At the END of a hydrocarbyl chain……..

An ##”aldehyde”## contains the ##R-C(=O)H## and thus is has to be at a chain terminus. On the other hand, a carbonyl group that is flanked by 2 alkyl groups (or non-hydrogen) groups is a ##”ketone”##. ##”Aldehydes”## tend to be much more reactive than ketones in that steric access to the carbonyl centre is greatly improved.