Q.Which suffix do appear in the IUPAC names of aldehydes and carboxylic acids ?

Both families are named by locating the longest chain through the carbonyl carbon and swapping the alkane's '-e' for '-al' (aldehyde) or '-one' (ketone). An aldehyde's carbonyl carbon is ALWAYS C1, since -CHO can only sit at a chain end; a ketone's carbonyl carbon instead gets the LOWEST possible locant among its two numbering directions, since it can sit anywhere along an interior chain -- e.g. pentan-2-one, not pentan-4-one. A second, ketonic carbonyl appearing as a substituent on a chain whose PRIMARY suffix is already '-al' (only one group can be the suffix) is cited instead as an 'oxo-' prefix with its own locant, e.g. 3-oxopentanal. Common names (forma …