1. Ethanol: preparation and physical properties
Ethanol (ethyl alcohol), C₂H₅OH or CH₃–CH₂–OH: an ethyl group joined to the hydroxyl group, –OH, the functional group of alcohols (general formula CₙH₂ₙ₊₁OH).
Preparation by hydrolysis of an alkyl halide: boil bromoethane with aqueous NaOH or KOH; OH⁻ replaces Br. (Alcoholic KOH gives ethene instead.)
Physical properties: colourless, volatile liquid; pleasant smell, burning taste; boils at 78 °C, freezes at about −114 °C; density 0.79 g cm⁻³ (lighter than water); miscible with water in all proportions (hydrogen bonding through –OH); good solvent; neutral to litmus.
| Property | Ethanol |
|---|---|
| Appearance | Colourless, volatile liquid |
| Boiling point | 78 °C |
| Density | 0.79 g cm⁻³ |
| Solubility in water | Miscible in all proportions |
| Litmus | Neutral |
Formulas
- Ethanol from bromoethane
Key terms
- Alcohol
An organic compound in which a hydroxyl group, –OH, is joined to a saturated carbon atom.
- Hydrolysis (of an alkyl halide)
Replacement of the halogen atom of an alkyl halide by –OH on boiling with an aqueous alkali.
2. Chemical properties of ethanol
- Combustion: burns with a blue, non-sooty flame; CO₂ and H₂O; much heat (fuel).
- Sodium: brisk effervescence of hydrogen; sodium ethoxide C₂H₅ONa is left (gentler than sodium with water).
- Esterification: warmed with ethanoic acid and a few drops of conc. H₂SO₄, gives ethyl ethanoate (fruity smell) and water; reversible.
- Dehydration: heated with excess conc. H₂SO₄ at about 170 °C, gives ethene.
| Reagent and condition | Product | Observation |
|---|---|---|
| Air, ignite | CO₂ + H₂O | Blue flame |
| Sodium | C₂H₅ONa + H₂ | Effervescence |
| Ethanoic acid, conc. H₂SO₄, warm | CH₃COOC₂H₅ + H₂O | Fruity smell |
| Excess conc. H₂SO₄, 170 °C | C₂H₄ + H₂O | Gas decolourises bromine water |
Formulas
- Combustion
- With sodium
- Esterification
- Dehydration
Key terms
- Sodium ethoxide
The compound C₂H₅ONa formed, with hydrogen, when sodium reacts with ethanol.
- Esterification
The reaction of a carboxylic acid with an alcohol, in the presence of concentrated sulphuric acid, to form an ester and water.
3. Denatured, methylated and spurious alcohol; uses of ethanol
Denatured alcohol: ethanol made unfit to drink by adding methanol, pyridine and a dye (such as copper sulphate). Industrial ethanol carries no liquor tax; denaturing prevents its misuse as a drink. Methylated spirit: ethanol denatured mainly with methanol.
Spurious alcohol: illicit liquor, often containing methanol. In the body methanol is oxidised to methanal and methanoic acid, which damage the optic nerve: blindness and even death.
Uses of ethanol: solvent (medicines, tinctures, perfumes, varnishes); fuel and petrol blend (E20); antiseptic and hand sanitiser; thermometer liquid for cold places (freezes at about −114 °C); making ethanoic acid, esters and ethene; alcoholic drinks.
| Use of ethanol | Property it depends on |
|---|---|
| Solvent in medicines and perfumes | Dissolves many organic substances |
| Fuel | Burns with a clean, hot flame |
| Sanitiser | Kills germs |
| Thermometers for cold places | Very low freezing point |
Key terms
- Denatured alcohol
Ethanol made unfit for drinking by adding poisonous or unpleasant substances such as methanol and pyridine.
- Spurious alcohol
Illegally made liquor, often contaminated with methanol, which can cause blindness or death.
4. Ethanoic (acetic) acid: structure and physical properties
Ethanoic acid, CH₃COOH: a methyl group joined to the carboxyl group, –COOH (the carboxyl carbon has one C=O and one C–OH). Molecular mass 60. Vinegar is a 5–8 per cent solution of it in water.
Physical properties: colourless liquid; pungent vinegar smell; sour taste; boils at 118 °C; melts at about 17 °C (16.6 °C); miscible with water; the pure acid is corrosive.
Glacial acetic acid: pure (anhydrous) ethanoic acid. Below about 17 °C it freezes into colourless, ice-like crystals, hence "glacial".
| Property | Ethanol | Ethanoic acid |
|---|---|---|
| Smell | Pleasant | Vinegar |
| Boiling point | 78 °C | 118 °C |
| Melting point | about −114 °C | about 17 °C |
| Blue litmus | No change | Red |
Key terms
- Carboxyl group
The functional group –COOH of carboxylic acids: a carbon atom with a C=O bond and a C–OH bond.
- Glacial acetic acid
Pure, water-free ethanoic acid, which freezes at about 17 °C to an ice-like solid.
5. Chemical properties of ethanoic acid
- Litmus: turns blue litmus red. It is a weak acid: only partly ionised in water (CH₃COOH ⇌ CH₃COO⁻ + H⁺).
- Alkalis: neutralisation gives a salt (ethanoate) and water.
- Alcohols (esterification): with ethanol and a few drops of conc. H₂SO₄ (warm), gives ethyl ethanoate; with methanol gives methyl ethanoate. Evidence: the mixture poured into water gives an oily layer with a sweet, fruity smell. Conc. H₂SO₄ acts as catalyst and dehydrating agent; the reaction is reversible.
| Reagent | Product | Type of reaction |
|---|---|---|
| Blue litmus | Turns red | Acidic nature |
| NaOH solution | Sodium ethanoate + water | Neutralisation |
| Ethanol, conc. H₂SO₄ | Ethyl ethanoate + water | Esterification |
Formulas
- With sodium hydroxide
- With potassium hydroxide
- With methanol: methyl ethanoate
Key terms
- Weak acid
An acid that is only partly ionised in water, so its solution contains relatively few hydrogen ions.
- Ester
A compound formed from a carboxylic acid and an alcohol, with the group –COO–; esters have fruity smells.
6. Uses of ethanoic acid
- Vinegar: flavouring food and preserving pickles (the acid stops microbes from growing).
- Esters: for perfumes and artificial food flavourings.
- Rubber industry: coagulating latex.
- Plastics and fibres: cellulose acetate (photographic film, rayon).
- Laboratory: a reagent and solvent; making ethanoates such as sodium ethanoate.
| Use | Why ethanoic acid |
|---|---|
| Preserving pickles | Acidic: stops microbes growing |
| Making esters | Reacts with alcohols |
| Coagulating latex | Acid makes rubber particles clump |
Key terms
- Vinegar
A dilute (about 5–8 per cent) solution of ethanoic acid in water, used in food.
Exam tips
Common mistakes
- Writing aqueous and alcoholic KOH as if they gave the same product with bromoethane.
- Unbalanced sodium equation: write 2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂.
- Calling ethanol acidic because it reacts with sodium (it is neutral to litmus).
- Writing hydrogen instead of water as a product of ethanoic acid with an alkali.
- Naming the ester with the acid part first (it is ethyl ethanoate, methyl ethanoate).
How marks are usually awarded
- Equation (1 mark): correct formulae, balanced, with the condition.
- Observation (1 mark): "brisk effervescence" with sodium; "fruity smell" for an ester; "blue litmus turns red" for the acid.
- Definitions (1 mark): denatured alcohol, glacial acetic acid — include the key reason.