Carbon forms millions of compounds. Learn the families of hydrocarbons, how they react, and two everyday compounds: ethanol and ethanoic acid.
A homologous series is a family of compounds with the same functional group and general formula, in which neighbouring members differ by CH₂. Alkanes (CₙH₂ₙ₊₂) have only single bonds, alkenes (CₙH₂ₙ) have a C=C double bond, and alkynes (CₙH₂ₙ₋₂) have a C≡C triple bond. Isomers have the same molecular formula but different structures.
Write the molecular formulae of the first four alkanes.
CH₄ (methane), C₂H₆ (ethane), C₃H₈ (propane) and C₄H₁₀ (butane), from CₙH₂ₙ₊₂
What is the molecular formula of the alkene with three carbon atoms?
C₃H₆ (propene), from CₙH₂ₙ with n = 3
What are isomers? Give an example.
Compounds with the same molecular formula but different structures; butane and 2-methylpropane are both C₄H₁₀
Alkanes burn (CH₄ + 2O₂ → CO₂ + 2H₂O) and undergo substitution, for example in sunlight CH₄ + Cl₂ → CH₃Cl + HCl. Alkenes undergo addition: C₂H₄ + Br₂ → C₂H₄Br₂ decolourises bromine, and C₂H₄ + H₂ → C₂H₆ needs a nickel catalyst. Ethyne is prepared from calcium carbide: CaC₂ + 2H₂O → C₂H₂ + Ca(OH)₂.
What is the difference between a substitution and an addition reaction?
In substitution an atom is replaced by another, as Cl replaces H in methane; in addition a molecule adds across a double or triple bond and nothing is lost, as in ethene with bromine
How can you distinguish ethane from ethene?
Shake each with bromine water: ethene decolourises it, but ethane does not
How is ethyne prepared in the laboratory?
By adding water to calcium carbide: CaC₂ + 2H₂O → C₂H₂ + Ca(OH)₂
Ethanol, C₂H₅OH, contains the –OH group and is made by fermenting sugar with yeast: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂. It burns (C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O) and reacts with sodium (2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂). Ethanoic acid, CH₃COOH, contains the –COOH group and is the acid in vinegar. With ethanol and a little concentrated sulphuric acid it forms an ester: CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O.
Write the equation for the fermentation of glucose.
C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ (using zymase from yeast)
What are the products when ethanoic acid reacts with ethanol, and what is the reaction called?
Ethyl ethanoate (an ester) and water; the reaction is esterification, catalysed by concentrated sulphuric acid
What mass of ethyl ethanoate could form from 60 g of ethanoic acid? (C = 12, H = 1, O = 16)
The molar masses are 60 for the acid and 88 for the ester, and one mole gives one mole, so 88 g (theoretical)
1. Give the general formula of the alkanes.
2. What is the molecular formula of the alkane with five carbon atoms?
3. What is a homologous series?
4. What are isomers?
5. Name the functional group in ethanol.
6. Which of ethane and ethene decolourises bromine water?
7. Write the equation for the substitution of methane with chlorine.
8. Name the ester formed from ethanoic acid and ethanol.
9. How is ethyne prepared from calcium carbide?
10. What mass of ethanol is obtained from 180 g of glucose by fermentation, in theory? (C = 12, H = 1, O = 16)