AlcoholsAQA GCSE Chemistry: Revision notes
Section 1
What is the functional group of alcohols?
Alcohols are organic compounds containing the hydroxyl functional group –OH bonded to a carbon atom. This –OH group is responsible for the characteristic properties of alcohols.
The general formula for alcohols is CₙH₂ₙ₊₂O, where the –OH group can be attached to different carbon chains, creating different alcohols. The –OH group is polar, which makes alcohols soluble in water and gives them relatively high boiling points compared to hydrocarbons of similar molecular weight.
Examiners expect you to identify the –OH group when shown a structure and to correctly distinguish alcohols from other organic compounds. Always label or highlight the functional group in your answers.
Section 2
How are alcohols named and what are their structures?
Alcohols are named using the suffix –ol. The main alcohols you must know are:
| Alcohol | Molecular formula | Structural formula | Key features |
|---|---|---|---|
| Methanol | CH₃OH | CH₃–OH | Simplest alcohol; toxic; used as solvent |
| Ethanol | C₂H₅OH | CH₃–CH₂–OH | Two carbons; widely used as fuel and drink |
| Propanol | C₃H₇OH | CH₃–CH₂–CH₂–OH | Three carbons; isomers possible (1-propanol shown) |
| Butanol | C₄H₉OH | CH₃–CH₂–CH₂–CH₂–OH | Four carbons; isomers possible (1-butanol shown) |
The –OH group is always written at the end of the name and attached to the carbon chain. For longer chains, the position of the –OH group may be numbered (e.g. 2-propanol), but for the four main alcohols you must know, the standard structures shown above are the primary isomers.
In exam questions, you may be asked to draw or identify structures. Always draw the –OH group clearly bonded to a specific carbon, and ensure your hydrogens and bonds are correct for each carbon.
If asked to name CH₃–CH₂–CH₂–OH, identify that it has three carbons in the main chain with an –OH group, making it propan-1-ol (or simply propanol). If the –OH were on the middle carbon, it would be propan-2-ol.
Section 3
What are the main uses of ethanol?
Ethanol has three major industrial and commercial uses:
-
As a solvent – Ethanol dissolves many organic and some inorganic compounds, making it valuable in pharmaceuticals, cosmetics, and laboratory applications. Its polarity due to the –OH group enables this solubility.
-
As a fuel – Ethanol burns completely in oxygen to produce carbon dioxide and water, releasing energy. It is increasingly used as a biofuel (a fuel produced from renewable biological sources), often mixed with petrol in vehicles. Biofuels reduce dependence on fossil fuels and lower carbon emissions when the renewable crop is regrown.
-
As a drink – Ethanol is the active ingredient in alcoholic beverages. It is safe for human consumption in controlled quantities, unlike methanol which is toxic.
Ethanol's versatility makes it one of the most economically important organic chemicals.
Examiners often ask why ethanol is used as a fuel or solvent. Answer by linking properties to uses: the –OH group makes it polar (solvent property), and complete combustion (fuel property) releases energy efficiently.
Think of ethanol as a 'Swiss Army knife' chemical – it has multiple different uses because its –OH group gives it special properties like polarity and the ability to undergo combustion.
Section 4
How is ethanol produced by fermentation and hydration?
There are two main industrial methods for producing ethanol:
Method 1: Fermentation (Anaerobic Process)
Equation: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂
- Reactant: Glucose (or other sugars) from renewable sources such as sugar cane or grain
- Conditions: Room temperature (~25–37°C), anaerobic (no oxygen), using yeast enzymes
- Products: Ethanol and carbon dioxide
- Advantages: Uses renewable resources; low energy cost; natural biological process
- Disadvantages: Slow reaction rate; produces dilute ethanol solution (requires distillation); product purity requires careful separation
Method 2: Hydration of Ethene (Addition Reaction)
Equation: C₂H₄ + H₂O → C₂H₅OH
- Reactants: Ethene (from cracking crude oil) and steam
- Conditions: High temperature (~300°C), high pressure (~70 atm), phosphoric acid catalyst
- Products: Ethanol directly
- Advantages: Fast reaction rate; produces pure ethanol; continuous industrial process
- Disadvantages: Uses non-renewable crude oil; high energy cost; requires specialised equipment
Comparison Table
| Factor | Fermentation | Hydration |
|---|---|---|
| Rate | Slow | Fast |
| Purity | Lower (dilute solution) | Higher (pure product) |
| Resources | Renewable (crops) | Non-renewable (crude oil) |
| Temperature | Room temperature (~30°C) | High (~300°C) |
| Pressure | Atmospheric | High (~70 atm) |
| Catalyst | Yeast enzymes | Phosphoric acid |
| Industrial use | Alcoholic drinks, some fuel | Large-scale fuel production |
Key insight: The choice of method depends on whether sustainability or efficiency is prioritised. Fermentation suits small-scale or beverage production; hydration suits large-scale industrial fuel production despite using non-renewable resources.
Examiners frequently ask you to compare the two methods. Structure your answer using clear headings (rate, purity, resources, conditions) and always reference the specific advantages and disadvantages relevant to the question context.
If asked 'Why might fermentation be preferred for producing ethanol for alcoholic drinks?', answer: 'Fermentation uses renewable resources (crops), is environmentally sustainable, and produces ethanol at lower cost with lower energy input than hydration. Although the product is less pure, distillation can separate it, and the natural process is acceptable for beverages.'
Section 5
What are the key reactions of alcohols?
Alcohols undergo several important chemical reactions that you must know:
1. Combustion (Oxidation)
General equation: CₙH₂ₙ₊₂O + excess O₂ → nCO₂ + (n+1)H₂O
Example (ethanol): C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O
- Alcohols burn completely in oxygen (or air) to produce carbon dioxide and water only
- This is a redox reaction – the alcohol is oxidised
- Combustion releases large amounts of energy, which is why alcohols are used as fuels
- Incomplete combustion (limited oxygen) produces carbon monoxide, which is toxic, and carbon (soot)
2. Reaction with Sodium Metal
General equation: 2ROH + 2Na → 2RONa + H₂
Example (ethanol): 2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂
- Alcohols react with sodium metal to produce a metal alkoxide (such as sodium ethoxide) and hydrogen gas
- This reaction is vigorous and exothermic (releases heat); sodium becomes hot and may ignite the hydrogen produced, creating a yellow flame
- The reaction demonstrates that the –OH group contains an acidic hydrogen atom (more acidic than in water)
- This test can identify alcohols or distinguish them from ethers
3. Oxidation to Carboxylic Acids
General equation: Primary alcohol + oxidising agent → carboxylic acid
Example (ethanol to ethanoic acid): C₂H₅OH + [O] → CH₃COOH
- Primary alcohols (–CH₂OH group) are oxidised to carboxylic acids
- Oxidising agents used include acidified potassium dichromate(VI), potassium permanganate, or microbial action (e.g. acetic acid bacteria oxidising ethanol to acetic acid in vinegar production)
- Microbial oxidation: Aerobic bacteria can oxidise ethanol to acetic acid under mild conditions; this is used commercially to produce vinegar
- The oxidation proceeds through an aldehyde intermediate (e.g. ethanal from ethanol) before forming the carboxylic acid
- Secondary alcohols can be oxidised to ketones, but tertiary alcohols cannot be oxidised
| Alcohol Type | Oxidation Product | Example |
|---|---|---|
| Primary | Carboxylic acid | Ethanol → Ethanoic acid |
| Secondary | Ketone | Propan-2-ol → Propanone |
| Tertiary | No oxidation | 2-methylpropan-2-ol → No reaction |
Key insight: The ability of an alcohol to be oxidised depends on its structure. Only those with available hydrogen atoms on the carbon bearing the –OH group can be oxidised further.
Examiners test oxidation reactions frequently. Always identify whether the alcohol is primary, secondary, or tertiary, then predict the correct product. For fermentation contexts, mention microbial oxidation of ethanol to acetic acid as a real-world application.
Students often forget that combustion of alcohols produces only CO₂ and H₂O (not carbon). Also, confusing primary, secondary, and tertiary alcohols leads to incorrect predictions of oxidation products. Practise identifying the type of alcohol from the structure.
Oxidation of ethanol: C₂H₅OH + [O] → CH₃CHO (ethanal, aldehyde intermediate) + [O] → CH₃COOH (ethanoic acid). In excess oxidising agent, the aldehyde is further oxidised to the carboxylic acid.
Must Know
- Alcohols contain the –OH functional group bonded to a carbon atom; the main alcohols are methanol (CH₃OH), ethanol (C₂H₅OH), propanol (C₃H₇OH), and butanol (C₄H₉OH)
- Ethanol uses: solvent (due to polarity), fuel and biofuel (complete combustion), and drink (toxic-free)
- Fermentation: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ using yeast at room temperature and anaerobic conditions; slow, renewable, but dilute product
- Hydration: C₂H₄ + H₂O → C₂H₅OH using phosphoric acid catalyst at ~300°C and ~70 atm; fast, pure product, but non-renewable resources and high energy
- Combustion: CₙH₂ₙ₊₂O + O₂ → CO₂ + H₂O (complete combustion only); all alcohols undergo this exothermic redox reaction
- Reaction with sodium: 2ROH + 2Na → 2RONa + H₂ (vigorous, exothermic, produces hydrogen gas and metal alkoxide)
- Oxidation of primary alcohols: Forms carboxylic acids using oxidising agents (dichromate, permanganate) or microbial action (e.g. acetic acid bacteria); secondary alcohols form ketones; tertiary alcohols do not oxidise
That's the notes covered.
Carry on to the next subtopic.