Study Guide

Nomenclature of organic compounds

IB Chemistry HLΒ· Topic 10 Organic Chemistry, 10.1 Introduction, 20.1 Types of organic reactionsΒ· 25 min read

1. IUPAC Rules for Simple Hydrocarbonsβ˜…β˜…β˜†β˜†β˜†β± 6 min

πŸ“˜ Definition

Hydrocarbon

An organic compound consisting only of carbon and hydrogen atoms, classified by the type of carbon-carbon bonds

Example:

Methane (CH4), ethene (C2H4), ethyne (C2H2)

Unbranched hydrocarbons follow a simple naming pattern: the prefix indicates the number of carbon atoms, and the suffix indicates bond type. Alkanes (all single bonds) use -ane, alkenes (at least one C=C) use -ene, and alkynes (at least one C≑C) use -yne. Position numbers are added for multiple bonds to indicate their starting carbon.

πŸ“ Worked Example

Name the unbranched 5-carbon hydrocarbon with one double bond between carbon 1 and 2

  1. 1

    Count the number of carbon atoms: 5 carbons uses the prefix pent-

  2. 2

    Identify bond type: one double bond means it is an alkene, so suffix is -ene

  3. 3

    Number the chain from the end closest to the double bond: the double bond starts at carbon 1

  4. 4

    Combine components to get the IUPAC name: pent-1-ene

2. Naming Branched & Substituted Hydrocarbonsβ˜…β˜…β˜†β˜†β˜†β± 8 min

For branched hydrocarbons, follow these key steps: 1) Find the longest continuous carbon chain (parent chain) that includes any multiple bonds. 2) Number the parent chain starting from the end closest to the first branch or multiple bond to get the lowest possible position numbers. 3) Name substituents (side chains) with the prefix + -yl, then list them alphabetically.

πŸ“˜ Definition

Alkyl Substituent

An alkane missing one hydrogen, used as a side chain attached to the parent hydrocarbon

Example:

Methyl (-CH3), ethyl (-CH2CH3)

πŸ“ Worked Example

Give the correct IUPAC name for an alkane with a 7-carbon parent chain, methyl groups on carbons 2 and 4, and an ethyl group on carbon 3

  1. 1

    List all substituents with their positions: 2-methyl, 4-methyl, 3-ethyl

  2. 2

    Order substituents alphabetically: ethyl comes before methyl (ignore position numbers)

  3. 3

    Combine identical substituents: two methyl groups become dimethyl

  4. 4

    Assemble the full IUPAC name: 3-ethyl-2,4-dimethylheptane

3. Naming Compounds with One Functional Groupβ˜…β˜…β˜…β˜†β˜†β± 6 min

When a compound contains one functional group, the suffix of the name changes to reflect the functional group. The parent chain must always include the functional group, and the chain is numbered to give the functional group the lowest possible position number.

πŸ“ Worked Example

Name a 4-carbon straight chain compound with a hydroxyl (alcohol) group on carbon 2

  1. 1

    Identify parent chain length: 4 carbons = prefix but-

  2. 2

    Identify functional group: hydroxyl = alcohol, so suffix is -ol at position 2

  3. 3

    Combine components to get the IUPAC name: butan-2-ol

4. Polyfunctional Compounds & Priority Rulesβ˜…β˜…β˜…β˜…β˜†β± 5 min

When a compound has more than one functional group, you must prioritize functional groups to determine which is the main group (gets the suffix) and which are named as substituents (use prefixes). Higher priority functional groups always get the lowest possible position number.

Priority (higher = first)

Functional group

Suffix

1

Carboxylic acid

-oic acid

3

Ketone

-one

5

Alcohol

-ol

7

Alkene

-ene

9

Alkane

-ane

πŸ“ Worked Example

Name a 5-carbon compound with a ketone on carbon 2 and a hydroxyl group on carbon 4

  1. 1

    Compare priority: ketone has higher priority than alcohol, so ketone is the main functional group

  2. 2

    Name the parent chain with the main functional group: 5 carbons + ketone on 2 = pentan-2-one

  3. 3

    Treat the lower priority alcohol as a hydroxy substituent at position 4

  4. 4

    Full IUPAC name: 4-hydroxypentan-2-one

5. Common Pitfalls

Wrong move:

Numbering the parent chain starting from the end closest to the first substituent instead of the highest priority functional group

Why:

Functional groups have higher priority than substituents, so they must get the lowest possible position number

Correct move:

Always number the chain to give the highest priority functional group the smallest position number, then minimize substituent positions

Wrong move:

Ordering substituents by position number instead of alphabetical order

Why:

Alphabetical order ignores position numbers and multiplying prefixes (di-, tri-) for alkyl substituents

Correct move:

List substituents alphabetically by name, ignoring multiplying prefixes, and add all position numbers

Wrong move:

Choosing the shortest carbon chain as the parent chain

Why:

The parent chain is defined as the longest continuous carbon chain containing the highest priority functional group

Correct move:

Count all possible continuous carbon chains to find the longest one that includes the main functional group

Wrong move:

Forgetting to add the position number of double/triple bonds

Why:

IB exam questions require position numbers for multiple bonds to award full marks

Correct move:

Always add the position number of the first carbon in the multiple bond before the -ene/-yne suffix

Wrong move:

Naming a terminal carbonyl group as a ketone

Why:

A carbonyl group on the end carbon is an aldehyde, not a ketone

Correct move:

Terminal carbonyl groups are aldehydes (suffix -al), internal carbonyl groups are ketones (suffix -one)

6. Quick Reference Cheatsheet

Carbon count

Prefix

Functional group

Suffix/Prefix

1

meth-

Alkane

-ane

2

eth-

Alkene

-ene

3

prop-

Alkyne

-yne

4

but-

Alcohol

-ol

5

pent-

Aldehyde

-al

6

hex-

Ketone

-one

7

hept-

Carboxylic acid

-oic acid

1-10

any

Halogen

halo- (prefix)

any

any

Hydroxyl (lower priority)

hydroxy- (prefix)

When this came up on past exams

AI-estimated based on syllabus patterns β€” cross-check with official past papers for accuracy. Use only as revision-focus signals.

  • 2025 Β· 1

    Name 3 functional groups from structures

  • 2024 Β· 2

    Draw structure from IUPAC name

  • 2023 Β· 1

    Correct IUPAC name multiple choice

Going deeper

What's Next

Nomenclature is the foundational language of organic chemistry, and consistent accurate naming is required for all subsequent IB Chemistry HL topics including isomerism, reaction mechanisms, and multi-step organic synthesis. Mastering IUPAC rules now will help you avoid confusion when interpreting reaction schemes and drawing products in exam questions. Naming questions are almost always present in both paper 1 and paper 2, so small errors here can cost you easy marks. Taking time to practice applying priority and numbering rules will pay off with consistent full marks on these routine questions.