The alcohol homologous series
Alcohols form a homologous series whose members share the general formula CnH2n+1OH. Every alcohol contains the hydroxyl functional group, -OH, which gives the family its similar chemical reactions. The first members are methanol (CH3OH), ethanol (C2H5OH) and propanol (C3H7OH). Ethanol is the most important alcohol at IGCSE.
Key idea / 要点
General formula: CnH2n+1OH. Functional group: -OH (hydroxyl). This -OH group controls the reactions of every member.
Making ethanol
Ethanol is made in two ways. Fermentation uses yeast on a sugar solution at about 30-40 °C in the absence of air; the enzymes in yeast convert glucose into ethanol and carbon dioxide: C6H12O6 → 2C2H5OH + 2CO2. This is a renewable, batch process. Alternatively, ethanol is made by the catalytic addition of steam to ethene using phosphoric acid as catalyst at about 300 °C and 60 atm: C2H4 + H2O → C2H5OH. This is a fast, continuous process using a finite resource.
Reactions of ethanol
Ethanol burns completely in plenty of air to give carbon dioxide and water, releasing energy, so it is used as a fuel. It can be oxidised by warming with acidified potassium dichromate(VI) (or by air/microbes) to form ethanoic acid, CH3COOH. Ethanol is also a widely used solvent.
Worked example / 例题
Balanced combustion: C2H5OH + 3O2 → 2CO2 + 3H2O. So 1 mol of ethanol needs 3 mol of oxygen. To burn 2 mol of ethanol you therefore need 2 × 3 = 6 mol of oxygen.
Remember / 记住
- Functional group of alcohols is -OH.
- Fermentation: yeast, 30-40 °C, no air, gives ethanol + CO2.
- Oxidation of ethanol gives ethanoic acid.