# Naming Organic Compounds

> Homologous series, functional groups, the IUPAC naming rules for chains, branches and functional groups, the ways to draw organic molecules, and structural isomers.

From HSC Chemistry on WhizzNotes, free study notes for NSW students: https://whizznotes.com/chemistry/naming-organic-compounds/. Updated 3 November 2026.
Year 11 and Year 12 boxes go deeper than the Year 10 text around them.

### Story time: Ten million names
Chemists have made or found more than ten million carbon compounds. Imagine if every one had a made-up name, like "spirit of wine" or "oil of wintergreen". Nobody could remember them, and nobody could tell what was inside a bottle from its label.

In 1892, chemists met in Geneva and agreed on a system. Today it is run by IUPAC (the International Union of Pure and Applied Chemistry). Like a street address that tells you the country, city, street and house number, an IUPAC name tells you the chain length, the type of compound, and where every piece is attached. Learn the rules once and you can name, or draw, almost any molecule.

**In short:** Organic compounds are named from their longest carbon chain (meth, eth, prop, but and so on) and an ending for their functional group, like -ane, -ene, -ol or -oic acid. Number the chain so the functional group gets the lowest number, and list any branches in alphabetical order.

## Organic chemistry and carbon

**Organic chemistry** is the chemistry of carbon compounds (apart from simple ones like $\mathrm{CO_2}$ and carbonates). Carbon is special because each carbon atom:

- makes **four** covalent bonds,
- can bond to other carbon atoms in long **chains** and **rings**,
- can form **single, double and triple** bonds.

## Homologous series

A **homologous series** is a family of compounds that:

- have the same **functional group**,
- have the same **general formula**,
- differ from the next member by $\mathrm{CH_2}$,
- show a steady **trend** in physical properties (like boiling point) and similar chemical properties.

## The prefix: chain length

The longest carbon chain gives the stem of the name.

| Carbons | Stem | Carbons | Stem |
|---|---|---|---|
| 1 | meth- | 5 | pent- |
| 2 | eth- | 6 | hex- |
| 3 | prop- | 7 | hept- |
| 4 | but- | 8 | oct- |

## The suffix: functional groups

| Series | Functional group | Ending | Example |
|---|---|---|---|
| Alkane | C−C only | -ane | propane $\mathrm{CH_3CH_2CH_3}$ |
| Alkene | C=C | -ene | propene $\mathrm{CH_2{=}CHCH_3}$ |
| Alkyne | C≡C | -yne | propyne |
| Haloalkane | −F, −Cl, −Br, −I | prefix fluoro-, chloro-, bromo-, iodo- | 1-chloropropane |
| Alcohol | −OH (hydroxyl) | -ol | propan-1-ol |
| Aldehyde | −CHO (at the end of the chain) | -al | propanal |
| Ketone | C=O (in the middle of the chain) | -one | propanone |
| Carboxylic acid | −COOH (carboxyl) | -oic acid | propanoic acid |
| Amine | −NH₂ (amino) | -amine | propan-1-amine |
| Amide | −CONH₂ | -amide | propanamide |
| Ester | −COO− | -yl -oate | methyl ethanoate |

## IUPAC naming rules

1. Find the **longest carbon chain** that contains the functional group. This gives the stem.
2. **Number** the chain from the end that gives the **functional group** the lowest number. If there is no functional group, give the branches the lowest numbers.
3. Name the **branches** (methyl $\mathrm{-CH_3}$, ethyl $\mathrm{-C_2H_5}$) and halogens as prefixes, with their positions.
4. List prefixes in **alphabetical order** (bromo before chloro before methyl). Use **di-, tri-, tetra-** for repeats, but ignore them when sorting.
5. Use **commas** between numbers and **hyphens** between numbers and words.

### Worked example: Naming step by step
$\mathrm{CH_3CH(CH_3)CH(OH)CH_3}$

- Longest chain with the −OH: 4 carbons, so **butan-**.
- Number from the end nearest the −OH: the −OH is on carbon 2, the methyl on carbon 3.
- Name: **3-methylbutan-2-ol**.

### Worked example: Common traps
- "2-ethylpropane" is wrong: the longest chain is really 4 carbons. It is **2-methylbutane**.
- "1-chloro-4-bromopentane" breaks two rules: prefixes go in alphabetical order, and the numbers should be lowest. It is **1-bromo-4-chloropentane**.
- Aldehydes and carboxylic acids are always on carbon 1, so no number is needed: **butanal**, **butanoic acid**.

**Esters** are named from the alcohol part (as an "-yl" word) then the acid part (as an "-oate" word). Ethanol + ethanoic acid gives **ethyl ethanoate**, $\mathrm{CH_3COOCH_2CH_3}$.

**Amides** with a group on the nitrogen use "N-": **N-methylpropanamide** has a methyl group on the N.

## Drawing organic molecules

| Type | Shows | Example (propan-1-ol) |
|---|---|---|
| Molecular formula | Number of each atom | $\mathrm{C_3H_8O}$ |
| Condensed structural | Groups in order | $\mathrm{CH_3CH_2CH_2OH}$ |
| Full structural | Every atom and bond | All C−H, C−C, C−O, O−H bonds drawn |
| Skeletal | Zigzag line; each corner or end is a carbon; H on carbon not shown | A zigzag of 3 points with OH on the end |

## Structural isomers

**Structural isomers** have the **same molecular formula** but **different structures**.

- **Chain isomers**: the carbon chain is branched differently. Butane and 2-methylpropane ($\mathrm{C_4H_{10}}$).
- **Position isomers**: the functional group is in a different place. Propan-1-ol and propan-2-ol.
- **Functional group isomers**: a different functional group altogether. Propanal and propanone ($\mathrm{C_3H_6O}$), or propanoic acid and methyl ethanoate ($\mathrm{C_3H_6O_2}$).

| Formula | Number of structural isomers |
|---|---|
| $\mathrm{C_4H_{10}}$ | 2 |
| $\mathrm{C_5H_{12}}$ | 3 |
| $\mathrm{C_6H_{14}}$ | 5 |
| $\mathrm{C_4H_{10}O}$ (alcohols only) | 4 |

**Mind map**

```text
Naming organic compounds
  Carbon
    Four bonds
    Chains and rings
  Homologous series
    Same functional group
    Differ by CH2
  Name parts
    Stem: chain length
    Ending: functional group
    Prefixes: branches, halogens
  Rules
    Longest chain
    Lowest numbers
    Alphabetical prefixes
  Formulas
    Molecular, condensed
    Structural, skeletal
  Isomers
    Chain
    Position
    Functional group
```

### Year 11: Counting isomers carefully
To count isomers, draw them in order and check for repeats. A molecule flipped or rotated is the **same** molecule.

**$\mathrm{C_5H_{12}}$:** pentane (straight), 2-methylbutane, 2,2-dimethylpropane. "3-methylbutane" is just 2-methylbutane numbered from the wrong end, so it is not new. Three isomers.

**$\mathrm{C_3H_6Cl_2}$:** 1,1-, 1,2-, 1,3- and 2,2-dichloropropane. Four isomers.

**$\mathrm{C_4H_{10}O}$ alcohols:** butan-1-ol, butan-2-ol, 2-methylpropan-1-ol and 2-methylpropan-2-ol. Four isomers.

**$\mathrm{C_2H_4Br_2}$:** 1,1- and 1,2-dibromoethane. Two isomers.

**$\mathrm{C_3H_6BrCl}$:** two different halogens give more choices: 1-bromo-1-chloro, 1-bromo-2-chloro, 1-bromo-3-chloro, 2-bromo-1-chloro and 2-bromo-2-chloropropane. Five isomers.

**$\mathrm{C_6H_{14}}$:** hexane, 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane and 2,3-dimethylbutane. Five chain isomers.

**Counting substitution products.** Count the different kinds of hydrogen. In 2,2-dimethylbutane, $\mathrm{(CH_3)_3CCH_2CH_3}$, there are three: the nine H on the three matching methyls, the $\mathrm{CH_2}$, and the end $\mathrm{CH_3}$. So three monochloro products.

### Year 12: Harder names
**Two functional groups.** The higher-priority group gets the ending; the other becomes a prefix. Priority: carboxylic acid > ester > amide > aldehyde > ketone > alcohol > amine > alkene. So $\mathrm{CH_3CH(OH)CH_2COOH}$ is **3-hydroxybutanoic acid**, and $\mathrm{CH_3COCH_2CH_2CH(OH)CH_3}$ is **5-hydroxyhexan-2-one**.

**Alkenes with branches.** Number to give the double bond the lowest number first: $\mathrm{CH_2{=}C(CH_3)CH(CH_3)CH_3}$ is **2,3-dimethylbut-1-ene**.

**Cis and trans.** But-2-ene has two forms: with both $\mathrm{CH_3}$ groups on the same side of the double bond (**cis**) or opposite sides (**trans**). The C=C bond cannot rotate, so they are different molecules. These are **stereoisomers**, not structural isomers.

**Halogens with a main group.** Number from the end that gives the main group the lowest number, then list halogens and branches alphabetically. In $\mathrm{CH_3CHBrCH_2C(CH_3)ICH_2CHO}$ the aldehyde carbon is C1, so the name is **5-bromo-3-iodo-3-methylhexanal**.

## Quick summary

- A homologous series has one functional group, one general formula, and members that differ by $\mathrm{CH_2}$.
- Stems: meth, eth, prop, but, pent, hex, hept, oct.
- Endings: -ane, -ene, -yne, -ol, -al, -one, -oic acid, -amine, -amide, -yl -oate.
- Longest chain, lowest numbers for the functional group, prefixes in alphabetical order.
- Structural isomers: same molecular formula, different structure (chain, position or functional group).

## Quiz

Try these questions from the chapter quiz.

1. How many covalent bonds does a carbon atom usually make?
   - A. 2
   - B. 3
   - C. 4
   - D. 6
   - Answer: C. 4. Carbon has four outer electrons to share.

2. The stem for a 5-carbon chain is:
   - A. but-
   - B. pent-
   - C. hex-
   - D. prop-
   - Answer: B. pent-. meth 1, eth 2, prop 3, but 4, pent 5.

3. Members of a homologous series differ by:
   - A. $\mathrm{CH_2}$
   - B. $\mathrm{CH_3}$
   - C. One carbon atom only
   - D. $\mathrm{H_2O}$
   - Answer: A. $\mathrm{CH_2}$. Each member has one more $\mathrm{CH_2}$ unit.

The whole chapter quiz is on the page, for signed-in students: https://whizznotes.com/chemistry/naming-organic-compounds/#quiz