Carboxylic Acids (Edexcel GCSE Chemistry)

• Carboxylic acids is the name given to compounds containing the functional group carboxyl, -COOH
• The naming of a carboxylic acid follows the pattern alkan + oic acid
• The names and structure of the first four carboxylic acids are shown below

The Names and Structures of the First Four Carboxylic Acids

Preparation of ethanoic acid

• Alcohols undergo oxidation to produce carboxylic acids when treated with oxidising agents
• When ethanol is heated with acidified potassium dichromate solution the ethanol oxidises to ethanoic acid
• The equation for the reaction is:

CH₃CH₂OH + [O]  →  CH₃COOH + H₂O

• The oxidising agent is represented by the symbol for oxygen in square brackets
• The reaction is slow so the mixture is heated to its boiling point for about an hour; to avoid the substances evaporating a condenser is placed above the reaction flask that prevents volatile liquids from escaping
• During the reaction the potassium dichromate turns from orange to green

Ethanol can be oxidised by heating it with potassium dichromate in sulfuric acid. The solution turns  from orange to green during the reaction

• Organic molecules that belong to the same homologous series react in the same way, so the products of those reactions can be predicted
• Homologous series are families or groups of organic compounds that have similar features and chemical properties due to them having the same functional group
• All members of a homologous series have:
  • The same general formula
  • The difference in the molecular formula between one member and the next is CH₂
  • Gradation in their physical properties
  • Same functional group
  • Similar chemical properties

• The chemistry of homologous series is therefore determined by the functional group
• We can use this to predict how other molecules in a homologous series will react
• Previously we have seen that ethanol can be oxidised to ethanoic acid using acidified potassium dichromate
• The next member of the series is propanol, so we can predict the reaction would be:

CH₃CH₂CH₂OH + [O]  →  CH₃CH₂COOH + H₂O

      propanol                   propanoic acid

• Although the homologous series allows us to predict what the reaction products should be, it tells us nothing about the rate or extent of the reaction
• For example, as the chain length increases in alcohols the combustion or oxidation reactions may be slower or incomplete as the carbon chain influences the reactivity of the functional group