −al, −one, −ol, −amine, −nitrile).
- Primary Prefix: Distinguishes cyclic from acyclic molecules (−cyclo− for alicyclic rings, −bicyclo−, −spiro−).
- Secondary Prefix: Identifies subordinate functional groups (halo, nitro, alkoxy) and alkyl branches attached as substituents to the parent chain.
2. Principal Functional Group Priority Hierarchy
When an organic molecule contains multiple functional groups, one group is selected as the principal functional group according to strict IUPAC precedence and designated as the secondary suffix. All remaining functional groups are relegated to substituent status and cited as secondary prefixes:
| Priority Rank | Functional Group Class | Formula Structure | Prefix Name (Subordinate) | Suffix Name (Principal) |
|---|---|---|---|---|
| 1 | Carboxylic Acid | −COOH | Carboxy− | −oic acid |
| 2 | Sulphonic Acid | −SO3H | Sulpho− | −sulphonic acid |
| 3 | Acid Anhydride | −(CO)−O−(CO)− | — | −oic anhydride |
| 4 | Ester | −COOR | Alkoxycarbonyl− | alkyl ... −oate |
| 5 | Acid Halide | −COX (X = Cl, Br) | Halocarbonyl− | −oyl halide |
| 6 | Amide | −CONH2 | Carbamoyl− | −amide |
| 7 | Nitrile (Cyanide) | −C≡N | Cyano− | −nitrile |
| 8 | Aldehyde | −CHO | Formyl− / Oxo− | −al |
| 9 | Ketone | >C=O | Oxo− / Keto− | −one |
| 10 | Alcohol | −OH | Hydroxy− | −ol |
| 11 | Thiol | −SH | Mercapto− | −thiol |
| 12 | Amine | −NH2 | Amino− | −amine |
| 13 | Alkene | −C=C− | — | −ene |
| 14 | Alkyne | −C≡C− | — | −yne |
| Subordinate | Halogen, Nitro, Alkoxy | −X, −NO2, −OR | Halo−, Nitro−, Alkoxy− | Always Prefixes |
3. Core IUPAC Rules for Branched Alkanes & Polyfunctionals
- Longest Chain Rule: Select the longest continuous carbon chain as the parent skeleton. If two chains possess equal length, select the one bearing the larger number of side-chains/substituents.
- Lowest Locant Rule: Number the parent carbon chain from the end that assigns the lowest numerical locant set to substituents at the first point of difference (e.g., set (2, 7, 8) is preferred over (3, 4, 9) because 2 < 3).
- Alphabetical Ordering: Different alkyl substituents are cited in strict alphabetical order (e.g. ethyl before methyl). Numerical multiplying prefixes like di-, tri-, tetra- are disregarded in alphabetization, but prefixes iso- and neo- are considered part of the alkyl name.
- Polyfunctional Directives:
- Number the chain starting from the terminal carbon giving the principal functional group the lowest possible locant.
- When three or more identical carbon-containing terminal functional groups (−COOH, −CHO, −CN) are linked directly to a parent carbon chain, none of their carbon atoms is included in the parent chain, and the suffix −carboxylic acid / −carbaldehyde / −carbonitrile is appended (e.g., Propane-1,2,3-tricarboxylic acid).
4. Nomenclature of Substituted Benzene Derivatives
- Monosubstituted Benzenes: Substituent prefix is prepended to 'benzene': Chlorobenzene, Nitrobenzene, Bromobenzene. Many common historical names are officially retained by IUPAC: Toluene (Methylbenzene), Aniline (Benzenamine), Phenol (Hydroxybenzene), Anisole (Methoxybenzene), Benzoic acid (Benzenecarboxylic acid), Benzaldehyde.
- Disubstituted Benzenes: Positional isomerism is indicated by locants (1,2- = ortho; 1,3- = meta; 1,4- = para). In IUPAC names, numerical locants are mandatory (e.g., 1,2-Dichlorobenzene, 1-Chloro-4-methylbenzene).
- Principal Functional Group Priority on Benzene: If a benzene ring carries a principal functional group, the ring carbon attached to that group is designated C1 (e.g., 4-Bromophenol, 2-Nitrobenzoic acid).
- Phenyl vs. Benzyl: When an aromatic ring is attached to an aliphatic chain containing a functional group or a longer chain (≥ 7 carbons), the benzene ring is treated as a substituent prefix termed Phenyl (−C6H5). The group −CH2C6H5 is termed Benzyl.
JEE & NEET Solved Practice Problems
Problem 1: Write the systematic IUPAC name for the following compound: CH3−CH(OH)−CH2−CO−CH2−COOH.
Solution:
1. Identify the functional groups present: −COOH (carboxylic acid), >C=O (ketone), and −OH (alcohol).
2. From the IUPAC priority table: −COOH > >C=O > −OH. Hence, −COOH is the principal functional group (suffix: −oic acid), while ketone and alcohol are subordinate prefixes (oxo− and hydroxy−).
3. Number the longest continuous chain starting from the carboxylic carbon as C1:
C1: COOH, C2: CH2, C3: C=O, C4: CH2, C5: CH(OH), C6: CH3 (6-carbon hexanoic acid parent).
4. Substituents with locants: 5-hydroxy and 3-oxo. Alphabetical arrangement: 'hydroxy' precedes 'oxo'.
IUPAC Name: 5-Hydroxy-3-oxohexanoic acid.
1. Identify the functional groups present: −COOH (carboxylic acid), >C=O (ketone), and −OH (alcohol).
2. From the IUPAC priority table: −COOH > >C=O > −OH. Hence, −COOH is the principal functional group (suffix: −oic acid), while ketone and alcohol are subordinate prefixes (oxo− and hydroxy−).
3. Number the longest continuous chain starting from the carboxylic carbon as C1:
C1: COOH, C2: CH2, C3: C=O, C4: CH2, C5: CH(OH), C6: CH3 (6-carbon hexanoic acid parent).
4. Substituents with locants: 5-hydroxy and 3-oxo. Alphabetical arrangement: 'hydroxy' precedes 'oxo'.
IUPAC Name: 5-Hydroxy-3-oxohexanoic acid.
Problem 2: Give the correct IUPAC name for the compound: CH≡C−CH2−CH=CH−CH3.
Solution:
1. The chain contains 6 carbons (hex-), with one double bond and one triple bond.
2. Numbering options:
- Left to right: 1-yne, 4-ene ⇒ locant set (1, 4).
- Right to left: 2-ene, 5-yne ⇒ locant set (2, 5).
3. By the lowest locant rule, (1, 4) is lower than (2, 5). Therefore, numbering starts from the left:
C1≡C2−C3H2−C4H=C5H−C6H3.
4. Combining 'ene' and 'yne' with terminal 'e' dropped before vowel 'y':
IUPAC Name: Hex-4-en-1-yne.
1. The chain contains 6 carbons (hex-), with one double bond and one triple bond.
2. Numbering options:
- Left to right: 1-yne, 4-ene ⇒ locant set (1, 4).
- Right to left: 2-ene, 5-yne ⇒ locant set (2, 5).
3. By the lowest locant rule, (1, 4) is lower than (2, 5). Therefore, numbering starts from the left:
C1≡C2−C3H2−C4H=C5H−C6H3.
4. Combining 'ene' and 'yne' with terminal 'e' dropped before vowel 'y':
IUPAC Name: Hex-4-en-1-yne.
Frequently Asked Questions
Q1. What is the structural anatomy of an IUPAC name?
A systematic IUPAC name consists of five key components arranged in sequence: Secondary Prefix (substituents/side-chains) + Primary Prefix (cyclo/bicyclo/spiro) + Root Word (longest continuous carbon chain length) + Primary Suffix (degree of saturation: -ane, -ene, -yne) + Secondary Suffix (principal functional group: -oic acid, -al, -one, -ol, -amine).
Q2. What is the strict decreasing priority order of principal functional groups in IUPAC naming?
The IUPAC priority order is: -COOH (carboxylic acid) > -SO3H (sulphonic acid) > -(CO)2O (acid anhydride) > -COOR (ester) > -COX (acid halide) > -CONH2 (amide) > -CN (nitrile) > -NC (isocyanide) > -CHO (aldehyde) > >C=O (ketone) > -OH (alcohol) > -SH (thiol) > -NH2 (amine) > -C=C- (alkene) > -C#C- (alkyne). All substituents like -NO2, -OR (alkoxy), and -X (halo) are always treated as prefixes.
Q3. How is a tie between a double bond and a triple bond resolved in numbering?
First, the carbon chain is numbered to give the lowest locant set to all unsaturations combined. If a tie occurs where a double bond and a triple bond receive identical locants from opposite ends of the chain, the double bond is given lower locant numbering priority because 'ene' precedes 'yne' alphabetically (e.g. pent-1-en-4-yne, NOT pent-4-en-1-yne).
Your progress
Saved on this device only — no account, no sign-in.