Section 1 of 5Alcohol Basics
Hydroxyl group
- All alcohols contain the hydroxyl (–OH) functional group, which is responsible for their characteristic reactions.
Properties
- Alcohols are colourless liquids that dissolve in water to form neutral solutions.
General formula
- The general formula of alcohols is CₙH₂ₙ₊₁OH, which describes the homologous series.
Naming
- Alcohol names follow the alkane system, with the final e replaced by ol, such as ethanol for two carbons.
Fuels
- The first four alcohols are commonly used as fuels, and ethanol burns readily with an almost invisible blue flame.
Section 2 of 5The First Four Alcohols
1
Count the longest carbon chain to find the prefix: meth-, eth-, prop- or but-.
2
Replace the final e of the alkane name with -ol to show the alcohol functional group.
3
Use a number to show which carbon carries the –OH group, such as the 1 in propan-1-ol.
4
Propan-1-ol and butan-1-ol have the –OH group on the first carbon atom of the chain.
| Name | Molecular formula | Structural formula |
|---|---|---|
| Methanol | CH₄O | CH₃OH |
| Ethanol | C₂H₆O | CH₃CH₂OH |
| Propan-1-ol | C₃H₈O | CH₃CH₂CH₂OH |
| Butan-1-ol | C₄H₁₀O | CH₃CH₂CH₂CH₂OH |
Section 3 of 5Uses of Alcohols
Uses of Alcohols
Fuel
- Ethanol burns cleanly and is used as a fuel in spirit burners in laboratories.
Solvent
- Alcohols dissolve many substances and are used as industrial solvents.
Drinks
- Ethanol is the alcohol in alcoholic drinks, and it can be produced by fermentation.
Oxidation
- Alcohols can be oxidised to carboxylic acids using suitable oxidising agents.
Safety
- Alcohols are flammable, so they should be heated with care and kept away from naked flames.
Section 4 of 5Worked Example
Question Give the molecular formula of propan-1-ol.
- Propan-1-ol has a chain of 3 carbon atoms (prop-)
- Using CₙH₂ₙ₊₁OH with n = 3 gives C₃H₇OH
- Counting the atoms gives C₃H₈O
Answer C₃H₈O
Section 5 of 5Exam Checklist & Tips
4.5.1 – Key Points for the Exam
- Alcohol An organic compound containing the –OH functional group.
- Hydroxyl group The –OH functional group found in alcohols.
- Homologous series A family of compounds with the same functional group and a general formula.
- Flammable Able to catch fire and burn easily.
- Solvent A liquid that dissolves other substances.
Common Mistakes
- Forgetting the –OH group when writing alcohol formulae, for example writing C₂H₆ for ethanol.
- Confusing the -ol ending of alcohols with the -ene ending of alkenes.
- Describing alcohols as acidic because they contain –OH; their solutions are neutral.
- Dropping the position number from names such as propan-1-ol when the position of the –OH group matters.