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.
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.
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.
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.
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.
Check Your Understanding
Use these short activities to check ketone functional groups, the -one suffix, cyclic ketones and carbonyl naming traps before moving on.
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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.