Carboxylic Acids

Specification Point 9.29C:

• Recall the formulae of molecules of the carboxylic acids, methanoic, ethanoic, propanoic and butanoic acids, and draw the structures of these molecules, showing all covalent bonds

• Carboxylic acids are a homologous series of compounds that have the general formula of C_nH_2n+ 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.

Specification Point 9.30C:

• Recall that the functional group in carboxylic acids is –COOH

• All carboxylic acids contain the carboxyl functional group (-COOH) which is the part of the acid molecule that is responsible for their characteristic reactions.
• As with other organic molecules, a large R represents the side chain which doesn´t  affect the chemistry of the molecule.

Diagram showing the -COOH group which characterizes the chemistry 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.

Specification Point 9.31C:

• Recall that ethanol can be oxidised to produce ethanoic acid and extend this to other alcohols (reagents not required)

• Ethanol can be oxidised using an oxidising agent to produce the carboxylic acid ethanoic acid:

C₂H₅OH + [O] → CH₃COOH + H₂O

• Note that this is an oxidation reaction, not combustion.
• This reaction occurs when a bottle of wine is left open for a few days as bacteria in the air (acetobacter) use atmospheric oxygen from air to oxidise the ethanol.
• The acidic, vinegary taste of wine which has been left open for several days is due to the presence of ethanoic acid.
• The same reaction will occur with any alcohol in the presence of an oxidising agent, producing the corresponding carboxylic acid.
• E.g. propanol can be oxidised to propanoic acid, hexanol can be oxidised to hexanoic acid etc.