# Nomenclature of organic compounds

> IB Chemistry HL · IB Chemistry HL
> Source: https://www.owlsprep.com/study/ib-chemistry-hl-u6-nomenclature-of-organic-compounds/

This module covers core IUPAC rules for naming common organic compounds, including hydrocarbons and key functional groups. You will learn to select parent chains, number substituent positions, and apply functional group priority for accurate naming.

**Prerequisites:** [Functional group identification](https://www.owlsprep.com/study/ib-chemistry-hl-u6-functional-group-identification/); [Organic structural formulas](https://www.owlsprep.com/study/ib-chemistry-hl-u6-organic-structural-formulas/)

## Learning objectives

- Apply IUPAC rules to name alkanes, alkenes, alkynes, and substituted alkanes
- Name organic compounds containing common functional groups
- Draw structural formulas from given IUPAC names
- Apply functional group priority rules for polyfunctional compounds

## IUPAC Rules for Simple Hydrocarbons

**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.

**Worked example:** Name the unbranched 5-carbon hydrocarbon with one double bond between carbon 1 and 2

1. Count the number of carbon atoms: 5 carbons uses the prefix `pent-`
2. Identify bond type: one double bond means it is an alkene, so suffix is `-ene`
3. Number the chain from the end closest to the double bond: the double bond starts at carbon 1
4. Combine components to get the IUPAC name: **pent-1-ene**

## Naming Branched & Substituted Hydrocarbons

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.

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

*Example:* Methyl (-CH3), ethyl (-CH2CH3)

**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. List all substituents with their positions: 2-methyl, 4-methyl, 3-ethyl
2. Order substituents alphabetically: ethyl comes before methyl (ignore position numbers)
3. Combine identical substituents: two methyl groups become *dimethyl*
4. Assemble the full IUPAC name: **3-ethyl-2,4-dimethylheptane**

## Naming Compounds with One Functional Group

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.

> **tip**
>
> Common functional group suffixes: alcohol (-ol), aldehyde (-al), ketone (-one), carboxylic acid (-oic acid), ester (-oate), amine (-amine)

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

1. Identify parent chain length: 4 carbons = prefix `but-`
2. Identify functional group: hydroxyl = alcohol, so suffix is `-ol` at position 2
3. Combine components to get the IUPAC name: **butan-2-ol**

## Polyfunctional Compounds & Priority Rules

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 |

**Worked example:** Name a 5-carbon compound with a ketone on carbon 2 and a hydroxyl group on carbon 4

1. Compare priority: ketone has higher priority than alcohol, so ketone is the main functional group
2. Name the parent chain with the main functional group: 5 carbons + ketone on 2 = pentan-2-one
3. Treat the lower priority alcohol as a `hydroxy` substituent at position 4
4. Full IUPAC name: **4-hydroxypentan-2-one**

## Common pitfalls

- **Wrong:** Numbering the parent chain starting from the end closest to the first substituent instead of the highest priority functional group
  - Why it fails: Functional groups have higher priority than substituents, so they must get the lowest possible position number
  - Correct: Always number the chain to give the highest priority functional group the smallest position number, then minimize substituent positions
- **Wrong:** Ordering substituents by position number instead of alphabetical order
  - Why it fails: Alphabetical order ignores position numbers and multiplying prefixes (di-, tri-) for alkyl substituents
  - Correct: List substituents alphabetically by name, ignoring multiplying prefixes, and add all position numbers
- **Wrong:** Choosing the shortest carbon chain as the parent chain
  - Why it fails: The parent chain is defined as the longest continuous carbon chain containing the highest priority functional group
  - Correct: Count all possible continuous carbon chains to find the longest one that includes the main functional group
- **Wrong:** Forgetting to add the position number of double/triple bonds
  - Why it fails: IB exam questions require position numbers for multiple bonds to award full marks
  - Correct: Always add the position number of the first carbon in the multiple bond before the -ene/-yne suffix
- **Wrong:** Naming a terminal carbonyl group as a ketone
  - Why it fails: A carbonyl group on the end carbon is an aldehyde, not a ketone
  - Correct: Terminal carbonyl groups are aldehydes (suffix -al), internal carbonyl groups are ketones (suffix -one)

## 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) |

## 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.

- [Types of organic reactions](https://www.owlsprep.com/study/ib-chemistry-hl-u6-types-of-organic-reactions/)
- [Redox reactions and electrochemistry basics](https://www.owlsprep.com/study/ib-chemistry-hl-u6-redox-reactions-and-electrochemistry-basics/)
- [Oxidation numbers and voltaic cells](https://www.owlsprep.com/study/ib-chemistry-hl-u6-oxidation-numbers-and-voltaic-cells/)

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