Aldehyde
A concise revision guide to aldehyde functional groups, the -al suffix, formyl- prefix, terminal carbonyl groups, dialdehyde naming and priority over alcohols in IUPAC names.
What Is an Aldehyde?
An aldehyde is an organic compound containing the -CHO functional group. This group contains a carbonyl group, written as C=O, with the carbonyl carbon also bonded to a hydrogen atom.
The aldehyde functional group must occur at the end of a carbon chain. This is because the carbonyl carbon in an aldehyde is bonded to one hydrogen atom and only one carbon atom from the rest of the molecule.
The carbonyl carbon is automatically treated as carbon 1, so a position number is normally not needed for a simple aldehyde suffix.
Key idea: An aldehyde contains a terminal -CHO group. The carbonyl carbon is automatically numbered as carbon 1.
The aldehyde functional group is terminal because the carbonyl carbon is bonded to a hydrogen atom and the rest of the carbon chain.
Naming Aldehydes Using the -al Suffix
The name of an aldehyde usually ends in the suffix -al. The final -e of the parent alkane is removed before adding -al.
For example, ethane becomes ethanal. The aldehyde carbon is automatically carbon 1, so the name does not need to be written as ethan-1-al.
| Homologous series | Functional group | Prefix | Suffix | Example |
|---|---|---|---|---|
| Aldehydes | -CHO | formyl- | -al | ethanal |
Ethanal has two carbon atoms and a terminal -CHO group, so its name uses eth- and the aldehyde suffix -al.
In skeletal formulae, the carbonyl group is shown at the end of the chain to show that the compound is an aldehyde.
Why Aldehydes Do Not Usually Need Position Numbers
For most aldehydes, the C=O group is on the first carbon of the carbon chain. This means the position of the aldehyde group is fixed by the functional group itself.
Because the carbonyl carbon is automatically numbered as carbon 1, a simple aldehyde name uses -al without a location number for the suffix.
Identify the longest chain containing -CHO
Include the carbonyl carbon of the aldehyde group as part of the parent chain.
Number from the aldehyde carbon
The aldehyde carbonyl carbon is carbon 1, so substituents are numbered from this end.
Add the -al suffix
Replace the final -e of the parent alkane name with -al, such as ethane to ethanal.
Exam focus: Do not place the aldehyde group in the middle of the chain. A terminal C=O group with an attached hydrogen is what makes the compound an aldehyde.
Molecules with Two Aldehyde Groups
If a molecule contains two aldehyde groups, the suffix includes di before -al. The parent alkane name keeps the final e before the dial suffix.
For example, a four-carbon chain with aldehyde groups at both ends is named as butanedial. In some teaching resources, you may see the simplified label butandial, but the key naming idea is that two aldehyde groups are shown using the dial ending.
Two terminal aldehyde groups are indicated by the dial ending, with one -CHO group at each end of the carbon chain.
| Number of aldehyde groups | Name ending | Meaning |
|---|---|---|
| One | -al | One terminal -CHO group is present. |
| Two | -dial | Two terminal -CHO groups are present. |
Aldehydes Have Higher Priority Than Alcohols
Aldehydes have a higher priority than alcohols in IUPAC naming. If a molecule contains both an aldehyde group and an -OH group, the aldehyde normally provides the main suffix -al.
The alcohol group is then named as a substituent using the prefix hydroxy-, rather than using the alcohol suffix -ol.
For example, 4-hydroxybutanal contains an aldehyde group at carbon 1 and an -OH group on carbon 4.
The aldehyde group controls the suffix, while the -OH group is named using the hydroxy- prefix because it has lower naming priority here.
Remember: If -CHO and -OH are both present, use -al for the aldehyde and hydroxy- for the alcohol group.
Check Your Understanding
Use these short activities to check aldehyde functional groups, the -al suffix, dialdehyde naming and functional group priority 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.
Copyright notice: This OLS revision content, including the explanations, layout, diagrams, tables and embedded learning structure, is authored for Online Learning System by Dr. Mohammed Al-Fatah. It may not be copied, reproduced, redistributed or adapted without written permission.