What type of isomerism do Ketones and Functional groups have?

I was doing a question where it asked me to compare the molecule Propanal and molecule X, which is an isomer of it;

X's structure is CH3COCH

And somehow the isomerism between these two is functional group and not positional, even though, they both have the same functional group. Can someone please explain why it's not positional isomerism, and actually functional group isomerism.

Reply 1

TypicalNerd

Original post by GuadaLupi

First and foremost, a functional group is a particular group of atoms on a molecule that is arranged in a particular manner, leading to characteristic chemical properties.

Ketones and aldehydes aren’t the same functional group. Yes, they both contain a C=O bond that allows them to undergo similar reactions, but the presence of a hydrogen on the same carbon as the one in the C=O bond in an aldehyde makes all the difference.

For example, try adding acidifed potassium dichromate (an oxidising agent) to separate samples of propanone and propanal. Propanal will react and turn the solution from orange to green, whereas propanone will not.

This is because the aldehyde functional group is capable of acting as a reducing agent, whereas the ketone functional group is not. This demonstrates that the aldehyde and ketone functional groups demonstrate different characteristic chemical properties and are therefore separate functional groups.

For positional isomerism to apply, you’d need to molecules to contain the same functional group, but this is clearly not the case.