Ketone
A concise revision guide to ketone functional groups, the carbonyl group, the -one suffix, position numbers, diones and common naming traps in organic chemistry.
What Is a Ketone?
A ketone is an organic compound containing a carbonyl group, written as C=O, where the carbonyl carbon is bonded to two carbon atoms.
This means a ketone has the general structural pattern RCO R’, often written as R-C(=O)-R’. The R groups may be alkyl groups or parts of a ring.
The carbonyl group is the functional group of ketones. It controls the naming pattern and helps identify the homologous series.
Key idea: In a ketone, the C=O group is within the carbon chain, not at the end of the chain.
The carbonyl carbon is attached to two carbon atoms, which distinguishes ketones from aldehydes.
Quick Check: Ketone or Not?
Decide quickly whether each structural formula is a ketone by looking at what the C=O carbon is bonded to.
Naming Ketones Using the -one Suffix
Ketones are usually named using the suffix -one. The parent chain is selected so that it includes the carbonyl carbon.
When a position number is needed, the carbon chain is numbered so that the carbonyl carbon receives the lowest possible number.
| Homologous series | Functional group | Prefix | Suffix |
|---|---|---|---|
| Ketones | C=O bonded to two carbon atoms | oxo- | -one |
Exam focus: The suffix -one identifies the compound as a ketone when the carbonyl group is the principal functional group.
Quick Check: Lowest Possible Number
Decide whether a student's name for a ketone is correct.
Position Numbers in Ketone Names
Some ketone names need a position number to show where the C=O group is located in the carbon chain.
This becomes especially important when there is more than one possible position for the carbonyl group. For example, in a five-carbon chain, the names pentan-2-one and pentan-3-one describe different structures.
For simple cases where only one ketone position is possible, the position number may be omitted in common A Level usage.
Remember: A ketone carbonyl cannot be on carbon 1 in an open-chain molecule, because that would place the C=O group at the end of the chain.
Quick Check: Name the Ketone
Type the IUPAC name of each ketone from its structural formula.
Quick Check: Counting Heptanones
Work out how many different straight-chain ketones have seven carbon atoms.
Cyclic Ketones
Ketones can also be found in cyclic molecules. In a cyclic ketone, the carbonyl carbon is part of the ring and is bonded to two other carbon atoms in the ring.
Cyclohexanone is a common example. The name combines the cyclohexane ring framework with the ketone suffix -one.
The carbonyl carbon is part of the ring, so the structure is named as a cyclic ketone.
Quick Check: Picture a Cyclic Ketone
Apply the idea of a cyclic ketone to a ring that the notes do not show.
Diones and Common Naming Traps
If a molecule contains two ketone carbonyl groups, the suffix can become -dione. The parent alkane name keeps the final e before adding the suffix, as in butane-2,3-dione.
Take care with carbonyl groups at the end of a chain. A terminal C=O group is not a ketone carbonyl because the carbonyl carbon is not bonded to two carbon atoms.
A carbonyl group at carbon 1 is terminal, so check whether the structure is really a ketone before using the -one suffix.
For ketone diones, each carbonyl carbon must be bonded to two carbon atoms, so terminal carbonyl groups need careful checking.
Exam focus: Do not identify a molecule as a ketone from the C=O group alone. Check what the carbonyl carbon is bonded to.
Quick Check: Diones and Traps
In each round pick the one accurate statement about a molecule with two C=O groups.
Quick Check: Explain the Naming Error
Write a short explanation of why a student's ketone name cannot be right.
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Some ionic radii are shown.
| Ion | Ionic radius / nm |
|---|---|
| Na+ | 0.102 |
| K+ | 0.138 |
| F− | 0.133 |
| Cl− | 0.180 |
Which compound has the strongest ionic bonding?
Explain why the metallic bonding in magnesium is much stronger than that in sodium.
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