Naming Carboxylic Acids

Naming carboxylic acids

Carboxylic acids are characterised by having a carboxyl group, which has the formula \(-\text{COOH}\). In a carboxyl group a carbon atom is double-bonded to an oxygen atom (a carbonyl group), and is also bonded to a hydroxyl (alcohol) group. The IUPAC suffix for a carboxylic acid is -oic acid.

Example:

Question

Give the IUPAC name and molecular formula for the following compound:

Identify the functional group

The compound has a \(-\text{COOH}\) group and is therefore a carboxylic acid. The suffix will be -oic acid.

Find the longest carbon chain that contains the functional group

There are five carbon atoms in the longest chain that contains the functional group, and only single bonds between carbon atoms. The prefix for this compound is pentan-.

Number the carbon atoms in the carbon chain

The carbon atoms will be numbered from right to left so that the carboxylic acid functional group has the lowest numbered carbon atom.

Look for any branched groups

There are no branched groups in this compound.

Combine the elements of the compound's name into a single word in the order of branched groups; prefix, name ending according to the functional group

The compound's name is pentanoic acid.

Reduce the structural representation to the molecular formula

There are 5 carbon atoms, 10 hydrogen atoms and 2 oxygen atoms so the molecular formula is \(\text{C}_{5}\text{H}_{10}\text{O}_{2}\).

Example:

Question

Draw the structural and condensed structural representations for the organic compound 2-ethylhexanoic acid.

Identify the functional group

The compound has the suffix -oic acid. It is therefore a carboxylic acid and has a \(-\text{COOH}\) group. This group can only be on carbon 1 (at the end of the carbon chain).

Find the longest carbon chain that contains the functional group

The prefix is hex- therefore there are 6 carbons in the longest chain.

Place the functional group as well as any branched groups

The \(-\text{COOH}\) group contains the first carbon atom. The ethyl group is attached to the second carbon atom.

Remember that the main carbon chain must contain the functional group. This is hexanoic acid and not a substituted heptane.

Combine this information and add the hydrogen atoms

Condense the structural representation

First condense the main chain, including the \(-\text{COOH}\) functional group:

\(\text{CH}_{3}\text{CH}_{2}\text{CH}_{2}\text{CH}_{2}\text{CHCOOH}\)

Then add the side chains (in brackets) on the relevant carbon atoms:

\(\text{CH}_{3}\text{CH}_{2}\text{CH}_{2}\text{CH}_{2}\text{CH}(\text{CH}_{2}\text{CH}_{3})\text{COOH}\)

Optional Activity: Building carboxylic acids

Using atomic model kits build the molecules of methanoic acid, ethanoic acid, butanoic acid, pentanoic acid and octanoic acid. If you don't have an atomic model kit remember that you can use jelly tots (or playdough) and toothpicks. Use different colour jelly tots to represent the different atoms and two toothpicks to represent double bonds.

You should see that all these compounds have a similar formula. Remember, they belong to the carboxylic acid homologous series. What is the general formula for this series?
Remember that carbon must have four bonds, oxygen must have two bonds and hydrogen can only have one bond. Thinking about this fact, is it possible to have the carboxylic acid (\(-\text{COOH}\)) group in a position other than the last (or first) carbon atom?

Naming Carboxylic Acids