Organic Chemistry • ICSE Class 10

IUPAC Nomenclature of Organic Compounds

Rules, Solved Examples & Board-Exam Practice Questions

By Dr. A. K. Saxena  |  CHEMSAK — Chemistry by Saxena A.K.

In this post

  1. Why This Topic Matters
  2. What Does IUPAC Mean?
  3. The Four Parts of an IUPAC Name
  4. Step 1 — Word Root (Number of Carbons)
  5. Step 2 — Numbering the Main Chain
  6. Step 3 — Primary Suffix
  7. Step 4 — Secondary Suffix
  8. Step 5 — Prefix
  9. Worked Examples
  10. Quick Reference Tables
  11. Practice Questions
  12. Exam Tip

Why This Topic Matters

If there's one organic chemistry skill that shows up in every single ICSE Class 10 board paper — 2008 through 2025 — it's IUPAC nomenclature. Sometimes you're asked to name a given structure. Sometimes you're asked to draw the structure for a given name. Either way, this topic alone can account for several marks on your paper, and it's completely rule-based — which means it's one of the easiest places to score full marks once you learn the system properly.

This post breaks down the IUPAC naming system exactly as it's taught in the ICSE Class 10 syllabus (Chapter 9: Organic Chemistry), with worked examples and real board-exam-style questions at the end.

What Does IUPAC Mean?

IUPAC stands for the International Union for Pure and Applied Chemistry. Before IUPAC rules existed, organic compounds were named based on their origin, properties, colour, or discoverer — for example, methane was called "marsh gas" because it was found in marshy areas, and methanoic acid was called "formic acid" because it was first obtained from red ants (formica is Latin for ant). This non-scientific naming became impossible to manage as the number of known organic compounds grew — organic compounds today outnumber all other known compounds roughly five times over.

IUPAC nomenclature fixes this by deriving every compound's name systematically from the name of the corresponding alkane.

The Four Parts of an IUPAC Name

Every IUPAC name is built from up to four parts, always in this order:

Prefix + Word Root + Primary Suffix + Secondary Suffix
PartWhat it represents
PrefixSide chains (alkyl groups) or halogen substituents
Word rootThe number of carbon atoms in the main chain
Primary suffixThe type of carbon–carbon bonding (single, double, or triple)
Secondary suffixThe functional group present
3-Bromo-but-2-en-1-al
  • Prefix → 3-Bromo (bromine at carbon 3)
  • Word root → but (4 carbons)
  • Primary suffix → ene (double bond, at position 2)
  • Secondary suffix → al (aldehyde group, at position 1)

Step 1: Identify the Word Root (Number of Carbons)

The word root simply tells you how many carbon atoms are in the main chain.

No. of C atomsWord root
1Meth
2Eth
3Prop
4But
5Pent
6Hex
7Hept
8Oct
9Non
10Dec
Rule for selecting the main chain Always pick the longest continuous carbon chain (LCCC) — the one with the maximum number of side chains or functional groups attached to it, not just the longest chain on paper.

Step 2: Numbering the Main Chain

Once you've picked the main chain, number the carbons using these rules:

  • Case 1 — Hydrocarbons (no functional group): Start numbering from the end closer to the side chain (alkyl group).
  • Case 2 — Functional group present: Start numbering from the end closer to the functional group.
  • Case 3 — Functional group carbon itself (e.g., –CHO, –COOH): The carbon of the functional group is always numbered 1.

These locant numbers are then used to indicate the position of side chains, double bonds, triple bonds, and functional groups in the final name.

Step 3: Primary Suffix — Based on Carbon–Carbon Bonding

Bond typePrimary suffix
C — C (single)-ane
C = C (double)-ene
C ≡ C (triple)-yne

So: but + ane = butane; but + ene = butene; but + yne = butyne.

Step 4: Secondary Suffix — Based on Functional Group

The secondary suffix is what tells you what kind of compound you're dealing with.

StructureFunctional GroupSecondary Suffix
–OHAlcohol-ol
–CHOAldehyde-al
>C=OKetone-one
–COOHCarboxylic acid-oic acid
Combining suffix + word root When the primary suffix ends in a consonant, the secondary suffix's vowel is retained (e.g., but + diene = butadiene). When the primary suffix starts with a vowel, an “e” from “-ane” is dropped before adding the secondary suffix:
  • Methane − e + ol = Methanol
  • Ethane − e + al = Ethanal
  • Propane − e + one = Propan-2-one
  • Ethane − e + oic acid = Ethanoic acid

Step 5: Prefix — Side Chains and Halogens

If side chains or halogen atoms are attached to the main chain, they're named as a prefix, placed before the word root, with a locant number showing their position.

SubstituentPrefix
–CH3Methyl
–C2H5Ethyl
–CH2CH2CH3Propyl
–CH(CH3)2Isopropyl
–ClChloro
–BrBromo
–IIodo

If the same group appears more than once, use a numerical prefix with locants for each position:

Number of identical groupsNumerical prefix
2Di
3Tri
4Tetra
5Penta

Putting It All Together — Worked Examples

Example 1: CH3–CH(CH3)–CH2–CH2–CH3
  • Main chain: 5 carbons → pent
  • No double/triple bond → ane
  • One methyl group at C-2 → 2-methyl
IUPAC name: 2-methylpentane
Example 2: H3C–CH2–CH2–CH(Cl)–CH2–CHO
  • Main chain (starting from the –CHO carbon as C-1): 6 carbons → hex
  • Numbering: CHO(1)–CH2(2)–CH(Cl)(3)–CH2(4)–CH2(5)–CH3(6), so chlorine falls at C-3 → 3-chloro
  • Aldehyde group → -al
IUPAC name: 3-chlorohexanal
Example 3: CH3–CH(OH)–CH3
  • Main chain: 3 carbons → prop
  • –OH at C-2 → -ol, numbering from the end nearer –OH
IUPAC name: Propan-2-ol

Quick Reference: Homologous Series and Their Names

SeriesGeneral FormulaBondSuffixFirst Member
AlkaneCnH2n+2C–C-aneMethane
AlkeneCnH2nC=C-eneEthene
AlkyneCnH2n-2C≡C-yneEthyne
Alkyl groupCnH2n+1C–C-ylMethyl

Common Name vs IUPAC Name — Don't Get These Mixed Up

Board papers often test whether you know both names for the same compound. Here's a summary table worth memorising:

Common NameIUPAC Name
EthyleneEthene
AcetyleneEthyne
Ethyl alcoholEthanol
Methyl alcoholMethanol
Formic acidMethanoic acid
Acetic acidEthanoic acid
AcetaldehydeEthanal
AcetonePropanone
Diethyl etherEthoxy ethane
Isobutane2-methylpropane
Neopentane2,2-dimethylpropane
n-butaneButane

Practice: Solve These (ICSE Board-Style)

Name these structures:

  1. H3C–CH(CH3)–CH2–COOH
  2. H3C–CH(OH)–CH3
  3. HC≡C–HC=CH2
  4. H3C–CH2–CH2–CH(Cl)–CH2–CHO

Draw the structural formula for:

  1. 2,2-dimethylpropane
  2. But-1,3-diene
  3. 2-methyl pentan-2-ol
  4. 3-methyl but-1-yne

(Adapted from ICSE Chemistry-10 textbook practice questions, Chapter 9.)

Exam Tip

Score more, faster In the ICSE board paper, nomenclature questions are rarely asked in isolation — they're usually combined with equation-writing or structure-drawing in the same question. So the fastest way to build speed is to practise going in both directions: structure → name, and name → structure, until both feel automatic. This single skill underpins almost every other reaction-writing question in the Organic Chemistry chapter, since you can't balance or complete an equation correctly if you can't name the product.
ICSE Chemistry Class X by Dr. A.K. Saxena — Shri Balaji Publication

Want to Go Deeper? Explore the Full Chapter

ICSE Chemistry – Class X, by Dr. A. K. Saxena (Shri Balaji Publication)

This post covers IUPAC nomenclature in depth, but it's just one part of Chapter 9 – Organic Chemistry. For complete, exam-ready coverage of the whole ICSE Class 10 Chemistry syllabus, this textbook includes:

  • Topicwise questions for practice
  • Concept-based & application-based questions
  • Examiner's Favourite – Top 111 Equations
  • Competency-based questions (Recall to Synthesis)
  • Genuine Past Year Questions (PYQs) from ICSE board papers
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