Aldehyde and ketone nomenclature when aldehyde/ketone group is bonded to an alkyl group The longest alkane chain is decane and, attached to the fifth carbon, is an ethyl group the end of which is double bonded to an oxygen atom. My best guess at naming it would be:
5-(ethyl-2-al)decane.

Here, the longest chain is again decane, but there is a propyl group and an oxygen atom double bonded not to the ends of the alkyl, but to carbon #2, which leads me to:
5-(propyl-2-one)decane
Now I have no clue as to whether I am right or not because I have never come across such molecules and I have just done what I found to be logical. Have I come to the correct conclusions?
The characteristic group $\ce{=O}$ corresponds to the substitutive prefix ‘oxo’.
Thus, the substitutive names that you are looking for are ‘5-(2-oxoethyl)decane’ and ‘5-(2-oxopropyl)decane’.

However, according to the current version of Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book), the preferred names for aldehydes and ketones derived from alkanes are constructed using the suffixes ‘al’ and ‘one’, respectively.

Furthermore, the selection of a preferred parent structure is based on the seniority of classes, which gives priority first to characteristic groups expressed as suffixes and then to parent hydrides when different classes are present.

Therefore, the preferred IUPAC names for the given compounds are ‘3-butyloctanal’ and ‘4-butylnonan-2-one’.