Topic 10 · page 15 of 27
Naming and classifying alcohols
An alcohol is named from its longest chain with the ending -ol and a number for the OH group. It is primary, secondary or tertiary according to the carbon bonded to OH, and that class decides what oxidation does to it.
Naming alcohols 10.15
An alcohol contains the –OH group bonded to a saturated carbon. The name ends in -ol.
MethodNaming an alcohol
- Find the longest carbon chain that includes the carbon bonded to the –OH group. It gives the stem: methanol, ethanol, propan-1-ol, butan-2-ol.
- Replace the final -e of the alkane with -ol and put the number of the OH carbon in front of the -ol: propan-1-ol, not propanol.
- Number the chain from the end that gives the OH carbon the lowest number.
- Branches go in front, in alphabetical order, with their own numbers: 2-methylpropan-2-ol.
- Two –OH groups give -diol: ethane-1,2-diol. The final -e of the alkane is kept before -diol.
| Structural formula | Name | Note |
|---|---|---|
| CH₃CH₂OH | ethanol | no number needed: both carbons are the same |
| CH₃CH₂CH₂OH | propan-1-ol | OH on the end carbon |
| CH₃CH(OH)CH₃ | propan-2-ol | OH on the middle carbon |
| CH₃CH₂CH(OH)CH₃ | butan-2-ol | numbered from the end that gives OH the lower number |
| (CH₃)₃COH | 2-methylpropan-2-ol | longest chain has three carbons |
| HOCH₂CH₂OH | ethane-1,2-diol | two OH groups |
Three ways to draw an alcohol 10.15
- Displayed: every atom and every bond, including the O–H bond.
- Skeletal: a zigzag of carbon vertices, with OH written at the carbon it is bonded to. No C–H bonds.
- Structural: CH₃CH(OH)CH₃, or CH₃CHOHCH₃. The OH in brackets shows that it is on the carbon, not part of the chain.
Primary, secondary and tertiary alcohols 10.16
The class depends on the carbon bonded to the –OH group, in the same way as for halogenoalkanes. Count the other carbon atoms bonded to it.
| Class | Carbon neighbours of the C–OH carbon | Hydrogens on the C–OH carbon | Example |
|---|---|---|---|
| Primary (1°) | 1 | 2 | butan-1-ol |
| Secondary (2°) | 2 | 1 | butan-2-ol |
| Tertiary (3°) | 3 | 0 | 2-methylpropan-2-ol |
Methanol is bonded to no other carbon. It behaves like a primary alcohol.
Worked exampleName and classify two alcohols
Name and classify (a) CH₃CH₂CH(OH)CH₂CH₃ and (b) (CH₃)₂CHCH₂OH.
- (a) The longest chain has five carbons (pentane). The OH is on carbon 3, in the middle, so it is pentan-3-ol. The C–OH carbon has two carbon neighbours: secondary.
- (b) The longest chain through the C–OH carbon is three carbons, with the OH on carbon 1 and a methyl on carbon 2: 2-methylpropan-1-ol. The C–OH carbon has one carbon neighbour: primary.
Check yourself
- Name CH₃CH₂CH₂CH₂OH and classify it.
- Name CH₃CH₂CH(OH)CH₃ and classify it.
- Name (CH₃)₂CHCH₂OH and classify it.
- Draw the displayed formula of butan-2-ol.
- Draw the skeletal formula of 2-methylbutan-2-ol and classify it.
- A student names CH₃CH(OH)CH₃ "propanol". Explain why this is not enough.
Answers cover these until you have tried the questions
- Butan-1-ol, primary: the C–OH carbon has one carbon neighbour.
- Butan-2-ol, secondary: the C–OH carbon has two carbon neighbours.
- 2-methylpropan-1-ol, primary: the C–OH carbon has one carbon neighbour (the CH).
- See the left-hand drawing in the figure below. Four carbons in a line; on carbon 2 an O–H group, and every other bond to a hydrogen atom.
- See the right-hand drawing in the figure below: a four-carbon zigzag with OH and CH₃ both on carbon 2. It is tertiary: the C–OH carbon has three carbon neighbours (carbons 1, 3 and the methyl).
- "Propanol" does not say where the OH is. There are two isomers, propan-1-ol (primary) and propan-2-ol (secondary), and they behave differently when oxidised. The number is needed.
