Carboxylic Acids (SQA National 5 Chemistry): Revision Note

The carboxylic acids

What are carboxylic acids?

• Carboxylic acids are a family of organic compounds

  • Like alcohols, they are not hydrocarbons because they contain oxygen

• For a full table of the first carboxylic acids and their structures, see the full & shortened structural formulae

• Carboxylic acids are a homologous series with four key facts:

1. They contain the -COOH (carboxyl) functional group

• All carboxylic acids contain a carboxyl group

  • This is a carbon atom double-bonded to one oxygen atom and single-bonded to an -OH group

• This is the functional group that gives this family its characteristic acidic properties.

2. They are represented by a general formula

• The general formula for the carboxylic acids homologous series is C_nH_2n+1COOH

  • This formula applies to saturated, straight-chain carboxylic acids

3. They are used to make preservatives, soaps, and medicines

• Carboxylic acids are important in manufacturing

• They are used as building blocks for:

  • Preservatives (to stop food from spoiling)

  • Soaps

  • A variety of medicines

4. A key example is ethanoic acid (vinegar)

• Vinegar is a dilute solution of ethanoic acid (CH₃COOH).

• It is a non-toxic acid

  • This is why it is used safely in food (as a preservative) and in household cleaning products

Names & formulae of carboxylic acids

• Like other organic compounds, carboxylic acids are named systematically

• There are three skills for carboxylic acids:

  • Naming carboxylic acids from their structure

  • Drawing carboxylic acids from their name

  • Determining the formula of a carboxylic acid

1. Naming carboxylic acids

• Naming carboxylic acids is straightforward as the functional group is always at the end of the chain

• The name of a straight-chain carboxylic acid is made of two parts:

  1. A prefix to show the number of carbon atoms

  2. The ending "-oic acid" to show it is a carboxylic acid

• The rules for naming carboxylic acids:

  1. Find the longest continuous chain of carbon atoms

     • This chain must contain the -COOH group

     • The carbon of the -COOH group must be included in the count

  2. Add the ending "-oic acid"

     • No number is needed because the carboxyl group is always on carbon 1

  3. If there are branches, you name the branches in the same way as alkanes

2. Drawing carboxylic acids

• You can work backwards from the name to draw the structure of a carboxylic acid

• The rules for drawing carboxylic acids:

  1. The -COOH functional group

     • "-oic acid" means one of the end carbons is a -COOH group

     • Draw this group first, showing the C=O and O-H bonds

  2. Identify the longest carbon chain

     • Identify the number of carbons in the longest chain

     • For example, "pent" means a 5-carbon chain

  3. Add the main carbon chain

     • Make sure to include the COOH carbon as one of the atoms counted in the chain

  4. Add the hydrogens to complete the structure

     • Make sure every carbon atom has exactly four bonds

3. The molecular formula of carboxylic acids

• The general formula of carboxylic acids can be used to determine the molecular formula