11.6 Alcohols

2026 Syllabus Objectives

  1. Core: Describe the manufacture of ethanol by:

    • (a) fermentation of aqueous glucose at 25–35°C in the presence of yeast and in the absence of oxygen
    • (b) catalytic addition of steam to ethene at 300°C and 6000 kPa / 60 atm in the presence of an acid catalyst

  2. Core: Describe the combustion of ethanol

  3. Core: State the uses of ethanol as:

    • (a) a solvent
    • (b) a fuel

  4. Supplement: Describe the advantages and disadvantages of the manufacture of ethanol by:

    • (a) fermentation
    • (b) catalytic addition of steam to ethene

What Are Alcohols? 🔬

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Alcohols are a homologous series of organic compounds that contain the hydroxyl functional group (–OH). They have the general formula CnH2n+1OH\mathrm{C}_n\mathrm{H}_{2n+1}\mathrm{OH}.

Ethanol (CH3CH2OH\mathrm{CH}_3\mathrm{CH}_2\mathrm{OH} or C2H5OH\mathrm{C}_2\mathrm{H}_5\mathrm{OH}) is one of the most important and well-known alcohols. It contains two carbon atoms, six hydrogen atoms, one oxygen atom, and the characteristic –OH functional group.

Structure of Ethanol

The ethanol molecule can be represented in several ways:

  • Molecular formula: C2H5OH\mathrm{C}_2\mathrm{H}_5\mathrm{OH}
  • Structural formula: Shows the arrangement of atoms with single covalent bonds between carbon, hydrogen, and oxygen atoms
  • 3D representation: Ball-and-stick models show the spatial orientation of the molecule

The bonding in ethanol involves:

  • Carbon-carbon single bonds
  • Carbon-hydrogen single bonds
  • Carbon-oxygen single bond
  • Oxygen-hydrogen single bond (the hydroxyl group)

Manufacture of Ethanol ⚗️

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Ethanol can be produced industrially using two different methods, each with distinct advantages and disadvantages.

Method 1: Fermentation 🧫

Fermentation is a reaction carried out using a living organism, usually yeast or bacteria, to produce a useful chemical compound. In the context of ethanol production, fermentation refers specifically to the production of ethanol from sugars.

The Fermentation Process

Fermentation involves the anaerobic respiration of sugars by yeast. This means the process occurs in the absence of oxygen. Yeasts are single-cell, living fungi that contain enzymes which catalyse the breakdown of sugar.

Key conditions for fermentation:

  • Temperature: 25–35°C (optimum temperature range for yeast enzyme activity)
  • Catalyst: Enzymes present in yeast
  • Atmosphere: Anaerobic (no oxygen present)
  • Raw material: Aqueous glucose solution

The Fermentation Equation

C6H12O6(aq)→yeast enzymes2C2H5OH(aq)+2CO2(g)\mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6(aq) \xrightarrow{\text{yeast enzymes}} 2\mathrm{C}_2\mathrm{H}_5\mathrm{OH}(aq) + 2\mathrm{CO}_2(g)

One molecule of glucose produces two molecules of ethanol and two molecules of carbon dioxide gas.

Important Features of Fermentation

Self-limiting process: Fermentation naturally stops when:

  • The ethanol concentration reaches approximately 14% (ethanol is toxic to yeast)
  • The sugar supply is exhausted
  • The yeast cells die

Laboratory setup: Fermentation can be carried out in a laboratory using a vessel equipped with an airlock. The airlock:

  • Allows carbon dioxide gas to escape from the vessel
  • Prevents airborne bacteria from entering
  • Maintains anaerobic conditions inside the vessel

Sources of sugar: Sugar is present in fruits, grains, plant sap, and nectar. For industrial production, specific forms of yeast are used to prevent unwanted side reactions.

Applications: Beyond ethanol production, fermentation is important in:

  • Biofuel production
  • Bread making (where carbon dioxide causes dough to rise)
  • Alcoholic beverage production

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