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
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
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
- Functional group of alcohols is -OH.
- Fermentation: yeast, 30-40 °C, no air, gives ethanol + CO2.
- Oxidation of ethanol gives ethanoic acid.