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This lesson covers three important homologous series of organic compounds — alcohols, carboxylic acids, and esters — as required by the AQA GCSE Chemistry specification (5.8.2). This is Higher Tier only [H] content. You need to know the functional groups, names, properties, and reactions of these compounds.
Alcohols are organic compounds that contain the hydroxyl functional group (–OH). The general formula for alcohols is CnH2n+1OH (sometimes written as CnH2n+2O).
| Name | Molecular Formula | Structural Formula | Number of Carbons |
|---|---|---|---|
| Methanol | CH3OH | CH3OH | 1 |
| Ethanol | C2H5OH | CH3CH2OH | 2 |
| Propanol | C3H7OH | CH3CH2CH2OH | 3 |
All alcohol names end in -ol.
Exam Tip: [H] The functional group of alcohols is –OH (hydroxyl). When drawing displayed formulae, make sure the –OH group is clearly shown bonded to a carbon atom. Do not confuse it with the hydroxide ion (OH-) found in alkalis.
| Property | Detail |
|---|---|
| State at room temperature | The first few alcohols are colourless liquids at room temperature |
| Solubility | The first few alcohols are soluble in water because the –OH group can form hydrogen bonds with water molecules |
| Volatility | Alcohols are volatile liquids — they evaporate easily |
| Flammability | Alcohols burn in air with a clean blue flame |
| Smell | Alcohols have a characteristic smell |
Alcohols burn completely in oxygen to produce carbon dioxide and water:
ethanol + oxygen → carbon dioxide + water
C2H5OH + 3O2 → 2CO2 + 3H2O
This is why ethanol can be used as a fuel (e.g., in spirit burners and as a biofuel blended with petrol).
Alcohols react with sodium metal to produce a sodium alkoxide and hydrogen gas:
ethanol + sodium → sodium ethoxide + hydrogen
2C2H5OH + 2Na → 2C2H5ONa + H2
The hydrogen gas can be tested with a burning splint — it burns with a squeaky pop.
Alcohols can be oxidised to form carboxylic acids. This can happen by:
graph LR
A["Ethanol<br/>C2H5OH<br/>(Alcohol)"] -->|"Oxidation"| B["Ethanoic Acid<br/>CH3COOH<br/>(Carboxylic Acid)"]
A -->|"Combustion"| C["CO2 + H2O"]
A -->|"+ Sodium"| D["Sodium Ethoxide<br/>+ Hydrogen"]
A -->|"+ Carboxylic Acid"| E["Ester + Water"]
style A fill:#2980b9,color:#fff
style B fill:#e67e22,color:#fff
style E fill:#27ae60,color:#fff
| Alcohol | Uses |
|---|---|
| Methanol | Industrial solvent, feedstock for making formaldehyde, fuel |
| Ethanol | Alcoholic drinks, solvent (e.g., in perfumes, aftershaves), fuel (biofuel), antiseptic |
| Propanol | Solvent, cleaning agent |
There are two industrial methods of producing ethanol:
| Method | Hydration of Ethene | Fermentation |
|---|---|---|
| Raw material | Ethene (from crude oil) | Sugars (from plants — glucose) |
| Conditions | Steam, 300°C, phosphoric acid catalyst, 60–70 atm | Yeast, 25–35°C, anaerobic conditions |
| Equation | C2H4 + H2O → C2H5OH | C6H12O6 → 2C2H5OH + 2CO2 |
| Rate | Fast (continuous process) | Slow (batch process, takes several days) |
| Purity | Pure ethanol produced directly | Impure product — must be distilled to purify |
| Sustainability | Uses non-renewable crude oil | Uses renewable plant material (carbon neutral) |
Exam Tip: [H] You may be asked to compare the two methods of making ethanol. Always discuss: raw material (renewable vs non-renewable), rate of reaction, purity of product, and atom economy. Fermentation is more sustainable; hydration produces a purer product faster.
Carboxylic acids are organic compounds that contain the carboxyl functional group (–COOH). The general formula is CnH2n+1COOH.
| Name | Molecular Formula | Structural Formula | Number of Carbons |
|---|---|---|---|
| Methanoic acid | HCOOH | HCOOH | 1 |
| Ethanoic acid | CH3COOH | CH3COOH | 2 |
| Propanoic acid | C2H5COOH | CH3CH2COOH | 3 |
All carboxylic acid names end in -anoic acid.
Carboxylic acids are weak acids. This means they:
| Property | Strong Acid (e.g. HCl) | Weak Acid (e.g. CH3COOH) |
|---|---|---|
| Ionisation | Fully ionises | Partially ionises |
| pH (same concentration) | Lower (e.g. pH 1) | Higher (e.g. pH 3–4) |
| Rate of reaction | Faster | Slower |
Exam Tip: [H] Do not confuse "weak" with "dilute." Weak refers to the degree of ionisation (partial). Dilute refers to the concentration (small amount of acid in a lot of water). A weak acid can be concentrated, and a strong acid can be dilute.
Carboxylic acids undergo the same typical reactions as other acids:
| Reaction | General Equation | Example with Ethanoic Acid |
|---|---|---|
| + Metal | acid + metal → salt + hydrogen | 2CH3COOH + Mg → (CH3COO)2Mg + H2 |
| + Metal carbonate | acid + carbonate → salt + water + CO2 | 2CH3COOH + Na2CO3 → 2CH3COONa + H2O + CO2 |
| + Metal oxide | acid + metal oxide → salt + water | 2CH3COOH + CuO → (CH3COO)2Cu + H2O |
| + Alkali | acid + alkali → salt + water | CH3COOH + NaOH → CH3COONa + H2O |
The salts of ethanoic acid are called ethanoates (e.g., sodium ethanoate, CH3COONa).
Esters are organic compounds formed by the reaction of an alcohol with a carboxylic acid in the presence of an acid catalyst (usually concentrated sulfuric acid). This reaction is called esterification (or condensation).
alcohol + carboxylic acid → ester + water
ethanol + ethanoic acid → ethyl ethanoate + water
C2H5OH + CH3COOH → CH3COOC2H5 + H2O
Ester names have two parts:
| Alcohol | Carboxylic Acid | Ester Name |
|---|---|---|
| Methanol | Ethanoic acid | Methyl ethanoate |
| Ethanol | Ethanoic acid | Ethyl ethanoate |
| Ethanol | Propanoic acid | Ethyl propanoate |
| Propanol | Methanoic acid | Propyl methanoate |
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