Nomenclature II Aldehydes & ketones II Dr. Rizwana

Aldehydes and ketones get their names from how the carbonyl group sits in the carbon chain—especially whether the carbonyl carbon is terminal (aldehydes) or internal (ketones). For aldehydes, the carbonyl carbon is always treated as carbon number 1, because the carbonyl group sits at the end of the chain. That terminal placement forces a consistent naming pattern: the parent alkane name is formed, then the “-e” is replaced with “-al” (for example, methane becomes methanal; ethane becomes ethanal). When substituents appear, they’re named as prefixes, and the numbering follows the rule that the carbonyl carbon remains carbon 1.

Ketones follow a different logic because the carbonyl carbon can appear anywhere along the chain. Instead of assuming a terminal position, naming requires specifying the carbonyl’s location with a number. The parent chain is named like an alkane, but the “-e” is replaced with “-one” (or “-anone” in the examples given). For ketones, the carbonyl carbon is not automatically carbon 1; the correct numbering is chosen so the carbonyl carbon receives the lowest possible number. That’s why the same carbon skeleton can yield different names depending on which end is numbered first.

The transcript also lays out how to handle multiple functional groups. When two aldehyde groups are present on the same parent chain, the naming shifts to a “di-” aldehyde pattern: the parent alkane is named and “-dial” is used to indicate two aldehyde groups. Substituents are still numbered and listed as prefixes, with the carbonyl positions built into the base name. For ketones, the presence of additional functional groups changes priorities: aldehyde vs ketone ordering is treated as a matter of preference, and the naming framework uses the parent chain associated with the higher-priority group.

Aromatic systems get special treatment too. When an aldehyde appears on a benzene ring, it’s not written as “-al” in the same way; instead, the naming uses “formyl” as the functional-group prefix (noted as “formyl” meaning “-CHO”). For carboxylic acids, the transcript emphasizes that carboxylic acid takes priority over ketones when both occur on the same aromatic ring, so the parent name is built around the acid (e.g., benzoic acid derivatives). Ketones on aromatic rings are then handled as substituents using “oxo” prefixes (the transcript explicitly uses “oxo” when ketones are expressed as prefixes rather than suffixes).

Across examples—ranging from simple aldehydes and ketones to halogenated, methyl-substituted, and multi-functional aromatic compounds—the core takeaway is consistent: aldehydes lock the carbonyl carbon to position 1, ketones demand the carbonyl position number, and mixed functional groups require choosing the parent based on priority before assigning the remaining groups as prefixes or suffixes.