Naming Organic Compounds
Section: 11. Organic Chemistry | Syllabus: Cambridge IGCSE Chemistry (0620)
The Naming System
An organic compound's name is built from a prefix (the number of carbon atoms in the chain) and a suffix (the functional group/homologous series).
| Number of carbons | Prefix |
|---|---|
| 1 | meth- |
| 2 | eth- |
| 3 | prop- |
| 4 | but- |
| Homologous series | Suffix | Example |
|---|---|---|
| Alkanes | -ane | methane, ethane |
| Alkenes | -ene | ethene |
| Alcohols | -anol | ethanol |
| Carboxylic acids | -oic acid | ethanoic acid |
The type of compound can also be identified in reverse: given a name ending in -ane, -ene, -ol, or -oic acid (or given a molecular/structural formula), the homologous series can be stated directly from the ending or from the functional group present.
Naming Position Isomers (Supplement)
For alkenes and alcohols with 4 or more carbons, a number is needed to show where the double bond or -OH group is positioned along the chain, since more than one position is possible.
| Name | Structural formula |
|---|---|
| but-1-ene | CH₃CH₂CH=CH₂ |
| but-2-ene | CH₃CH=CHCH₃ |
| propan-1-ol | CH₃CH₂CH₂OH |
| propan-2-ol | CH₃CH(OH)CH₃ |
| butan-1-ol | CH₃CH₂CH₂CH₂OH |
| butan-2-ol | CH₃CH₂CH(OH)CH₃ |
The number always gives the position of the carbon atom that carries the double bond or the -OH group, counting from the end of the chain that gives the lowest possible number.
Worked Example
- Question (based on a real exam question): what is the structural formula of propan-1-ol?
- Answer: CH₃CH₂CH₂OH. C₃H₇OH is only a condensed molecular formula and does not show the chain explicitly; C₃H₈O is the molecular formula; CnH2n+1OH is the general formula for the whole alcohol series, not one specific compound - only the structural formula shows the -OH is on carbon 1 of a 3-carbon chain.
Identifying Compounds from Displayed Formulae
Given a displayed formula, the name and type of compound can be identified by looking for the functional group (C=C for alkenes, -OH for alcohols, -COOH for carboxylic acids) and counting the carbons.
Common Mistakes
- ❌ Matching a displayed formula to the wrong name by miscounting carbons or misreading the functional group - e.g. mistaking a 3-carbon alkene (propene, CH₃CH=CH₂) for "ethene" (only 2 carbons), or mistaking ethanol's C-C-O-H formula for "ethanoic acid" (which needs a C=O group)
- ✓ always count the carbons in the longest chain first, then check exactly which functional group is drawn (a lone -O-H is an alcohol; a C=O next to an -O-H is a carboxylic acid)
Interactive revision notes, videos and practice questions load below.