12.3

IUPAC Nomenclature of Aldehydes

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IUPAC names of aldehydes are obtained by replacing the final -e of the longest –CHO group containing the parent hydrocarbon with ‑al.

In acyclic aldehydes, parent chain numbering begins from the aldehydic carbon. However, due to the terminal position of the aldehyde group, the locant value of this carbon is not part of the name.

In cyclic compounds comprising the aldehyde group directly attached to a ring system, ‘carbaldehyde’ is suffixed to the parent name.

The ring carbon adjacent to the aldehydic carbon is considered carbon-1, and the numbering continues in the direction that gives a lower value to the next substituent on the ring.

If a molecule has multiple functional groups, it is named based on the priorities of the different groups.

For instance, if the aldehyde group is part of the parent chain containing a higher priority group, it is prefixed as ‘oxo’ to the parent name.

However, if the aldehyde group is not part of the parent chain, the prefix ‘formyl’ is used with the parent name.

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Aldehydes are named based on the systematic nomenclature rules set by the IUPAC. For acyclic aldehydes, the longest carbon chain containing the aldehydic (–CHO) group is considered the parent chain. The aldehyde is named by replacing the last letter “e” in the hydrocarbon name with “al”. For instance, a simple, seven-carbon-membered acyclic aldehyde is called heptanal, derived from heptane. The carbon chain is numbered starting from the aldehydic carbon, although the aldehydic carbon’s locant value—1—is not included in the name. Names of any substituents that are present on the carbon chain are included as prefixes to the parent names, along with their corresponding locant values.

Chemical structure diagram of hexanal, aldehyde chain, organic chemistry molecule study.

Figure 1: Heptanal

Cyclic aldehydes are named by appending the word “carbaldehyde” at the end of the parent name of the ring. For instance, an aldehyde with the –CHO group linked to cyclohexane is called cyclohexanecarbaldehyde. The numbering of the carbon chain in a cyclic aldehyde does not start from the aldehydic carbon; instead, it begins from the ring carbon adjacent to it and continues in the direction that gives the lowest value to a substituent, for example, 2-ethylcyclohexanecarbaldehyde.

Chemical structure of cyclohexanone; molecular diagram for organic chemistry education.

Figure 2: 2-Ethylcyclohexanecarbaldehyde

If a molecule has different functional groups, acids get the highest priority, followed by esters, aldehydes, ketones, and alcohols. When a compound contains an aldehydic group and a higher priority group in the parent chain, the aldehydic group is considered a substituent; that is, the numbering of the parent carbon chain begins from the higher priority group. The presence of the aldehydic group is indicated by prefixing “oxo” to the parent name along with its locant value if the aldehydic group is at the end of the parent chain; for example, 3-oxopropanoic acid, where the carbon atom of the aldehydic group is part of the three-carbon parent chain.

Organic molecule, glyoxylic acid structure, chemical formula, diagram, carbonyl, carboxyl groups.

Figure 3: 3-Oxopropanoic acid

However, if the aldehydic group is a substituent of the parent chain, it is also represented as a formyl group; for example, 2-formylbenzoic acid, where the aldehydic group is the lower-priority substituent of benzene.

Chemical structure of phthalic acid, diagram, organic chemistry, structural formula.

Figure 4: 2-Formylbenzoic acid

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IUPAC Nomenclature

Aldehyde Naming

Acyclic Aldehydes

Cyclic Aldehydes

Functional Group Priority

Carbon Chain Numbering

Substituent Locants