Naming Ketones
Naming ketones
Tip:
Ketone is pronounced keytone. Therefore propanone is pronounced propanown.
If the carbonyl group is in the middle of the carbon chain, the compound is called a ketone. A ketone has the suffix -one.
Fact:
Note that butanone can only be butan-2-one. If the carbonyl group were on carbon 1 it would be an aldehyde, while if it were on carbon 3 we would simply count from the other side of the molecule. The 2 is still required for IUPAC naming however.
Example:
Question
Give the IUPAC name and molecular formula for the following compound:
Identify the functional group
The compound has a C\(=\)O (carbonyl) group and no other functional groups. It is therefore either an aldehyde or a ketone. The carbonyl group is not at the end of the chain. Therefore the compound is a ketone and the suffix will be -one.
Find the longest carbon chain containing the functional group
There are four carbons in the longest chain that contains the functional group, and only single carbon-carbon bonds. The prefix for this compound will be butan-.
Number the carbon atoms in the carbon chain
The carbon atoms will be numbered from left to right so that the carbon atom of the ketone group has the lowest number possible (-2-one).
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 butan-2-one or 2-butanone.
Reduce the structural representation to the molecular formula
There are 4 carbon atoms, 8 hydrogen atoms and 1 oxygen atom so the molecular formula is \(\text{C}_{4}\text{H}_{8}\text{O}\).
(Remember that there is no structural information given by the molecular formula)
Example:
Question
Give the IUPAC name for the following compound:
Identify the functional group
The compound has a C\(=\)O (carbonyl) group and no other functional groups. It is therefore either an aldehyde or a ketone. The carbonyl group is not at the end of the chain. Therefore the compound is a ketone and the suffix will be -one.
Find the longest carbon chain that contains the functional group
The longest carbon chain that contains the functional group has four carbon atoms in it, and only single bonds. The prefix for this compound will be butan-.
Number the carbon atoms in the carbon chain
The carbon atoms will be numbered from left to right so that the carbon atom of the ketone functional group has the lowest possible number. The suffix will be -2-one.
Look for any branched groups
There is a branched group on carbon 3. This group has only one carbon atom. The branched group is attached to the third carbon atom (3-methyl).
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 3-methylbutan-2-one.
Example:
Question
Draw the structural and condensed structural representations for the organic compound 3-methylpentanal.
Identify the functional group
The compound has the suffix -al. It is therefore an aldehyde and has a C\(=\)O (carbonyl) group on the first carbon atom.
Find the longest carbon chain that contains the functional group
The prefix is pent- so there are 5 carbon atoms in the longest chain.
Place the functional group as well as any branched groups
There is a C\(=\)O (carbonyl) group at the first carbon atom and a methyl group attached to the third carbon atom (3-methyl).
Combine this information and add the hydrogen atoms
Condense the structural representation
First condense the main chain including the carbonyl group oxygen atom: \(\text{CHOCH}_{2}\text{CHCH}_{2}\text{CH}_{3}\)
Then add the side chains on the relevant carbon atoms: \(\text{CHOCH}_{2}\text{CH}(\text{CH}_{3})\text{CH}_{2}\text{CH}_{3}\)
Example:
Question
Draw the structural and condensed structural representations for the organic compound 3-methylpentan-2-one.
Identify the functional group
The compound has the suffix -one. It is therefore a ketone and has a C\(=\)O (carbonyl) group. This group cannot be on the first carbon atom.
Find the longest carbon chain that contains the functional group
The prefix is pent- therefore there are 5 carbon atoms in the longest chain.
Place the functional group as well as any branched groups
There is a C\(=\)O (carbonyl) group at the second carbon atom (-2-one) and a methyl group attached to the third carbon atom (3-methyl).
Combine this information and add the hydrogen atoms
Condense the structural representation
First condense the main chain including the carbonyl group oxygen atom: \(\text{CH}_{3}\text{COCHCH}_{2}\text{CH}_{3}\)
Then add the side chains on the relevant carbon atoms: \(\text{CH}_{3}\text{COCH}(\text{CH}_{3})\text{CH}_{2}\text{CH}_{3}\)
This lesson is part of:
Organic Molecules