aldehyde must end

[Tasnim Sazzad 4B]

Why must an aldehyde be at the end of a carbon chain?

[Nitin Joseph]

This is because an Aldehyde functional group looks like this: CHO, where carbon is double bonded to oxygen, and single bonded to hydrogen. This carbon is also connected to another carbon atom, like this: C-C-C-CHO. So it's got all 4 valences filled (1 by carbon, 1 by hydrogen, 2 by oxygen), so it has no more room for any more carbons, so it has to be at the end of a chain.

If it was in the middle of a chain, two of its valences would be occupied by carbon atoms on either side, so it has only 2 valences left to accommodate an oxygen double bond and a hydrogen single bond, which it cannot do.

[Chem_Mod]

Specifically, an aldehyde is a carbonyl attached to both a carbon and a hydrogen, and the hydrogen on one side is what makes it at the end of the chain. If the carbonyl is between two carbons, it is then a ketone. If it is between a carbon and oxygen, it is then a carboxylate. What is on either side of the ketone is what dictates its name.