IUPAC Nomenclature of Organic Structures

01: Introduction to Organic Nomenclature
🔹 1.1 What is Nomenclature?

Nomenclature is the system of naming chemical compounds in a structured and standardized way. In organic chemistry, nomenclature is important to:

  • Clearly identify a compound without ambiguity.
  • Communicate information about its structure, functional groups, and carbon skeleton.

In the early days of chemistry, compounds were named based on their source or properties, such as:

  • Formic acid (from ants – Formica)
  • Acetic acid (from vinegar – Acetum)

However, as the number of organic compounds increased rapidly, these common (trivial) names became confusing and inconsistent. So a universal system was developed.

IUPAC stands for the International Union of Pure and Applied Chemistry. It is a global organization responsible for:

  • Standardizing chemical terminology
  • Creating rules for naming compounds
  • Updating nomenclature guidelines regularly

The IUPAC system of nomenclature ensures that each compound has a unique and meaningful name, understood by chemists worldwide.

There are three main types of naming in organic chemistry:

Type

Description

Example

Trivial Name

Historical or common name

Acetic acid

IUPAC Name

Systematic and standardized name

Ethanoic acid

Derived Name

Based on the name of another compound

Methyl alcohol (from methane)

An IUPAC name generally consists of three parts:

  1. Prefix – Substituents or side chains (e.g., methyl, chloro)
  2. Parent Name – Longest continuous carbon chain (e.g., but-, pent-)
  3. Suffix – Functional group (e.g., -ane, -ol, -al, -oic acid)

Example:

2-methylbutanoic acid

  • Prefix: 2-methyl
  • Parent: but
  • Suffix: anoic acid (carboxylic acid)
  • Organic nomenclature allows precise naming of molecules.
  • IUPAC is the official body for setting naming rules.
  • Systematic names follow a prefix-parent-suffix structure.
  • Trivial names still exist but are discouraged in scientific writing.

Q1. What does IUPAC stand for?

Q2. Give the IUPAC name of acetic acid.

Q3. Write the three components of an IUPAC name.

Q4. Why are trivial names not preferred in modern chemistry?

🔹 2.1 Introduction

To name organic compounds systematically, the IUPAC provides a set of rules. These rules help in writing unique, logical names that reflect the structure of the compound.

An IUPAC name is written in the following format:

Prefix + Word root (Parent Chain) + Primary Suffix + Secondary Suffix

  • Prefix: Describes substituents or side chains.
  • Word Root (Parent Chain): Based on the longest carbon chain.
  • Primary Suffix: Indicates the type of carbon-carbon bond (-ane, -ene, -yne).
  • Secondary Suffix: Indicates the main functional group (-ol, -al, -oic acid, etc.).

✅ Step 1: 

Select the Longest Continuous Carbon Chain

  • This chain must include:
    • The principal functional group
    • Double/triple bonds
  • This chain forms the parent hydrocarbon.

📝 Example:

For

CH3–CH2–CH=CH–CH3  

The longest chain = 5 carbons → pent-

✅ Step 2: 

Number the Carbon Chain

  • Number the chain so that:
    • The principal functional group gets the lowest number.
    • If no functional group, then:
      • Give the lowest number to the double/triple bond.
      • Then to substituents.

📝 In the example above:

Double bond starts at C-2 → So start numbering from left: 2-pentene

✅ Step 3: 

Identify and Name the Substituents

  • Groups like –CH₃ (methyl), –Cl (chloro), –NO₂ (nitro) are substituents.
  • List them in alphabetical order.

📝 Example:

CH₃–CH(Cl)–CH₂–CH₃ → 2-chlorobutane

✅ Step 4: 

Name the Compound

Combine the prefix, parent name, and suffix properly.

🧠 Important Rules:

  • Use hyphens (-) to separate numbers from words.
  • Use commas (,) between multiple numbers.
  • Do not put a space between parts of the name.

No. of Carbon Atoms

Word Root

1

Meth-

2

Eth-

3

Prop-

4

But-

5

Pent-

6

Hex-

7

Hept-

8

Oct-

9

Non-

10

Dec-

Bond Type

Primary Suffix

All single bonds

-ane

One double bond

-ene

One triple bond

-yne

Two double bonds

-diene

Two triple bonds

-diyne

Functional Group

Secondary Suffix

Prefix (if not principal group)

Alcohol (–OH)

-ol

hydroxy-

Aldehyde (–CHO)

-al

formyl-

Ketone (–CO–)

-one

oxo-

Carboxylic Acid (–COOH)

-oic acid

carboxy-

Amine (–NH₂)

-amine

amino-

Ester (–COOR)

-oate

alkoxycarbonyl-

  1. CH₃–CH₂–CH₂–OH → 1-propanol
  2. CH₃–CH(CH₃)–CH₃ → 2-methylpropane
  3. CH₂=CH–CH₂–OH → 3-hydroxyprop-1-ene
  4. CH₃–CH=CH–COOH → But-2-enoic acid
  • Follow the steps: Chain → Number → Substituents → Bonds → Functional Group
  • Use correct prefixes/suffixes and hyphenation rules
  • Always give priority to the principal functional group when numbering

Q1. Give the IUPAC name of: CH₃–CH₂–CH(CH₃)–CH₂–CH₃

Q2. Name the compound: CH₃–CH=CH–CH₃

Q3. Identify the word root and suffix in: 3-methylpentan-2-ol

Q4. Write the structure of 2-butanone.

🔹 3.1 Introduction

Organic compounds made of only carbon and hydrogen are called hydrocarbons. They are classified as:

Type

Bond Type

General Formula

Alkanes

Only single bonds

CₙH₂ₙ₊₂

Alkenes

At least one double bond

CₙH₂ₙ

Alkynes

At least one triple bond

CₙH₂ₙ₋₂

Their IUPAC names are based on the number of carbon atoms, the type of bond(s), and the position of these bonds.

✅ General Formula: CₙH₂ₙ₊₂

Alkanes contain only single bonds.

The suffix used is ”-ane”.

📝 Rules:

  1. Select the longest continuous carbon chain.
  2. Number the chain from the end nearest to any substituent.
  3. Name and number the substituents (e.g., methyl, ethyl, etc.).
  4. Arrange substituents alphabetically.
  5. Use prefixes like di-, tri- for repeated groups.

🧪 Examples:

  • CH₄ → Methane
  • CH₃–CH₂–CH₃ → Propane
  • CH₃–CH(CH₃)–CH₃ → 2-methylpropane
  • CH₃–CH₂–CH(CH₃)–CH₃ → 3-methylpentane

✅ General Formula: CₙH₂ₙ

Alkenes contain at least one double bond.

The suffix used is ”-ene”.

📝 Rules:

  1. Choose the longest chain containing the double bond.
  2. Number the chain from the end nearest the double bond.
  3. Give the position of the double bond (smallest possible number).
  4. Name and number any substituents.
  5. Use “diene”, “triene” etc. for multiple double bonds.

🧪 Examples:

  • CH₂=CH₂ → Ethene
  • CH₂=CH–CH₃ → Propene
  • CH₃–CH=CH–CH₃ → But-2-ene
  • CH₂=CH–CH=CH₂ → Buta-1,3-diene

✅ General Formula: CₙH₂ₙ₋₂

Alkynes contain at least one triple bond.

The suffix used is ”-yne”.

📝 Rules:

  1. Choose the longest chain containing the triple bond.
  2. Number the chain such that the triple bond gets the lowest number.
  3. Name and number substituents if any.
  4. Use “diyne”, “triyne” for multiple triple bonds.

🧪 Examples:

  • CH≡CH → Ethyne (acetylene)
  • CH≡C–CH₃ → Propyne
  • CH₃–C≡C–CH₃ → But-2-yne
  • CH≡C–CH=CH₂ → But-1-en-3-yne

When a compound contains both double and triple bonds:

  • Use the suffix “-en-yne”.
  • The chain is numbered to give the lowest possible number to the double or triple bond (whichever comes first).
  • If both have the same number, the double bond gets priority in naming.

🧪 Example:

  • CH₂=CH–C≡CH → But-1-en-3-yne
  • CH₃–CH=CH–C≡CH → Pent-2-en-4-yne

Compound Type

Suffix

Example Name

Alkane

-ane

Butane

Alkene

-ene

But-1-ene

Alkyne

-yne

But-1-yne

Alkene + Alkyne

-en-yne

But-1-en-3-yne

Q1. Give the IUPAC name of:

    CH₃–CH₂–CH=CH₂

Q2. Name the following compound:

    CH₃–C≡C–CH₃

Q3. Write the structure of 2-methylbut-1-ene

Q4. Give the IUPAC name of a compound with both a double and a triple bond in a 4-carbon chain.

🔹 4.1 What is a Functional Group?

A functional group is an atom or group of atoms that imparts specific chemical properties to an organic compound. It defines the class and reactivity of the molecule.

🧪 Examples of Functional Groups:

  • –OH (alcohol)
  • –CHO (aldehyde)
  • –COOH (carboxylic acid)
  • –NH₂ (amine)
  • –Cl, –Br, –NO₂ (substituents)
  • The principal functional group determines the suffix in the compound’s name.
  • Other functional groups are written as prefixes.
  • The carbon chain is numbered such that the principal functional group gets the lowest possible number.

Functional Group

As Principal Group (Suffix)

As Substituent (Prefix)

–OH (Alcohol)

-ol

hydroxy-

–CHO (Aldehyde)

-al

formyl-

>C=O (Ketone)

-one

oxo-

–COOH (Carboxylic acid)

-oic acid

carboxy-

–COOR (Ester)

-oate

alkoxycarbonyl-

–CONH₂ (Amide)

-amide

carbamoyl-

–NH₂ (Amine)

-amine

amino-

–CN (Nitrile)

-nitrile

cyano-

–NO₂ (Nitro group)

—

nitro-

Halogens (F, Cl, Br, I)

—

fluoro-, chloro-, etc.

When more than one functional group is present, use the following order to choose the principal functional group:

Priority

Functional Group

Suffix

1

–COOH (Carboxylic acid)

-oic acid

2

–SO₃H (Sulfonic acid)

-sulfonic acid

3

–COOR (Ester)

-oate

4

–COX (Acyl halide)

-oyl halide

5

–CONH₂ (Amide)

-amide

6

–CN (Nitrile)

-nitrile

7

–CHO (Aldehyde)

-al

8

>C=O (Ketone)

-one

9

–OH (Alcohol)

-ol

10

–NH₂ (Amine)

-amine

Note: Lower priority groups are named as prefixes.

  • Number the carbon chain such that the principal functional group gets the lowest possible number.
  • Multiple functional groups: Choose one as the principal, rest are prefixes.
  • Double/triple bonds get priority after functional groups.

Example 1:

CH₃–CH₂–CH₂–OH → 1-propanol

(–OH is the only group, gets suffix “-ol”)

 

Example 2:

CH₃–CH(OH)–CH₂–COOH

→ 3-hydroxybutanoic acid

(Carboxylic acid is principal, –OH is a prefix)

 

Example 3:

CH₃–CO–CH₂–CH₂–NH₂

→ 4-aminobutan-2-one

(Ketone gets suffix “-one”, amine is a prefix)

  • Functional groups define the reactivity and class of a compound.
  • One principal group gets the suffix; others become prefixes.
  • Follow the priority list to decide the suffix when multiple groups are present.
  • Number the chain to give the principal group the lowest number.

Q1. What is the IUPAC name of CH₃–CH(OH)–CH₂–CHO?

Q2. Name the compound: CH₃–CH(NH₂)–COOH

Q3. Write the IUPAC name of a compound containing both –OH and –CHO groups.

Q4. Arrange the following in order of decreasing priority:  –COOH, –OH, –CHO, –NH₂

🔹 5.1 Introduction

In this section, we apply IUPAC rules to name compounds that contain one or more functional groups. The name must indicate:

  • The type of functional group
  • Its position in the carbon chain
  • Any additional substituents or multiple bonds

We follow the rules learned in earlier chapters, especially:

  • Priority order
  • Suffix/prefix usage
  • Correct numbering
  • Suffix: -ol
  • Number the chain such that –OH gets the lowest number.

🧪 Examples:

  1. CH₃–CH₂–CH₂–OH → Propan-1-ol
  2. CH₃–CH(OH)–CH₃ → Propan-2-ol
  3. CH₃–CH(OH)–CH₂–CH₃ → Butan-2-ol
  • Suffix: -al
  • The –CHO group is always terminal and must be included in the parent chain.
  • Gets position 1, no need to write number.

🧪 Examples:

  1. CH₃–CHO → Ethanal
  2. CH₃–CH₂–CHO → Propanal
  3. CH₃–CH(Cl)–CH₂–CHO → 3-chlorobutanal
  • Suffix: -one
  • Carbonyl group should get the lowest possible number.

🧪 Examples:

  1. CH₃–CO–CH₃ → Propan-2-one (Common: Acetone)
  2. CH₃–CH₂–CO–CH₃ → Butan-2-one
  3. CH₃–CO–CH₂–CH₃ → Butan-2-one
  • Suffix: -oic acid
  • This group is always terminal and has highest priority.
  • Always gets position 1.

🧪 Examples:

  1. CH₃–COOH → Ethanoic acid
  2. CH₃–CH₂–COOH → Propanoic acid
  3. CH₃–CH(CH₃)–COOH → 2-methylpropanoic acid
  • Suffix: -amine
  • If not principal, use prefix amino-.

🧪 Examples:

  1. CH₃–CH₂–NH₂ → Ethylamine or Ethan-1-amine
  2. CH₃–CH(NH₂)–CH₃ → Propan-2-amine
  3. CH₃–CH(NH₂)–COOH → 2-aminopropanoic acid (Here, –COOH is principal)
  • Suffix: -nitrile
  • Nitrile carbon is part of the parent chain, counted as carbon number 1.

🧪 Examples:

  1. CH₃–CN → Ethanenitrile
  2. CH₃–CH₂–CN → Propanenitrile

Rule:

  • Choose the highest priority group to write as suffix.
  • Others are written as prefixes.

🧪 Example 1:

CH₃–CH(OH)–CH₂–COOH

  • –COOH is higher priority → suffix = -oic acid
  • –OH becomes prefix = hydroxy-
  • ✅ Name: 3-hydroxybutanoic acid

🧪 Example 2:

CH₃–CH(NH₂)–COOH

  • –COOH is the principal group
  • ✅ Name: 2-aminopropanoic acid

🧪 Example 3

HO–CH₂–CH=CH–COOH

  • –COOH gets suffix -oic acid
  • –OH becomes prefix hydroxy-
  • Double bond also shown
  • ✅ Name: 4-hydroxybut-2-enoic acid

Functional Group

Suffix

Prefix

Priority (↓)

–COOH

-oic acid

carboxy-

1

–CN

-nitrile

cyano-

2

–CHO

-al

formyl-

3

>C=O

-one

oxo-

4

–OH

-ol

hydroxy-

5

–NH₂

-amine

amino-

6

Halogens, –NO₂

—

chloro-, nitro-

Lowest

Q1. Write the IUPAC name of CH₃–CH(OH)–CH₂–CHO

Q2. Name the compound: CH₃–CH(NH₂)–CH₂–COOH

Q3. Give the IUPAC name of HO–CH₂–CH₂–COOH

Q4. Identify the principal group and give the name of: CH₃–CO–CH₂–CH₂–OH

🔹 6.1 Introduction

Organic compounds can have two or more different functional groups. In such cases, the IUPAC naming system follows strict rules to assign:

  • One principal functional group (gets the suffix),
  • The others as prefixes,
  • A numbering system that gives the principal group the lowest possible number.

Use the priority order of functional groups. The highest priority group becomes the suffix, and all other groups are named as prefixes.

📌 Priority Order (from highest to lowest):

  1. –COOH (Carboxylic acid)
  2. –SO₃H (Sulfonic acid)
  3. –COOR (Ester)
  4. –COCl (Acyl halide)
  5. –CONH₂ (Amide)
  6. –CN (Nitrile)
  7. –CHO (Aldehyde)
  8. >C=O (Ketone)
  9. –OH (Alcohol)
  10. –NH₂ (Amine)
  11. Halogens, –NO₂, etc. (Lowest)
  1. Identify all functional groups present.
  2. Select the principal functional group (highest priority).
  3. Number the longest carbon chain to give the principal group the lowest number.
  4. Name the principal group using suffix, others as prefixes.
  5. Use correct locants (position numbers) for each group.

🧪 Example 1: CH₃–CH(OH)–CH₂–COOH

  • Functional groups: –COOH (carboxylic acid), –OH (alcohol)
  • –COOH has higher priority → suffix: -oic acid
  • –OH becomes prefix: hydroxy-
  • Numbering: from COOH end → OH is on carbon 3
    ✅ Name: 3-hydroxybutanoic acid

🧪 Example 2: CH₃–CO–CH₂–CH₂–OH

  • Functional groups: >C=O (ketone), –OH (alcohol)
  • Ketone has higher priority → suffix: -one
  • Alcohol becomes prefix: hydroxy-
  • Numbering: ketone at position 2, OH at position 4
    ✅ Name: 4-hydroxybutan-2-one

🧪 Example 3: CH₃–CH(NH₂)–COOH

  • Functional groups: –COOH (carboxylic acid), –NH₂ (amine)
  • –COOH gets suffix → amine is prefix
    ✅ Name: 2-aminopropanoic acid

🧪 Example 4: HO–CH₂–CH=CH–COOH

  • Functional groups: –COOH, –OH, and a double bond
  • Principal group: –COOH → suffix: -oic acid
  • OH → prefix: hydroxy-, alkene → -en-
  • Numbering: COOH is carbon 1
    ✅ Name: 4-hydroxybut-2-enoic acid

When three or more functional groups are present:

  • Still choose one principal group (highest priority).
  • Others are treated as prefixes.
  • Ensure clear numbering and use proper order of prefixes alphabetically.

🧪 Example: HO–CH₂–CH(NH₂)–COOH

  • Groups: –COOH, –OH, –NH₂
  • Priority: –COOH > –OH > –NH₂
    ✅ Name: 2-amino-3-hydroxypropanoic acid

If two or more identical lower-priority groups are present, use:

  • di-, tri-, tetra- etc.
  • Combine with correct positions and alphabetical order.

🧪 Example: CH₃–CH(OH)–CH(OH)–COOH

  • Principal group: –COOH
  • Two –OH groups at C2 and C3
    ✅ Name: 2,3-dihydroxybutanoic acid

Step

Description

1

Identify all functional groups.

2

Select principal group using the priority table.

3

Number the chain so the principal group gets the lowest number.

4

Use correct suffix for principal, prefixes for others.

5

Alphabetically arrange prefixes; use correct locants.

Q1. Write IUPAC name of: CH₃–CH(OH)–CH(NH₂)–COOH

Q2. Identify the principal group and name: CH₃–CH(Cl)–CH₂–CHO

Q3. Write the name of: CH₃–CH₂–CO–CH(NH₂)–CH₂–OH

Q4. Arrange the following groups in decreasing order of priority: –OH, –CHO, –COOH, –NH₂, –CN

🔹 7.1 Introduction

Organic compounds can form rings or cycles, either aliphatic (non-aromatic) or aromatic (benzene-like). Their nomenclature involves special rules to represent the ring system, substituents, and any functional groups.

  1. Use the prefix “cyclo-” before the name of the corresponding alkane.
  2. Substituents are placed in alphabetical order.
  3. Number the ring to give substituents the lowest possible numbers.

🧪 Examples:

  • Cyclohexane – A 6-membered carbon ring
  • Methylcyclopentane – A methyl group on cyclopentane
  • 1,2-dimethylcyclobutane – Two methyls on adjacent carbons in a 4-membered ring
  • Functional groups are given priority for numbering.
  • The ring is numbered so the principal group gets position 1.

🧪 Example:

Cyclohexanol → Ring with an –OH group

1-chloro-2-methylcyclopentanol → OH at position 1, Cl at 2, CH₃ at 2

  • Benzene is the parent name.
  • Substituents are indicated with numbers or common names when standard.

🧪 Examples:

Compound

IUPAC Name

C₆H₅–CH₃

Methylbenzene or Toluene

C₆H₅–OH

Hydroxybenzene or Phenol

C₆H₅–NH₂

Aminobenzene or Aniline

When two groups are on the benzene ring:

  • Use prefixes:
    • 1,2- or ortho- (o-)
    • 1,3- or meta- (m-)
    • 1,4- or para- (p-)
  • Number the ring to give lowest locants.

🧪 Examples:

  • 1,3-dinitrobenzene or m-dinitrobenzene
  • 2-bromotoluene – Br is on position 2 relative to CH₃

Use the priority rules for selecting the suffix and prefixes:

  • –COOH, –SO₃H, –OH, –NO₂ are commonly found on benzene rings.

🧪 Examples:

  • Benzoic acid – COOH on benzene
  • 2-hydroxybenzoic acid – OH at position 2
  • 3-nitrophenol – NO₂ at position 3 of phenol

Bicyclic compounds contain two fused or bridged rings.

  • Use the prefix bicyclo[a.b.c]alkane
  • a, b, c = number of carbons in each bridge (excluding the two bridgehead carbons), written in descending order
  • Total number of carbons = name of parent alkane

🧪 Example

  • Bicyclo[2.2.1]heptane (Common: norbornane)
  • Numbering begins at a bridgehead carbon, goes around the longest path first.
  • Substituents are given numbers based on this order.

🔸 Summary Table

Compound Type

Prefix/Suffix

Special Rules

Cycloalkanes

cyclo- + alkane name

Ring numbering for lowest locants

Aromatic rings

benzene, phenyl-

o-/m-/p- or numeric positions

Bicyclic alkanes

bicyclo[a.b.c]alkane

Use bridge length for naming

🔹

Q1. Name the compound

A six-membered ring with an OH and CH₃ group at 1 and 2 positions.

Q2. Give the IUPAC name of:

    C₆H₄(NO₂)₂ with NO₂ groups at 1 and 3 positions.

Q3. Name:

    Bicyclo[2.2.2]octane with a methyl group at position 1.

Q4. Write the name of a compound with a carboxylic acid and an OH group on a benzene ring at positions 1 and 2.

🔹 8.1 Introduction

🔹 8.1 Introduction

While IUPAC nomenclature provides a systematic and universal naming system, many organic compounds are still widely known by their common or trivial names. These names are often historical or based on the source or properties of the compound.

In this section, we compare IUPAC names with commonly used trivial names, especially for:

  • Simple hydrocarbons
  • Aromatic compounds
  • Carboxylic acids, alcohols, aldehydes
  • Important natural or industrial compounds

Common Name

IUPAC Name

Structure

Acetylene

Ethyne

HC≡CH

Isobutane

2-methylpropane

CH₃CH(CH₃)CH₃

Isobutylene

2-methylpropene

CH₂=C(CH₃)CH₃

Common Name

IUPAC Name

Structure

Methyl alcohol

Methanol

CH₃OH

Ethyl alcohol

Ethanol

CH₃CH₂OH

Isopropyl alcohol

Propan-2-ol

CH₃CHOHCH₃

Common Name

IUPAC Name

Structure

Formaldehyde

Methanal

HCHO

Acetaldehyde

Ethanal

CH₃CHO

Acetone

Propan-2-one

CH₃COCH₃

Common Name

IUPAC Name

Source / Notes

Formic acid

Methanoic acid

Found in ants

Acetic acid

Ethanoic acid

Found in vinegar

Propionic acid

Propanoic acid

Found in dairy products

Butyric acid

Butanoic acid

Found in butter

Benzoic acid

Benzenecarboxylic acid

Aromatic acid

Common Name

IUPAC Name

Structure/Use

Toluene

Methylbenzene

CH₃–C₆H₅

Aniline

Aminobenzene

NH₂–C₆H₅

Phenol

Hydroxybenzene

OH–C₆H₅

Xylene

Dimethylbenzene

CH₃–C₆H₄–CH₃

Common Name

IUPAC Name

Structure

Acetyl chloride

Ethanoyl chloride

CH₃COCl

Methyl acetate

Methyl ethanoate

CH₃COOCH₃

Acetamide

Ethanamide

CH₃CONH₂

Feature

Trivial Name

IUPAC Name

Systematic rules

❌ No

✅ Yes

Universally accepted

❌ Not always

✅ Globally used

Easy to remember

✅ Often

❌ Sometimes complex

Indicates structure

❌ Rarely

✅ Always

  • In common language or industry
  • For very simple or well-known compounds
  • When the IUPAC name is too long or confusing

Q1. Give the IUPAC names of the following:

  • (a) Acetone
  • (b) Isobutane
  • (c) Acetic acid

Q2. Match the following:

  • (i) Methanoic acid →
  • (ii) Ethanoic acid →
  • (iii) Propan-2-ol →

Options: A. Acetic acid, B. Isopropyl alcohol, C. Formic acid

Q3. Write the common names of:

  • (a) Methanal
  • (b) Methylbenzene
  • (c) Ethanamide