Alcohols Carboxylic Acids and Esters
Alcohols
Alcohols are a homologous series with the functional group −OH (a hydroxyl group). Their general formula is:
CₙH₂ₙ₊₁OH
| Name | Formula | Structure |
|---|---|---|
| Methanol | CH₃OH | 1 carbon + OH |
| Ethanol | C₂H₅OH | 2 carbons + OH |
| Propanol | C₃H₇OH | 3 carbons + OH |
| Butanol | C₄H₉OH | 4 carbons + OH |
Properties of Alcohols
- Dissolve in water to form a neutral solution (pH 7)
- Flammable — burn in air to produce CO₂ and H₂O
- Used as fuels and solvents
Reactions of Alcohols
Combustion:
C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O
Ethanol burns with a clean blue flame and is used as a fuel (e.g. in spirit burners).
Reaction with sodium:
2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂
Produces hydrogen gas (less vigorous than sodium with water).
Oxidation:
Alcohols can be oxidised to form carboxylic acids. This can happen by:
- Reacting with an oxidising agent such as acidified potassium dichromate(VI) — the orange solution turns green
- Microbial oxidation — when wine is left open, bacteria oxidise ethanol to ethanoic acid (vinegar)
ethanol + [O] → ethanoic acid + water
C₂H₅OH + 2[O] → CH₃COOH + H₂O
Making Ethanol
Fermentation:
glucose → ethanol + carbon dioxide
C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂
Conditions: yeast (enzyme zymase), temperature 25–35°C, anaerobic (no oxygen).
Hydration of ethene:
C₂H₄ + H₂O → C₂H₅OH
Conditions: phosphoric acid catalyst, 300°C, 60 atm.
| Fermentation | Hydration | |
|---|---|---|
| Rate | Slow (batch) | Fast (continuous) |
| Raw material | Sugar/glucose (renewable) | Ethene from crude oil (non-renewable) |
| Atom economy | 51% | 100% |
| Purity | Impure (needs distillation) | Pure |
| Temperature | Low (25–35°C) | High (300°C) |
Carboxylic Acids
Carboxylic acids are a homologous series with the functional group −COOH (a carboxyl group). Their general formula is:
CₙH₂ₙ₊₁COOH
| Name | Formula |
|---|---|
| Methanoic acid | HCOOH |
| Ethanoic acid | CH₃COOH |
| Propanoic acid | C₂H₅COOH |
Properties of Carboxylic Acids
- Dissolve in water to form acidic solutions (pH below 7)
- They are weak acids — they only partially ionise in water
- CH₃COOH ⇌ CH₃COO⁻ + H⁺ (equilibrium lies to the left)
Reactions (Same as Other Acids, but Slower)
With metals:
2CH₃COOH + Mg → (CH₃COO)₂Mg + H₂
With carbonates:
2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂
With alkalis (neutralisation):
CH₃COOH + NaOH → CH₃COONa + H₂O
Because carboxylic acids are weak acids, these reactions are slower than with strong acids at the same concentration.
Esters
Esters are formed when a carboxylic acid reacts with an alcohol in the presence of an acid catalyst (usually concentrated sulfuric acid).
carboxylic acid + alcohol → ester + water
This is a condensation reaction (a small molecule, water, is lost).
Example:
ethanoic acid + ethanol → ethyl ethanoate + water
CH₃COOH + C₂H₅OH → CH₃COOC₂H₅ + H₂O
Naming Esters
The name has two parts:
1. First part comes from the alcohol (ethanol → ethyl)
2. Second part comes from the acid (ethanoic acid → ethanoate)
So ethanol + ethanoic acid → ethyl ethanoate
methanol + propanoic acid → methyl propanoate
Properties and Uses of Esters
- Volatile — they evaporate easily
- Sweet, fruity smells — used in perfumes, food flavourings
- Used as solvents in nail varnish removers and paints
Exam Tips
- Know the functional groups: −OH for alcohols, −COOH for carboxylic acids, −COO− for esters
- Carboxylic acids are weak acids — they partially ionise (use ⇌)
- Esters are made from acid + alcohol with a sulfuric acid catalyst
- Know both methods of making ethanol and be able to compare them
- Oxidation of alcohols turns potassium dichromate from orange to green