Formulae of 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 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
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.
Oxidation of Ethanol
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.
Edexcel GCSE Chemistry Notes
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