Question

Aliphatic aldehydes dont show position isomerism..why?

Answer 1

Aldehyde group is -CHO. Due to the presence of the hydrogen, the aldehyde can only be present at the terminal of the alkyl chain, or else it will become a ketone if placed in between. So, since aldedhydic group can only be placed at either ends of the chain, no position isomerism is observed.

Answer 2

Because all of them are 'chain-terminating-functional-groups'.

Let me explain that for you, Aldehydes always end the chain, there is no C ahead of the Aldehyde.

R-C-C-C=O
             |
            H
Let this be an aldehyde, now look at the Carbonyl Carbon (That's the Carbon which has got a double bond with Oxygen). For it to be an aldehyde, it has to have a double bond with Oxygen,a  single bond with Hydrogen and a Single bond with Carbon to make sure, its part of the chain