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.
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.
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.
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.
Figure 4: 2-Formylbenzoic acid
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