IUPAC Nomenclature of Aldehydes

Acyclic aldehydes are named by identifying the longest carbon chain containing the aldehydic (–CHO) group and replacing the final 'e' of the parent hydrocarbon name with 'al'. For example, a seven-carbon chain becomes heptanal. Numbering starts from the aldehydic carbon, though its locant value of 1 is not included in the name. Substituents are added as prefixes with their locant values.

Cyclic aldehydes are named by appending 'carbaldehyde' to the parent ring name. For instance, an aldehyde attached to cyclohexane is called cyclohexanecarbaldehyde. Numbering begins from the ring carbon adjacent to the aldehydic carbon and proceeds in the direction giving the lowest locant to any substituent, such as in 2-ethylcyclohexanecarbaldehyde.

The aldehydic carbon is always at the terminal position in acyclic aldehydes, so its locant is inherently 1. Since this position is fixed and unambiguous, including the locant would be redundant. The naming system omits it to simplify nomenclature while maintaining clarity about the aldehyde's location.

When a molecule contains both an aldehyde and a higher priority group like a carboxylic acid, the aldehyde becomes a substituent. If the aldehydic carbon is part of the parent chain, it is indicated with the prefix 'oxo' and its locant value, as in 3-oxopropanoic acid. If the aldehyde is not part of the parent chain, the prefix 'formyl' is used instead.

In cyclic aldehydes, numbering begins at the ring carbon adjacent to the aldehydic carbon and continues in the direction that assigns the lowest locant value to the next substituent on the ring. This priority rule ensures unambiguous naming and consistent identification of compound structures.

The prefix 'formyl' is used when an aldehyde group is a substituent on a parent chain containing a higher priority functional group and is not part of the main parent chain. For example, 2-formylbenzoic acid has the aldehydic group as a lower-priority substituent attached to benzene.

Functional group priority determines which group defines the parent chain. Carboxylic acids have the highest priority, followed by esters, aldehydes, ketones, and alcohols. When an aldehyde has lower priority than another group, it becomes a substituent named with 'oxo' or 'formyl' prefixes rather than determining the parent chain name.