Alcohols (AQA GCSE Chemistry): Revision Notes
Alcohols
What are alcohols?
Alcohols are a family of organic compounds that have many important uses. They work as solvents (substances that dissolve other materials) and fuels. The most well-known alcohol is ethanol, which is found in alcoholic drinks like wine and beer.
Alcohols are found everywhere in our daily lives - from the hand sanitizer we use to the fuel in our cars. Understanding their properties helps explain why they're so versatile and useful.
Structure of alcohols
All alcohols share the same key feature - they contain a functional group called -OH. This is made up of one oxygen atom bonded to one hydrogen atom. This -OH group is what makes a molecule an alcohol.
Because alcohols all have the same functional group, they form what we call a homologous series. This means they:
Key characteristics of a homologous series:
- Have the same functional group (-OH)
- Show similar chemical behaviour
- Have properties that change gradually as the molecules get bigger
Naming alcohols
You need to recognise alcohols from their names and chemical formulas:
- Methanol: CH₃OH (the smallest alcohol)
- Ethanol: CH₃CH₂OH (found in alcoholic drinks)
- Propanol: CH₃CH₂CH₂OH
- Butanol: CH₃CH₂CH₂CH₂OH
All alcohol names end in -ol, which helps you spot them easily.
Pattern Recognition: Alcohol Naming
Notice the pattern as we go up the series:
- Methanol: 1 carbon atom
- Ethanol: 2 carbon atoms
- Propanol: 3 carbon atoms
- Butanol: 4 carbon atoms
Each alcohol has one more CH₂ group than the previous one, but all keep the -OH functional group.
Properties of alcohols
Alcohols have several important properties that make them useful in many applications:
- They dissolve in water to make neutral solutions
- They react with sodium metal to produce hydrogen gas
- They burn in air to make carbon dioxide and water
- They can be oxidised to form carboxylic acids
- They react with carboxylic acids to make esters
The ability of alcohols to dissolve in water is due to the -OH group, which can form hydrogen bonds with water molecules. This property makes alcohols excellent solvents for many substances.
Reactions of alcohols
Combustion (burning)
When alcohols burn in plenty of air, they produce carbon dioxide and water. For example:
Or more specifically:
This is why alcohols can be used as fuels.
Other reactions
Alcohols can react with:
- Sodium - to produce hydrogen gas
- Carboxylic acids - to make ester compounds
Worked Example: Ethanol Combustion
When ethanol burns completely in oxygen:
Step 1: Write the word equation Ethanol + oxygen → carbon dioxide + water
Step 2: Write the chemical equation
This reaction releases energy, making ethanol useful as a fuel.
Formation of ethanol
Ethanol can be made in two main ways:
Method 1: Fermentation
This is how alcoholic drinks are made:
- Start with a sugar solution
- Add yeast (contains enzymes that break down sugar)
- Keep in a warm place (around 30-40°C)
- Prevent oxygen from getting in (anaerobic conditions)
The yeast converts the sugar into ethanol and carbon dioxide.
Critical Conditions for Fermentation:
- Temperature must be warm but not too hot (yeast dies above ~50°C)
- Oxygen must be excluded - oxygen would cause the yeast to produce carbon dioxide and water instead of ethanol
- pH should be slightly acidic for optimal yeast activity
Method 2: Addition reaction
Ethanol can also be made industrially:
- React steam with ethene (an alkene)
- Use a catalyst to speed up the reaction
- This happens at high temperature and pressure
The industrial method is faster and can produce larger quantities of ethanol, but the fermentation method is more environmentally friendly as it uses renewable resources (plant sugars) rather than fossil fuels (ethene from oil).
Key Points to Remember:
- All alcohols contain the -OH functional group - this is what makes them alcohols
- Alcohol names end in -ol (methanol, ethanol, propanol, butanol)
- Alcohols burn in air to produce carbon dioxide and water
- Ethanol can be made by fermentation of sugar with yeast in warm, oxygen-free conditions
- Alcohols form a homologous series with similar properties
- The -OH group gives alcohols their ability to dissolve in water and react with metals like sodium