Ethanoic Acid & Esterification Reactions (Cambridge International IGCSE Chemistry)

Formation of ethanoic acid

Spec point — 11.7.2 · Describe the formation of ethanoic acid by the oxidation of ethanol: (a) with acidified aqueous potassium manganate(VII) (b) by bacterial oxidation during vinegar production

Formation of ethanoic acid

Extended tier only

• Two methods used to make carboxylic acids are:

  • Oxidation by fermentation
  • Using oxidising agents

• The microbial oxidation (fermentation) of ethanol will produce a weak solution of vinegar (ethanoic acid)

• This occurs when a bottle of wine is opened as bacteria in the air (acetobacter) will use atmospheric oxygen from air to oxidise the ethanol in the wine

C₂H₅OH (aq) + O₂ (g) → CH₃COOH (aq)+ H₂O (l)

• The acidic, vinegary taste of wine which has been left open for several days is due to the presence of ethanoic acid
• Alternatively, oxidising agent potassium manganate(VII) can be used
• This involves heating ethanol with acidified potassium manganate(VII) in the presence of an acid
• The heating is performed under reflux which involves heating the reaction mixture in a vessel with a condenser attached to the top
• The condenser prevents the volatile alcohol from escaping the reaction vessel as alcohols have low boiling points
• The equation for the reaction is:

CH₃CH₂OH (aq) + 2[O] → CH₃COOH (aq) + H₂O (l)

• The solution will change from purple to colourless
• The oxidising agent is represented by the symbol for oxygen in square brackets

Diagram showing the experimental setup for the oxidation with KMnO₄ using reflux apparatus

Ethanol is heated with acidified potassium manganate(VII) in the presence of an acid

Esterification

Spec point — 11.7.3 · Describe the reaction of a carboxylic acid with an alcohol using an acid catalyst to form an ester

Extended tier only

• Alcohols and carboxylic acids react to make esters in esterification reactions
• Esters are compounds with the functional group R-COO-R

• Esters are sweet-smelling oily liquids used in food flavourings and perfumes
• An acid catalyst is required for an esterification reaction
• Ethanoic acid will react with ethanol in the presence of concentrated sulfuric acid (catalyst) to form ethyl ethanoate:

CH₃COOH (aq) + C₂H₅OH (aq) ⇌ CH₃COOC₂H₅ (aq) + H₂O (l)

Diagram showing the formation of ethyl ethanoate

During this esterification reaction, a molecule of water is also produced

Naming Esters

• An ester is made from an alcohol and carboxylic acid

• The first part of the name indicates the length of the carbon chain in the alcohol, and it ends with the letters ‘- yl’

• The second part of the name indicates the length of the carbon chain in the carboxylic acid, and it ends with the letters ‘- oate’

  • E.g. the ester formed from pent anol and butanoic acid is called pentyl butanoate

Diagram showing the origin of each carbon chain in ester; this ester is ethyl butanoate

Summary table of some esters and the compounds they are formed from