The Carboxylic Acid Functional Group

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

CnH2n+ 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.
  • The first four carboxylic acids are methanoic acid, ethanoic acid, propanoic acid and butanoic acid.

Carboxylic Acid - Functional Group, IGCSE & GCSE Chemistry revision notes

Diagram of he side chain and -OH group in ethanol which characterizes its chemistry

The First Four Carboxylic Acids

  • 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.

Carboxylic Acids- The First Four, IGCSE & GCSE Chemistry revision notes

Table showing the formulae and structures of the first four carboxylic acids

Properties of Carboxylic Acids

  • The carboxylic acids behave like other acids.
  • They dissolve in water and partially ionise producing weakly acidic solutions with pH values of below 7.
  • They react with metals to form salt and water 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.

Reactions of Carboxylic Acids

  • The salts formed by the reaction of carboxylic acids all end –anoate.
  • So methanoic acid forms a salta called methanoate, ethanoic a salt called ethanoate etc.
  • Carboxylic acids are organic acids which deionise slightly, so they are weak acids.
  • 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.
  • Ethanoic acid will react with ethanol in the presence of concentrated sulfuric acid (catalyst) to form ethyl ethanoate:

CH3COOH + C2H5OH → CH3COOC2H5 + H2O

Preparing-Ethyl-Ethanoate_, IGCSE & GCSE Chemistry revision notes

Diagram showing the formation of ethyl ethanoate

AQA GCSE Chemistry Notes

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Morgan Curtin Chemistry

Author: Morgan

Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.