Functional isomerism is a type of structural isomerism in which compounds have the same molecular formula but different functional groups. In the case of acetone (CH3COCH3) and propanal (CH3CH2CHO), they both have the molecular formula C3H6O but differ in their functional groups.

Acetone, also known as propanone, is a ketone with the functional group being a carbonyl group (C=O) bonded to two alkyl groups (methyl groups in this case). On the other hand, propanal, also known as propionaldehyde, is an aldehyde with the functional group being a carbonyl group bonded to one alkyl group (ethyl group in this case) and one hydrogen atom.

To better understand why acetone and propanal are functional isomers, let's break down their structural formulas:

Acetone:
CH3COCH3
- The carbonyl group (C=O) is located in the middle of the molecule.
- It is surrounded by two methyl (CH3) groups.

Propanal:
CH3CH2CHO
- The carbonyl group (C=O) is located at the end of the molecule.
- It is surrounded by one ethyl (CH3CH2) group and one hydrogen atom.

As we can see, despite having the same molecular formula, acetone and propanal differ in the arrangement of their functional groups. Acetone has a carbonyl group surrounded by two alkyl groups, while propanal has a carbonyl group surrounded by one alkyl group and one hydrogen atom. This difference in functional groups results in different chemical properties and reactivities for the two compounds.

In conclusion, acetone and propanal are functional isomers because they have the same molecular formula (C3H6O) but different functional groups. Acetone is a ketone, while propanal is an aldehyde, and their structural formulas reflect the difference in the arrangement of their functional groups.