Carboxylic Acids (AQA GCSE Chemistry)

• Carboxylic acids are a homologous series of compounds that have the general formula of

C_nH_2n+1 COOH 

• They differ by one -CH₂ in the molecular formulae from one member to the next
• They show a gradation in their physical properties:
  • Boiling points increase with increased carbon chain length
  • Viscosity increases with increased carbon chain length
• They have similar chemical properties

Diagram of the general structure of a carboxylic acid. The R- represents a varying hydrocarbon chain

• The names and structure of the first four carboxylic acids are shown below
• In terms of naming, the same system is used as for alkanes and alkenes, with the final ‘e’ being replaced with ‘oic’ and then the word acid

The structures and formulae of the first four carboxylic acids

• The carboxylic acids behave like other acids
• They react with metals to form a salt and hydrogen and with carbonates to form a salt, water and carbon dioxide gas
• They take part in neutralisation reactions to produce salt and water
• Ethanoic acid (also called acetic acid) is the acid used to make vinegar, which contains around 5% by volume ethanoic acid
• The salts formed by the reaction of carboxylic acids all end –anoate
• So methanoic acid forms a salt called methanoate, ethanoic a salt called ethanoate etc
• In the reaction with metals, a metal salt and hydrogen gas are produced
• For example in the reaction of ethanoic acid with magnesium, the salt magnesium ethanoate is formed:

2CH₃COOH + Mg → (CH₃COO)₂Mg + H₂

• In the reaction with hydroxides a salt and water are formed in a neutralisation reaction
• For example in reaction with potassium hydroxide the salt potassium propanoate is formed by reaction with propanoic acid:

CH₃CH₂COOH + KOH → CH₃CH₂COOK + H₂O

• In the reaction with carbonates a metal salt, water and carbon dioxide gas are produced
• For example in reaction with potassium carbonate the salt potassium butanoate is formed by reaction with butanoic acid:

2CH₃CH₂CH₂COOH + K₂CO₃ → 2CH₃CH₂CH₂COOK + H₂O + CO₂

• Alcohols and carboxylic acids react to make esters in esterification reactions in the presence of an acid catalyst, usually concentrated sulfuric acid
• Esters are compounds with the functional group R-COO-R
• Esters are sweet smelling oily liquids used in food flavourings and perfumes
• They are volatile, meaning they vapourise easily
• Ethanoic acid will react with ethanol in the presence of concentrated sulfuric acid (catalyst) to form ethyl ethanoate:

CH₃COOH + C₂H₅OH → CH₃COOC₂H₅ + H₂O

Diagram showing the formation of ethyl ethanoate